Meteorology Handouts

 

METEOROLOGY AND OCEANOGRAPHY TABLE OF CONTENTS

CHAPTER 1. 2.

TITLE

PAGE

Shipborne meteorological instruments The atmosphere, its composition and Physical properties

3 4 5 6 7 8

Atmospheric pressure Wind Cloud and precipitation Visibility The wind and pressure systems over the ocean Structure of depressions

9 10 11 12

Anticyclones and other pressure systems Weather services for shipping Recording and reporting weather observations Weather forecasting

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METEOROLOGY

CHAPTER 1 Shipborne meteorological instruments Shipborne Meteorological Instruments 

Shipborne Instruments

In order to get as complete a picture as possible of the weather, careful observations should be made. Many of the observations are made visually; for example, the form of clouds, and direction of the wind. Instruments make other observations; for instance one cannot find the pressure or the relative humidity although one may guess the air temperature. Various instruments have been designed to observe the different phenomena. The principal ones are pressure, temperature and wind velocity, whilst others have been designed to measure sunshine hours, density of the water and rainfall.

This is constructed by filling a tube, about 1 metre long with mercury. 2

 

The end of the tube is temporarily closed and is inverted and placed into a reservoir of mercury. When the closure is removed it will be seen that the level of mercury fills in the tube. The space above the mercury at the top of the tube is known as a Torricellian vacuum. If in air bubble were to get into this space it would depress the mercury (as the vacuum would no longer be complete) and an incorrect reading would result. To prevent this, an air trap is incorporated in the tube. A capillary tube is connects the reservoir and the tube.

The mercurial barometer is liable to error on account of the following: CAPILLARITY The surface tension of the mercury forms a MENISCUS and readings should always be taken at the top of this. CAPACITY The height of the barometer should be taken from the top of o f the mercury in the cistern to the top of the mercury in the marine tube. If the pressure increases, the level of the mercury in the cistern falls, so that the new measurements cannot be taken from a fixed point. Adjusting the distance between the graduations compensates compensates this error. PUMPING Due to the constant change in height above mean sea level of a barometer on a vessel in a seaway, the there will tend to be a continual change of reading. This movement of the mercury will make it difficult to get in accurate Reading. Gusty winds can also cause 3

 

pumping. Fitting the capillary tube above the air trap considerably reduces the effect of pumping. (If pumping is present try to get a mean reading) HEIGHT All readings should be corrected to sea level. Increase of height means a decrease of pressure by approximately 1 millibar for every 10 m. This correction can be made by tables or by the Gold Slide LATITUDE Due to the earth being somewhat ‘flattened’ at the poles, mercury weighs more at the poles

than at the equator. For equal atmospheric pressures the barometer would appear to read less at the poles and more at the equator. Readings should all be corrected for mean latitude of 45 . The correction can be made by tables or by the gold slide. 

TEMPERATURE The column of mercury will expand, with an increase of temperature and contract with a decrease of temperature in exactly the same way as does a thermometer. All readings must be reduced to a standard temperature, which is 285 K in the case of most 

millibar barometers. The correction can be made by tables or by the Gold Slide N.B. The attached thermometer should always be read before the barometer as otherwise heat from the observers body may’ give a false reading.

The temperature at which a barometer reads correctly is known as the FIDUCIAL TEMPERATURE. In lat. 45  at sea level this is the same as the standard temperature but at sea level in lat. 

57  the FIDUCIAL temperature would be 291 K, and in latitude 21  at sea level it, would be 

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273 K. Whereas at 20m above sea level in 45 it would be 297 K 

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OBSERVATIONAL ERRORS The barometer should always be upright; it is the vertical height of the column of mercury that balances the column of air. If the barometer is not upright, too high reading is obtained. The back and front of the vernier must be on the same level as the observer’s eye, otherwise the reading will be too high. GOLD SLIDE Is not made of gold but since its inventor was Lt. Col. Gold, so it is called such. This gives a rapid means of getting the latitude, height and temperature correction. To use the slide set the height of the barometer above sea level, against the latitude and read off the correction opposite the top of the mercury in the thermometer.

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ANEROID BAROMETER This is a very much more robust and compact instrument than the mercurial barometer. The main component is a vacuum box, which is partially exhausted of air. An increase of atmospheric pressure compresses this box causing the pointer on the dial (via the lever system) to register a higher pressure. Converse occurs with a decrease of pressure.

As there is no mercury in this barometer there are no corrections for latitude or temperature, but a height correction must be applied. There is an adjustment screw on the back of the instrument to take out any index error. The greater the area of the vacuum v acuum box the greater the accuracy of the instrument. It is usual to give the barometer a light tap before reading this helps to free the fine chain, which may stick if pressure changes are only small. It is simpler to transport and to read while temperature correction is unnecessary. Height corrections can be ‘built in’ by resetting the datum on the instrument. A pressure choke can

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be attached if rapid height variations, leading to rapid pressure variations, are e expected; xpected; this smoothes the variations to negligible ne gligible amounts. BAROGRAPH This is a recording aneroid barometer. A lever system connects the vacuum pile to a pen arm which makes a mark on the chart on the drum which is driven d riven by clockwork. The drum is wound and the chart changed weekly. The prime purpose of the barograph is to record the pressure tendency.

TEMPERATURE AND ITS MEASUREMENT

The forecasting of weather depends as much on knowledge of temperature as of pressure. The instruments for measuring temperature all depend on the expansion and contraction of liquids or metals when heated and cooled. The thermometer in its simplest form consists of a capillary tube on the end of which is a bulb filled with mercury. This thermometer is graduated by placing it in pure melting ice and marking the position p osition of the mercury, then placing it in boiling distilled water and again marking the position of the mercury. The barometric pressure in each case should be 760mm of mercury. These two points are known as the fixed points of the thermometer. The part of the tube between these points is then divided into a number of equal divisions. The number of divisions depends on the scale to be used. The various scales are:

Conversions from one scale to another can be made by: 6

 

Celsius:

- (Fahrenheit  - 32 x 5/9 5/9  

Fahrenheit:

- (Celsius  x 9/5) + 32  

Absolute:

- Celsius  + 273  

Reamur:

- Celsius  x 4/5

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At (-) 40  both the Celsius and the Fahrenheit scale readings are the same. 

PSYCHROMETER OR HYGROMETER Mason’s Hygrometer consists of two thermometers mounted side by side in a Stevenson’s

screen. One is a dry bulb thermometer, the other a wet bulb thermometer. Cambric or muslin is wrapped round the bulb of the wet bulb and it is kept moist by means of a piece of cotton wick leading to a container of distilled water. The evaporation of water requires heat and this is taken from round the wet bulb, which, unless the air is saturated, shows a lower reading than the dry bulb. Ensure that no finger touches the muslin; the oil from the finger would prevent the muslin from acting like a capillary tube and drawing the water up. The screen and thermometers should be hung up to windward away from local draughts or warm air currents and away from direct sunlight. It definitely should not be placed within an enclosed room. WHIRLING PSYCHROMETER.

More accurate readings can be obtained by using a whirling psychrometer. This looks rather like a rattle. The whirling ensures a steady flow of air over the two bulbs By entering tables with the dry bulb temperature and the difference between the wet and dry bulbs as, arguments the dew point and relative humidity can be found. The number of whirls to be given should be adequate to cool down the wet bulb thermometer. Ensure that no finger touches the muslin; the oil from the finger would prevent the muslin from acting like a capillary tube and drawing the water up.

WIND MEASURING INSTRUMENTS

The Robinson Cup Anemometer consists of four hemispherical cups fixed to the ends of rod set 90  from each other in a horizontal plane. The spindle, to which the rods are attached, is 

connected to a tachometer and from the number of revolutions made in a given time the ‘run’ of the wind can be calculated. 

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The anemometer assembly is generally fitted on top of a mast or a place without any obstruction to the wind, the signals from this is led to the wheelhouse repeaters by cables. Different calibration may be used to read the wind speed. Note that the wind as observed is the relative wind and not the true wind, which has to be calculated. Equipment fitted with a gyro feed can however give true wind also, the calculations being done by microprocessors within the equipment. HYDROMETER

The hydrometer works on Archimedes’ principle that a floating body displaces its own weight of the liquid in which it floats. It consists of a float chamber through which passes a stem, the lower end of Which is weighted so that it floats upright. The upper end is graduated to read re ad the density/ specific gravity of water. These are generally made of nickel plated brass and should be kept clean.

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The hydrometer is used to measure the density of the water and is one of the most important measuring instruments on board a vessel. As soon as the vessel arrives in port the density of the water is noted. If the dock water is connected to a river or is a river port then the water density should be taken at two levels surface and half the draft, and the full draft an average of the densities will give the correct draft. All bulk loaders should be aware of the necessity of taking the density especially at the 3 levels if the river is experiencing e xperiencing any heavy rainfall.

CHAPTER 2 The atmosphere, its composition and Physical properties 

 Atmosphere and Atmospheric Pressure  The Atmosphere

Most of the weather changes take place in the lower layer of the atmosphere, which is known as the TROPOSPHERE. TROPOSPHERE. This layer extends about 11 miles-high over the equatorial regions and about 5 miles high over the Polar Regions. The layer above the troposphere is known as the STRATOSPHERE where there is little water vapour and the lower part appears to be isothermal. There is a rise in temperature towards its upper limit.

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The boundary between the troposphere and the stratosphere is known as the TROPOPAUSE. About 20 miles above the earth the stratosphere gives way to the OZONOSPHERE, where there is a high concentration of ozone which absorbs u ultra ltra violet radiation. The IONOSPHERE starts about 50 miles above the earth. The density of the atmosphere decreases with height. Approximate percentage volumes of the various gases forming dry air in the troposphere are nitrogen 78%, oxygen 21%, argon 0.9%, carbon dioxide 0.03%, with the balance being made up of traces of hydrogen, helium, neon, krypton, radon, xenon and ozone. As most of the weather changes take place in the troposphere, it is in this region that the changes in temperature arc most important. Main features of the troposphere As altitude increases, air pressure decreases due to the decreased weight of air above. With

less pressure, the density decreases. More than three-fourths of the air is concentrated within a layer averaging about 7 statute miles thick, called the troposphere. This is the region of most “weather,” as the term is commonly understood. 

The top of the troposphere is marked by a thin transition zone called the tropopause, immediately above which is the stratosphere. Beyond this lie several other layers having hav ing distinctive characteristics. characteristics. The average height of the tropopause ranges from about 5 miles or less at high latitudes to about 10 miles at low latitudes.

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Sun as energy source

The sun, at a distance of about. 93,000,000 miles and at a temperature of about 6,000 C, is 

the principal source of light and heat for the earth. The heat from the sun travels to the earth in the form of short wave radiation, which passes through the atmosphere without appreciably warming it. On striking the earth some of the heat will be absorbed to warm the earth. The heat received at the earth from the sun is known as INSOLATION. The amount of insolation per unit area varies with latitude. It can be seen in the sketch below that a band of rays has to heat a very much larger area in a high latitude than it does in a low latitude.

The increase in the temperature of the earth will depend, amongst other things, on the amount of insolation and the specific heat of the earth. Assuming that the insolation is constant, a surface with a high specific heat warms and cools les s quickly than a surface with a low specific heat. For instance, the sea temperature in non-tidal waters hardly changes in the 24 hours. A contributing factor to this small change is its depth. It will be found that, t hat, in general, sea temperatures are less than the temperatures of adjacent land by day and greater by night. There is a similar difference in summer and winter. The amount of insolation will depend on the sun’s altitude and the degree of cloud cover. It  

may be noted that the SOLAR CONSTANT, which is the intensity of solar radiation at the outer boundary of the atmosphere, is 1.39kw/m2. The height of the station under consideration above sea level will be a factor influencing the heating, as will the prevailing wind. Air which his flowed over warm surfaces will have a smaller cooling effect than air which has flowed over cold surfaces. As the earth is a warm body it radiates heat. This radiation will cool the surface. 11

 

If the earth is radiating heat at a greater rate than it is receiving it, the net result will be a cooling of the surface. The converse is also true. In the diagram below the curves of insolation and radiation are shown to increase and decrease at a regular rate for case of illustration.

Quite clearly the temperature falls until point A is reached when it begins to rise. The rise continues to point B and then, once again, the temperature falls. It will be noted that the lowest temperature occurs shortly after sunrise whereas the maximum temperature occurs about 1400 hours local time, and not when insolation is minimum and maximum. A similar lag takes place on an annual basis. In northern latitudes the lowest temperatures do not usually occur until February and the maximum temperatures occur in July and early August. The sea temperature lags even further behind the sun. The minimum occurring in March and the maximum in September. It must be understood that all the above general statements can be considerably modified by local weather conditions, particularly cloud amounts. Nature of solar radiation

Heat is transferred to the air surrounding the Earth by one or more of the following means: 1.

Radiation

2.

Conduction

3.

Convection

4.

Turbulence.

S.  RADIATION. The sun’s rays being in the shorter wavelength do not appreciably heat up the air, rather the

heating is done of the water vapour particles in the atmosphere,

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The sun does however heat the Earth, and the Earth after getting heated up re-radiates heat. This earth heat is in the form of long waves. If there is no cloud cover then the heat is transferred to space, but with a cloud cover the heat is radiated back on the earth. Thus on a cloudless night the heat radiated out by the Earth is considerable and a lot of surface cooling takes place. 2.

CONDUCTION.

The heat is transferred from particle to particle. Thus, air, which is in contact with a warm surface, is warmed; this occurs by day when the sun su n is shining. Air, which is in contact with a cold surface, is cooled; this occurs on clear nights. Air masses acquire their characteristics by the same process. 3.

CONVECTION.

Air, which is warmed, expands and consequently its density decreases. This makes it lighter than the un-warmed air surrounding it and the warmer air rises. Convectional processes take large amounts of warm air and water vapour from the surface to the upper levels. When the water vapour condenses into water drops and precipitation occurs, the latent heat remains. Most of the atmospheric heating takes place in this way. Conversely, air, which is colder than the surrounding air, has a greater density, and as it is heavier, it sinks. An example of this is the Katabatic wind.

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4.

TURBULENCE.

Air, which flows over a rough surface tends to be deflected d eflected upwards. The Rising air will be replaced by some air from levels up to 600m giving an interchange of air between the surface and 600m. The rising air carries its warmth (acquired by conduction) with it, and the falling brings its coolness. LAPSE RATE

Increase of height above sea level generally means a decrease of temperature. The rate at which the temperature changes with height is known as the lapse rate, an average value being about 0.7°C/100m. ENVIRONMENT LAPSE RATE The lapse rate can vary and the environment lapse rate will be referred to when a particular air mass is under consideration. This rate will be dependent on many factors and will vary with the altitude, although it is usually between the S.A.L.R. and D.A.L.R. Plotting temperatures of the air at various heights will give environment curves.

ADIABATIC TEMPERATURE CHANGE If a volume of air rises to a region of lower pressure, the volume will increase and following the gas laws the temperature of the volume will fall. The change of temperature is solely due to expansion or contraction and no heat has been given to or received from the adjacent air. This change of temperature is known as an Adiabatic Temperature Change. DRY ADIABATIC LAPSE.RATE (D.A.L.R.). When dry air is forced to rise it has been found that it decreases its temperature by 1°C/100m and this is known as the Dry Adiabatic Lapse Rate. With dry air, which is forced down, an increase at a similar rate is found. Dry air is any air, which is not saturated. SATURATED ADIABATIC LAPSE RATE (S.A.L.R.) Latent heat is the heat necessary to change 1 kilogram of water to 1 kilogram of Vapour at saturation temperature. If a quantity of water changes to vapour an amount of latent heat will have been required. This sort of thing will happen with air at a temperature above dew point (note however that the higher the temperature of the air the more water vapour it will normally contain- unless some artificial means have been employed to reduce the water vapour content). The latent heat that has been originally required to change the water to its vapour state has not been lost but is retained in the mass of the water vapour. Now when the water vapour 14

 

is cooled and the water condenses back the latent heat is released rele ased and this is known as the latent heat of condensation. The latent heat of vapourisation is generally accepted as 540 calories/gramme of water. When the cooling of the air to its dew point temperature is caused by the air rising r ising the latent heat released will modify the overall cooling rate. For the air that is rising: D.A.L.R. Latent Heat S.A.L.R.

(-) 1.0°C/100m (+) 0.5°C/100m (average for sea level in middle latitudes) (-) 0.5°C/100m

The S.A.L.R. is a variable quantity depending on the latent heat of the condensing water vapour. The S.A.L.R. is low near the equator and high in Polar Po lar Regions. It will also increase with height.

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STABLE AIR. If air, which his been forced to rise (or fall) from its initial level tends to return to that level it is said to be stable. This condition occurs when the environment lapse rate is less than the S.A.L.R.

The figure shows in black the environment curve of some tropical air, whose lapse rate is about 0.3°C/100m. Suppose some dry air at point A is forced upwards, it will cool at the D.A.L.R. and follow the red line. When the rising air reaches the level BB1 it will be much colder than the environment or surrounding air. As it is colder it is denser, and will want to return to A, a tendency shown by the dotted line. If instead of the air rising from A, it fell, then at the level CC, the failing air would have warmed adiabatically to a considerably higher temperature than the environment or surrounding air. It would then be less dense and the tendency would be for it to return to A as indicated. If the air forced to rise or fall from A were saturated instead of dry, the path indicated by the green line would be followed. Again, for the same reasons as are given above, the tendency is for the air to return to A. 16

 

UNSTABLE AIR. If air, which has been forced to rise (or fall) from its initial level tends to continue its upward (or downward) movement, it is said to be unstable. This condition occurs when the environment lapse rate is greater than the D.A.L.R.

The figure shows in black the environment curve of some polar air whose lapse rate is about 1.2°C/100m. Suppose some saturated air at point A is forced upward it will cool at the S.A.L.R. and follow the green line. When this rising air reaches the level BB1, it will be warmer than the environment or surrounding air. As it is warmer it will be less dense and the tendency is for it to continue upwards. If, instead of the air rising from A, it fell, then at the level CC1, the falling air would be at a lower temperature than the environment or surrounding air. It would then be denser and it would continue to fall. If the air forced to rise or fill from A were dry instead of saturated, the path indicated by the red line would be followed. Again, for the reasons given above, the rising or falling air continues to rise or fall. NEUTRAL AIR.

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This condition occurs when the environment lapse rate is the same as the D.A.L.R., if dry d ry air is under consideration, or the. S.A.L.R. in the case of saturated air. The air which has been forced to rise or fall will have no tendency to continue its upward or downward movement nor will it have any tendency to return to its original position. CONDITIONAL INSTABILITY. This occurs when stable conditions exist for dry air and unstable conditions exist when the air is, saturated. The figure illustrates this.

The environment curve has a lapse rate between the S.A.L.R. and the D.A.L.R. The red line representing dry air forced up or down follows f ollows a similar pattern to earlier figure for stable air, whilst the green line representing saturated air forced up or down follows a similar pattern to figure for unstable air. This conditionally unstable condition is quite common, the air below the condensation level being stable and that above being unstable TEMPERATURE INVERSION The usual tendency for the temperature to fall with height is sometimes reversed. When the temperature increases with height an inversion is said to exist. An inversion also exists if a layer of air is isothermal. Inversions may occur at any level at all. They frequently, occur at ground level on clear nights. They also occur just above cloud layers. As an inversion gives rise to very stable conditions, convection and turbulence are damped out and there will be no upward movement of air through the inversion. 18

 

Inversions may be caused in anticyclone conditions by subsiding air warming adiabatically to a temperature above that of the lower layers of air.

The figure shows a curve for the environment where there is an inversion above the surface. Dry and saturated air forced up from A will clearly return showing stable conditions, whereas if forced up from B (or down from A) unstable conditions exist. Atmospheric Pressure

The atmosphere by reason of its weight exerts a pressure on the surface of the earth. This pressure is normally measured in millibars, the mean value at sea level being b eing around 1013 mb. This pressure is in certain places semi-permanently above the mean, while in other places it is semi permanently below the mean. These places are referred to as regions of high and low pressure respectively. There are also temporary areas of high or low pressure. CHAPTER 3

Wind   Wind

Because of the rotation of the earth, air, which is drawn towards a centre of low pressure, is deflected to the right in the N hemisphere and to the left in the S hemisphere. The result is an anti-clockwise circulation of wind around an area of low pressure in the N hemisphere and a clockwise circulation in the S hemisphere. Circulations around areas of low pressure are termed cyclonic. Conversely, the wind circulates in a clockwise direction around an area of high pressure in the N hemisphere and in an anti-clockwise direction in the S hemisphere, such circulations being termed anticyclone. 19

 

The strength of the wind at any given time depends de pends upon the pressure gradient, i.e. on the spacing of the isobars. Isobars are lines which join together places which at the same time have equal barometric pressure (reduced to sea level) and are analogous to the contour lines of a map; the closer they are together the greater the pressure gradient and the stronger the resulting wind. Surface friction has two effects on the wind. Firstly it causes a reduction in the strength of the wind at the surface and secondly it causes the wind to be deflected some 10° to 20° across the isobars, inwards towards the centre of low pressure or outwards away from the centre of high pressure. Buys Ballot’s Law sums this up as follows: 

If you face the wind the centre of low pressure will be from 90° to 135° on your right hand in the N hemisphere, and on your left hand in the S hemisphere. The diagram below shows the distribution of pressure and the winds, which would result over a featureless earth.

The effect of the distribution of land and sea s ea

The effect of large landmasses is to modify considerably the areas of pressure and the wind, as shown in the diagram. The belts of high-pressure around 30°N and 30°S are split into separate cells of high pressure (anticyclones) situated over the E part of each of the oceans. The belt of low pressure around 60°N is similarly modified into separate areas of low pressure situated near Iceland and the Aleutian Islands.

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In the S hemisphere there is little or no land in the area covered by this low low-pressure -pressure belt and consequently it extends almost without interruption around the earth. Superimposed upon these modifications there is a tendency for pressure to become relatively high over landmasses in the winter and relatively low in the summer. Such seasonal changes in pressure distribution can produce large-scale modifications to winds over neighbouring oceanic regions, a notable example being the monsoon wind circulation over the Indian Ocean. de clination Effects of variations in the sun’s declination. The annual movement of the sun in declination causes the belts of pressure and their associated winds to move towards each pole during its summer. This oscillation amounts to some 4° of latitude and it lags some 6 to 8 weeks behind the sun. The Equatorial Trough (Doldrums) is an area of low pressure situated between the Trade Winds of the two hemispheres. Characteristic features of this area are light and variable winds alternating with squalls, heavy rain and thunderstorms. The Trough varies greatly in width both daily and seasonally. The type of weather experienced also varies considerably. At times a ship may cross the Trough and experience fine weather, while on another crossing squalls and thunderstorms may be encountered. Weather in the Trough is generally worst when the Trades are strongest. This is a highly simplified account of an area where the weather is complicated and not completely understood.

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Consider the two columns of air AB and CD in the figure. These are of equal cross sectional area, height and temperature, so that the pressures at A and C are equal, as are those at B and D. Suppose the column AB is warmed it will expand to AE, if the area remains constant, and although the pressure at A will remain unaltered, the pressure at B will have increased relative to that at D, and to equalize these air will flow from B to D. This transfer of air from column AB to CD will cause a drop in pressure p ressure at A and a rise at C, and air will now flow from C to A. This illustrates the vertical circulation of air in that the out flowing or diverging air from C is replaced by subsidizing air from above, whilst the inflowing or converging air at A escapes to the upper levels and there completes the circulation back to D. The rate of flow of air from C to A will depend on the pressure gradient between C and A. The movement of air straight from a high pressure to a low pressure only occurs either very close to the equator or in places where the high h igh and low-pressure areas are small and their centres are near to each other. Elsewhere, due to the rotation of the earth, there is a deflective force, which prevents the air flowing directly from high pressure to low. This force is known as the GEOSTROPHIC GEOSTROPHIC or CORIOLIS force It must be emphasized that for the GEOSTROPHIC wind which blows at and above 600m, the surface wind speed is about 2/3 that of the geostrophic geostrop hic wind. 22

 

It is useful to know that for equal pressure gradients the wind velocity in the tropics (lat. 15 ) is tree times as great as that in temperate latitudes (52 ) 

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If the isobars are curved as in a high or low-pressure area, a further force will come into being. This force, an outward force from the centre of high or low pressure is called the CYCLOSTROPHIC FORCE. CYCLOSTROPHIC The angle, which the wind makes with the isobars, is known as the ANGLE OF INDRAUGHT. The direction of the true wind should be noted in the logbook and in weather messages. It may be estimated by noting the direction of the waves. The approximate speed of the wind may be estimated from the appearance of the sea (see Beaufort scale). The apparent wind direction and the speed is the resultant of the ship’s course and speed and the true wind direction and speed. If any two of the foregoing are known the third can be found by plotting a triangle of velocities. Example: Vessel Course: 022.5 (T) Speed: 11.5k 

Apparent Wind: 6 points on the port bow - 067.5  Speed: Estimated at 15k 

Required true wind direction and speed.

AB represents the ship’s course and speed –  this also represents the apparent wind caused

by the movement of the ship, thus the arrowhead is in opposite direction to the course. 23

 

AC represents the apparent wind direction and speed. A double arrowhead is marked on this since this is the resultant of the true wind and the wind caused by the movement of the ship. Joining the points B to C would give the true wind direction and the distance resolved would give the speed of the true wind Thus for the example above we would have a true wind – West and speed of 15k. WIND ROSES arc found on climatological charts and they depict the frequency and strength of the winds, blowing from the various directions. There are many forms of wind rose, each having its own features. The following is a Baillie Rose, which is comprehensive and simple.

From the circle to the inner end of the wind arrow represents a scale of 5%. A thin line represents wind force 1 - 3; a double line represents forces 4 - 7, and a broad line force 8 upwards. The figures in the centre show the number of observations, the percentage frequency of variables and the percentage frequency of calms in that order. Thus the figure shows North winds 4% light, 10% moderate and 2% gales; NE Winds 10% light, 9%-moderate E winds 5% light, 4% gales S winds 5% light, 5% moderate SW winds 6% moderate, 7% gales W winds, 2% light, 4% moderate, 9% gales NW winds 3% moderate, 12% gales. The total number of observation is 376, the frequency of variables v ariables is 1% and there is a 2% frequency of calms.

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Beaufort Wind Scale Beaufort

Wind limit of wind

Scale

speed (knots) 

Description & Sea Criterion 

Number 

1

Light air 1-3k

Ripples with the appearance of scales are formed but without foam crests.

2

Light breeze 4-6k

Small wavelets, still short but more pronounced, crests have a glassy appearance and do not break.

3

Gentle breeze 7-10k

Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.

4

Moderate breeze 11-

Small waves, becoming longer; fairly frequent white horses.

16k 5

Fresh breeze 17-21k

Moderate waves, taking a more pronounced long form; many white horses are formed. (Chance of some spray).

6

Strong breeze22-21k

Large waves be-gin to form; the white foam crests arc more extensive everywhere (probably some spray).

7

Near gale 28-33k

Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. (spindrift begins to be seen).

8

Gale 34-40k

Moderately high waves of greater length; edges of crests break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.

9

Strong gale 41- 47k

High waves. Dense streaks of foam along the direction of the wind. Sea begins to roll. Spray may affect visibility.

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Storm 48-55k

High waves with long overhanging crests. The resulting foam in great patches is blown in dense white streaks along the direction of the wind. On the whole the surface of the sea takes a white appearance. The rolling of the sea becomes heavy and a nd shock like. Visibility is affected.

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Violent storm 56-63k

Exceptionally high waves. (Small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the t he edges of the wave crests are blown into froth. Visibility Affected.

12

Hurricane 64-71k

The air is filled with foam and spray. Sea completely white with driving spray.; visibility very seriously affected.

There is an extended scale for exceptional wind as follows   Force

Knots

13

72 - 80

14

81 - 89

15

90 – 99

25

 

16

100-109

17

110-118

CHAPTER 4

Cloud and precipitation

Cloud and Precipitation  Cloud and Precipitation

When air is cooled below its dew point the water vapour therein starts to condense out into water droplets. The water droplets form either fog or cloud depending on the process by which the air is cooled. It is generally understood that the fog is formed when the cooling of the air takes place at the surface by conductive processes, whereas cloud generally forms above the surface due to the adiabatic cooling of the rising air. The cooling of the air occurs as the air is forced to rise as follows (a)

At a warm front

(b)

At a cold front

(c)

By convectional processes

(d)

By high ground

(e)

By turbulence

Contributory causes in cloud formation may be (i)

Radiation of heat by water vapour in the atmosphere

(ii)

Mixing of two masses of nearly saturated air at different temperatures.

Clouds are classed as being either high above 6000 metres, medium 2500 - 6000 metres or low below 2500 metres. There are several types in each class, the main ones being b eing shown in the following table:

CLASS 

LOW 

STRATIFORM 

FREE CONVECTIVE 

LIMITED CONVECTIVE 

(Stable air

(Unstable air

(Air conditions tending

Conditions)

conditions)

to be unstable)

Stratus (St) 

Cumulus (Cu) 

Stratocumulus (Sc)  26

Nimbostratus

(Ns)   

The water vapour mentioned earlier cannot condense into large size water do unless condensation nuclei are present. These nuclei, which may be salt or dust particles, are always present although the numbers will vary from about 1000 per cm3 over the sea to about 150,000 per cm3 over industrial areas. Water drops of less than 0.1mm diameter cannot reach the ground as they evaporate on the way. These drops are cloud drops and their average diameter is 0.01mm. 0 .01mm. Drops over 0.1mm are drizzle drops and over 0.5mm are raindrops. The maximum size to which a drop can grow is 5.5mm. The process of growth of these water drops has brought forth many theories one of which is that in the original stages of condensation taking place, some drops form larger than the rest. Their growth due to the attraction of the smaller drops is then assured. Rain is usually described. as being either Frontal, Convectional or Orthographic. This is in accordance with the process of cloud formation; Appreciable rainfall is unlikely with turbulent cloud. FRONTAL RAIN occurs in a warm front. The precipitation commences from Altostratus cloud and increases in intensity as the lower nimbostratus cloud comes in. Where the front is not so marked occasional precipitation, might occur. This has the same characteristics as frontal rain and lasts less than 30 minutes. CONVECTIONAL RAIN falls when the strong up currents, within the cumulus type cloud, cease or are so reduced that they can no longer keep the rain drops within the cloud. It may be noted that an up current of 8m/s will keep the maximum sized raindrops from falling. If this velocity is slightly reduced, only the largest such drops will fall, and the smaller drops will remain in the cloud. A feature of conventional type rain is that rainfall always starts heavily and often with little warning. Another name for convectional rain is showers. These last less than 30 minutes although showers frequently merge to give a period of prolonged rainfall. ORTHOGRAPHIC ORTHOGRA PHIC RAIN. This falls on the weather side of mountain ranges and gives some of the heaviest rain known.

27

 

SNOW. When water vapour condenses at a temperature below freezing point ice crystals form. These join together to form snowflakes. If the air temperature is much above freezing point the flakes will melt. In general the temperature near the surface must not exceed 3 C 

for snow to fall. It should be understood that at temperatures less than 3 C rain is still very 

likely and is not necessarily replaced by snow. SLEET. This is partially melted snow. THE CUMULONIMBUS CLOUD (Cb) This is a cloud of great vertical development formed in unstable conditions and may extend up to 13000m from a base which may be as low as 500m. The Cb cloud may be formed At a cold front, At a mountain range, When there is considerable heating of the ground, or When cold air flows over a warm surface. However, by whichever method it is formed it will still have its characteristic appearance. Air currents at high levels may, and often do, produce in ‘anvil’. Within the cloud will be

water drops and ice crystals and in between the two there may be super cooled water drops. These latter are water drops in their liquid state at a temperature below freezing. As long as they do not contact anything they can remain liquid at temperatures down to (-) ( -) 40 C. If, 

however, they touch anything, they immediately freeze into globules of clear ice. HAIL. Due to the turbulence within the Cb cloud ice crystals frequently contact the Super cooled water drops. When this occurs the super cooled drop freezes to give a layer of clear ice round the ice crystal. This is the start of the hailstone, which will then grow as convection currents carry it through the cloud. During this process the stone acquires alternate cores, one of clear ice (from the super cooled drops) and one opaque ice (from the ice crystals). If a hailstone is cut open the various layers can be seen, and the number of times it has been carried up and down can be estimated. When the hailstone can no longer be supported within the cloud it will fall to the ground. The hailstone varies considerably in size being anything from 5mm to 50mm in diameter. THUNDERSTORM When raindrops reach their maximum size of 5.5mm they break up. This splitting gives the air a negative electrical charge whilst the raindrop attains a positive electrical charge. 28

 

This action takes place within the Cb cloud and it is found that there is a large positive charge near the top of the cloud. At the bottom there is a large negative charge with a small positive charge due to the raindrops. When there is sufficient difference of potential between the top and bottom of the cloud, or between two clouds, or between a cloud and the earth, there is a visible electrical discharge, which is called LIGHTNING. If air is cooled below its dew point by conduction (i.e. Contact with a cold surface) then the water vapour will condense into droplets on or near that surface. The radiation from the earth on cloudless nights will cause considerable surface cooling with the possible deposit of water droplets if the air near the earth e arth is cooled until saturated. DEW. The water droplets deposited on the surface by the above process. Clear skies and practically no wind are necessary conditions for its formation. HOAR FROST. Formation is the same as for dew except that the dew point temperature is below freezing. The freezing of water droplets, which were deposited as dew, may also form hoar frost. MIST is similar to fog but here the visibility is 1000m to 2000m. It should be noted that HAZE giving the same visibility as mist is formed of dust particles not water droplets.

CHAPTER 6 Visibility

Visibility   FOG

FOG occurs when visibility is reduced to less than 1000 meters, due to the below mentioned causes: Fog is caused by the cooling of air in contact with the surface to a temperature at which it can no longer maintain, in an invisible state, the water vapour, which is present in it. Condensation of this vapour into minute, though visible, droplets produces fog. The type of fog depends upon the means by which the air is cooled. For details of specific areas, the Sailing Directions should be consulted. Sea or Advection Fog is associated with moist and relatively warm air flowing over a cold sea surface and is the main type of fog experienced at sea. This occurs over the sea when warm tropical maritime air is cooled below its dew point by conduction from a cold sea surface. It is most common in the late spring and early summer, when sea temperature is at its lowest compared with air temperature. To produce fog by this means the rate of cooling of 29

 

the air must be high. This only occurs frequently and on a large scale, either near cold currents and at a season when the prevailing wind transports warm, moist air over them, or elsewhere where the sea temperature is appreciably ably lower than that of the air which blows over land. The likelihood of fog occurring may be judged if graphs of the air dew point temperatures and the sea temperatures are drawn and plotted against time. In the graph below fog is likely to occur about 2000 hours, if the present trends are maintained. A change of course to the southward would alter conditions considerably. In general terms fog is likely to occur if tropical maritime air crosses seven sea isotherms from its source. Sea fog occurs mainly in the summer months where the areas have cold sea currents flowing through them. Examples of the former are the fogs, which occur off Newfoundland, off California and between Japan and the Aleutian Islands; the cold currents involved being the Labrador, California and Oya Shio, respectively. The latter type is represented by the spring and summer fop in the SW approaches to the English Channel. Aids to forecasting sea fog are given later. Frontal Fog may occur near an occlusion or ahead of a warm front and is due to the evaporation of the warm raindrops into the cold air beneath the frontal surface raising the relative humidity to saturation point. It occurs in temperate and high latitudes and is confined to a relatively narrow belt not usually more than 50 miles in width. Arctic Sea Smoke, or Frost Smoke, is normally confined to high latitudes and occurs when very cold air flows over a much warmer sea surface, when intense evaporation takes place from the relatively warm sea. The moisture thus evaporated is immediately chilled by contact with the cold air and condensed to form fog, giving the sea the appearance of steaming. This type of fog is often encountered where a cold wind is blowing off of f ice or snow on to a relatively warm sea. This frequently occurs in winter over rivers and also off the East Coasts of continents. RADIATION FOG Occurs over land at night when radiation from the earth has cooled its surface to below the dew point of the air adjacent to it. Conduction cools this air until condensation takes place. Slight turbulence (wind 3-7 knots) will carry this cold saturated air to higher levels and will bring to the surface drier air, which will in turn be cooled and saturated. As the process continues the whole lower layer of air will become saturated. Radiation Fog forms over lowlying land on clear nights (conditions for maximum radiation) especially during winter months. Radiation fog is thickest during the latter part of the night and early part of the day.

30

 

Occasionally it drifts out to sea but is found no farther than 10 to 15 miles offshore as the sea surface temperature is relatively high which causes the water droplets to evaporate. A temperature inversion is often associated with radiation fog, which, acting as a lid keeps the fog low down and causes it to thicken. Stronger winds will disperse the fog as will the heat from the sun. This type of fog may occur on the banks of rivers and usually disperses in the morning daylight. Forecasting sea fog. Warnings of the likely formation of sea fog may be obtained by frequent observations of air and sea surface temperature; if the sea surface temperature falls below the dew point, fog is almost certain to form. The following procedure is recommended whenever the temperature of the air is higher than, or almost equal to that of the sea, especially at night when approaching fog cannot be seen until shortly before entering it.

Sea and air (both dry and wet bulb) temperatures should be observed ob served at least every minutes and the sea surface temperature and dewpoint temperature plotted against time. If the curves converge fog may be expected when they coincide. The example shows that by 2200 there is a probability of running into fog about 2300, assuming that the sea surface temperature continues to fall at the same rate. A reliable warning of fog will be given when the dewpoint is within 5°C of the sea surface temperature. To avoid fog a course should be set for warmer waters. 31

 

CHAPTER 7

The wind and pressure systems over the ocean

Wind and Pressure Systems  The diagram below shows the distribution of pressure and the winds, which would result over a featureless earth.

The Trade Winds blow on either side of the Equatorial Trough, NE’ly in the N hemisphere and SE’ly in the S hemisphere.

The Trades blow with great persistence and each embraces a zone of some 1,200 miles of latitude. Trade winds, however, do not blow throughout this zone. The South West Monsoon winds blow instead off part of the West coast of Africa, in the North Indian Ocean and in the W part of the North Pacific Ocean. The trades move slightly north and south with the sun; their approximate limits are as follows: February

August

32

 

Atlantic Doldrums

000° - 002°N

005°N - 010°N

N.E. Trade

002°N - 025°N 010°N - 030°N

S.E. Trade

000° - 030°S

Pacific Doldrums

004°N - 008°N 008°N - 012°N

N.E. Trade

008°N - 025°N 012°N - 030°N

S. E. Trade

004°N - 030°S

008°N - 025°S

Indian S.E. Trade

015°S - 030°S

000° - 025°S

005°N - 025°S

The ANTI-TRADES are winds, which blow above about 2500 metres in the opposite direction to the trades on the surface.

Climatic Chart for July The average strength of the Trades is about Force 4, though variations occur between different oceans and at different seasons. The weather in Trade Wind zones is generally fair with small-detached cumulus clouds. On the E sides of the oceans cloud amounts and rainfall are small, while on the W sides cloud amounts are larger and rainfall is frequent, being a maximum in the summer months. 33

 

Cloud amounts and the frequency and intensity of rain all increase towards the Equatorial Trough. Poor visibility often occurs at the E end of the Trade Wind zones, due partly to mist or fog forming over the cold currents and partly to sand and dust being carried out to sea by prevailing offshore winds. At the W end of the zones visibility is good, except when reduced in rain. Fog is rare. In certain seasons and in certain localities the generally fair weather of the Trades is liable to be interrupted by tropical storms. These are described in detail later. The Variables

Over the areas covered by the oceanic anticyclones, between the Trade winds and the Westerlies further toward the poles, there exist zones of light and variable winds which are known as The Variables, and the N area is sometimes known as the Horse Latitudes (30°40°). The weather in these zones is generally fair with small amounts of cloud and rain. The Westerlies

On the polar sides of the oceanic anticyclones lie zones where the wind direction becomes predominantly W’ly. Unlike the Trades, these winds known as The Westerlies are far from

permanent. The continual passage of depressions from W to E across these zones causes the wind to vary greatly in both direction and strength. Gales are frequent, especially in w inter. The weather changes rapidly and fine weather is seldom prolonged. Gales are so frequent frequ ent in the S hemisphere that the zone, S of 40°S, has been named the Roaring Forties. In the N hemisphere fog is common in the W parts of the oceans in this zone in summer.

34

 

Climatic Chart for January

Areas where fog is likely and those where ice may be encountered are shown on the below in the climatic charts: The Polar Regions which lie on the polar side of the Westerlies are mainly un-navigable on account of ice. The prevailing wind is generally from an E’ly direction and gales are common

in winter, though less so than in the zones of the Westerlies. The weather is usually cloudy and fog is frequent in summer. SEASONAL WINDS AND MONSOONS

Over certain parts of the oceans the general distribution of pressure and wind in the zones described above is greatly modified by the seasonal heating and cooling of adjacent large, landmasses. The annual range of sea temperature in the open ocean is comparatively small, whereas large landmasses become hot in summer and cold in winter. This alternate heating and cooling of the land results in the formation of areas of low and high-pressure respectively. This redistribution of pressure results in a seasonal reversal of the prevailing wind over the adjacent oceans. The most important oceanic areas subject to these seasonal winds are the Indian Ocean, West Pacific Ocean and those adjacent to the coast of West Africa.

35

 

LAND AND SEA BREEZES

The regular alternation of land and sea breezes is a well-known well -known feature of most tropical and sub-tropical coasts and large islands. These breezes also occur at times in temperate latitudes in fine weather in the summer, though they are here much weaker and less well marked than is the case in lower latitudes. The cause of these breezes is the unequal heating heat ing and cooling of the land and sea. By day the surface of the land rapidly acquires heat from the sun whereas the sea temperature remains virtually unaffected. The heat of the land is communicated to the air in contact with it, which expands and rises. Air from over the sea flows in to take its place, producing an onshore wind known as a sea breeze. By night, land rapidly loses heat by radiation and becomes much colder than the adjacent sea. The air over the land is chilled, becomes denser and heavier and flows out to sea under the influence of gravity, producing an offshore wind known as a land breeze. Sea breezes usually set in late in the forenoon and reach maximum strength, about force 4 (occasionally they reach force 5 or even 6), around 1400. They die away around sunset. Land breezes are usually less well-marked and weaker than sea breezes. The effect of these breezes may be to deviate the prevailing wind, reinforce it, neutralize it or even reverse it. The following factors favour the formation of well-marked land and sea breezes: (a)

A dry desert coast as opposed to forests or swamps.

(b)   (b)

High ground near the coast.

(c)  (c) 

A weak prevailing wind.

(d)   (d)

A clear or partly cloudy sky.

A cold current along the coast also has the effect of favouring the establishment of a wellmarked sea breeze. Small islands less than 5 to 10 miles in diameter will not usually produce land and sea breezes. As the land is heated during the daytime the air over it will be heated by conduction. This heating causes a decrease in the density of the air, and the pressure falls. The sea temperature remains more or less the same and the pressure p ressure over it is high compared with that over the land. The pressure gradient is sufficient for air to flow from over the sea to the land; this is the sea breeze.

36

 

The sea breeze (about force 3  – 4) sets in during the morning, reaches its maximum strength about 1400 hours and then dies away towards sunset. After sunset the land cools rapidly and the air above it also cools and its density increases giving rise to an increase in pressure. The pressure over the sea is now low compared with that over the land. The pressure gradient causes air to flow from the land to the sea; this is the LAND BREEZE. The land breeze (generally very light compared to sea breeze) sets in shortly after sunset and continues until dawn. N.E. MONSOON OF THE CHINA SEAS AND INDIAN OCEAN

During the northern winter the Asian continent is cooled and an intense high pressure area forms over Eastern Siberia. The winds circulating round this form the N.E. Monsoon. In the northern part of the China Sea the pressure pre ssure gradient is large and winds are likely to be North Westerly force 6 - 7, further south where the pressure gradient is smaller the winds will be northerly force 5 - 6. In the Bay of Bengal and Arabian Sea winds are N.E.’ly force 3 4. Rainfall of the showery type is likely on windward coasts especially in the China Sea. As the air is of Polar origin the relative humidity will be low. The N.E. monsoon becomes established in early December and continues until April. S.W. MONSOON OF THE INDIAN OCEAN AND CHINA SEA

During the northern summer the Asian continent is warmed and the high pressure over Siberia declines and is replaced by a low pressure over N.W. India. There is a pressure gradient between the high pressure over the South Indian Ocean and the lows over Africa and India. This causes, the S.E. Trade to blow up to the equator, and on crossing it, the wind near the African coast circulates round the low over Africa and brings rainfall to East Africa. Away from the coast the wind is deflected to the right and blows from a southwesterly direction. In the Arabian Sea the wind is S.W. force 7 - 8, In the Bay of Bengal S.W. winds of force 6 - 7 can be expected, whilst in the southern part of the China Sea, the wind is S’ly force 4 - 5. The northern part of China Sea has h as light S.E.’ly winds. 

The air forming the S.W. Monsoon his traveled over thousands of miles of ocean and is saturated. This results in heavy rain, especially near the coasts, and poor visibility. 37

 

The S.W. monsoon sets in during the beginning of June and lasts until the beginning of October. KATABATIC EFFECTS

Katabatic winds are offshore winds caused by the drainage of cold air from high ground gro und under the influence of gravity. In temperate and high latitudes, where snow-covered mountains back the coast, intense radiation occurs and this causes cold air to accumulate over the high ground. A light offshore wind suffices to start this mass of cold air moving down the seaward facing slopes with gathering momentum reaching the coast without warning, as a strong Or even gale force wind, endangering small craft or ships at anchor. The following are some of the areas where Katabatic winds are common: Greenland, Norway, N Adriatic, E Black Sea and Antarctica.

When the earth’s surface cools by night, the air next to the surface is also cooled and thus

its density is increased. If this cold air is on high ground there is a tendency for it to sink down- to lower ground. If the high ground is a cliff top or a high coastal plateau, the down flowing cold-air will move horizontally when it reaches sea level.

The ANABATIC WIND is less noticeable as it blows up the sides of valleys with considerably less force than the KATABATIC wind. The air at the bottom of the valley is warmed by

38

 

conduction from the heated land during the day, and this air, being less dense than the air above it, takes the easiest path to the top of the valley by following the warm sides.

CHAPTER 8 Structure of depressions 

Structure of Depressions  DEPRESSIONS

A depression, also known for synoptic purposes as a low, appears on a synoptic chart as a series of isobars roughly circular or oval in shape, surrounding an area of low pressure. Depressions are frequent at sea in middle latitudes and are responsible for most strong winds and unsettled weather, though not all depressions are accompanied by strong winds. Depressions vary much in size and depth. One may be only 100 miles in diameter and another over 2,000; one may have a central pressure of 960 millibars and another 1,000 millibars. In the N (S) hemisphere the winds blow around an area of low pressure in an anti anti-clockwise -clockwise (clockwise) direction. There is a slight inclination across the isobars towards the lower pressure. The strength of the wind is closely related to the gradient across the isobars, the closer the isobars the stronger the wind. Depressions may move in any direction though many move in an E direction, at speeds varying from nearly stationary to 40 knots. Occasionally, during the most active stage of its existence, a low may move as fast as 60 knots. Lows normally last around 4 to 6 days and slow down when filling. The following is a brief general description of depressions and the associated weather in temperate or middle latitudes of the two hemispheres. It must be emphasized, however, that individual depressions in different localities differ considerably from one another according to the temperature and humidity of the air currents of which they are composed and the nature of the surface over which they are traveling. A falling barometer indicates the approach of a depression. In the N (S) hemisphere, if a depression is approaching from the W and passing to the N (S) of the ship, clouds appear on the W horizon, the wind shifts to a SW (NW) or S (N) direction and freshens, the cloud layer gradually lowers and finally drizzle, rain or snow begins. If the depression is not occluded, after a period of continuous rain or snow there is a veer (backing) of the wind at the warm front. In the warm sector, the temperature rises, the rain or snow eases or stops, visibility is usually moderate and the sky overcast with low cloud.

39

 

The passage of the cold front is marked by the approach from the W of a thick bank of cloud (which however cannot usually be seen because of the customary low overcast sky in the warm sector), a further veer (backing) of wind to W or NW (SW) sometimes with a sudden squall, rising pressure, fall of temperature, squally showers of rain, hail or snow, and improved visibility except during showers. The squally, showery weather with a further veer (backing) of wind and a drop in temperature may recur while the depression recedes owing to the passage of another cold co ld front or occlusion. If the depression is occluded, the occlusion is preceded by the cloud of the warm front; there may be a period of continuous rain mainly in front of and at the line of the occlusion, or a shorter period of heavy rain mainly behind the occlusion, according as the air in front of the occlusion is colder or warmer than the air behind it. There may be a sudden veer (backing) of wind at the occlusion. Often another depression follows 12 to 24 hours later, in which event the barometer begins to fall again and the wind backs towards SW (NW), or even S (N). If a depression travelling E or NE (SE) is passing S (N) of the ship, the winds in front of it are E and they back (veer) though NE (SE) to N (S) or NW (SW); changes of direction are not llikely ikely to be so sudden as on the S (N) side of the depression. In the rain area there is often a long period of continuous rain and unpleasant weather with low cloud. In winter in the colder regions the weather is cold and raw and precipitation is often in the form of snow. Winds may be temporarily light and variable near the centre of a depression but rapid changes to strong or gale force winds are likely as pressure begins to rise and the low moves away. Sometimes in the circulation of a large depression, usually on the equatorial side and often on the cold front, a secondary depression develops, traveling in the same direction as the primary but usually more rapidly. The secondary often deepens while the original depression fills. Between the primary and the secondary depressions, the winds are not as a rule strong but on the further side of the secondary, usually the S (N) side, winds are likely to be strong and they may reach gale force. Thus the development of a secondary may cause gales farther from the t he primary than was thought likely, while there may be only light winds where gales were expected. This is shown in the following diagram:

40

 

Depression Northern Hemisphere: Warm air is lighter than cold air and rises over the cold air ahead of the warm front as shown in the diagram: This causes condensation of the water vapour in the warm air, forming at first cloud and later drizzle or continuous steady rain. The cloud spreads out ahead of the warm front f ront and the highest cloud, cirrus, is often about 500 miles ahead of it. At the rear boundary of the warm sector, known as the cold front, the cold air is pushing under the warm air forcing the latter to ascend rapidly. This process is sometimes violent enough to produce squalls. The rapid ascent of the warm air causes the moisture to condense in the form of cumulonimbus clouds (shower clouds), from which heavy showers may fall. OCCLUSION The cold front moves faster than the warm front and gradually overtakes it, causing the warm air to be lifted up from the surface. When this happens the depression is said to be occluded and the fronts have merged into a single front, known as an occlusion.

41

 

Occlusion Northern Hemisphere:

Occlusion Northern Hemisphere:

42

 

FRONT If two air masses from different regions, such as the polar and tropical regions, are brought together, the surface boundary where they meet is known as a front. Further there is a tendency for waves to form on this front and some of these waves develop into depressions. This is shown in the diagram below:

After sometime this surface boundary slowly takes the form of a depression and has a circulation.

The part of the front marked AB is called a cold front as along it cold air is replacing warm air. The part marked BC is the warm front since along this front warm air is replacing cold air.

43

 

Oceanic depressions usually have one or more fronts extending from their centres, each

front representing a belt of bad weather, ac companied by a veer (backing) of wind, which marks the change from the weather characteristic of one air mass to that of the other. During the first two or three days of its existence a depression has a warm and a cold front, the area between the two being known as the warm sector because the air has come from a warmer locality than that which is outside the sector. This is shown in the following diagram: Depression Northern Hemisphere:

Warm air is lighter than cold air and rises over the cold air ahead of the warm front as shown in the diagram: 44

 

These are intense depressions occurring in tropical latitudes accompanied by high winds and heavy seas. Although the pressure at the centre of a tropical storm is comparable to that of an intense mid- latitude depression, the diameter of a tropical storm is much smaller (some 500 miles compared with 1,500 miles), and therefore the pressure gradients and the wind speeds correspondingly greater.

CHAPTER 9 Anticyclones and other pressure systems

 Anticyclone and Other Pressure Systems  ANTICYCLONE

Over the E sides of the oceans the movement of anticyclones, which are also known for synoptic purposes as highs, is generally slow and erratic and the anticyclone may remain stationary for several days giving settled weather. The pressure gradient is usually slight, the winds are light and the weather is often fine or partly cloudy, but in winter overcast skies are common, producing gloomy conditions. Precipitation is, however, rare except on the outskirts of an anticyclone. Over the W parts of the oceans anticyclones are more likely to move quickly and consequently the weather is more changeable. Movement is generally towards the E. LOCAL WEATHER

LOCAL MODIFICATION OF THE WEATHER NEAR THE COAST. The following notes indicate how weather near the coast is likely to be modified by the shape of the land: 45

 

If the coast is formed by steep cliffs, or if the ground rises rapidly inland, onshore winds are usually deflected to blow nearly parallel to the coast, and with increased force. Near headlands, or islands with steep cliffs, there may be large and sudden changes in the direction and speed of the wind. In a strait, especially if it is narrow and the sides steep, the wind will tend to blow along the strait in the direction most nearly corresponding to the general wind direction in the area, even though these two directions may differ considerably. Where the strait narrows the wind will increase. Similarly, in a fjord or other narrow steep-sided inlet there is a tendency for the wind to blow along the inlet. When a strong wind blows directly towards a very steep coast, there is usually a narrow belt of contrary, gusty winds close to the coast. Where there is high ground near the coast, offshore winds are liable to be squally, especially when the air is appreciably colder than the sea, and also when the wind over the open sea is force 5 or more. Land and sea breezes occur at certain latitudes. Radiation fog sometimes forms near the shore. Isobaric Systems

Anticyclone is the name given to a system of high pressure. The wind circulation around this system will be clockwise in the Northern hemisphere and anticlockwise in the Southern hemisphere. Anticyclones are associated vital small pressure gradients and consequently co nsequently light Winds. As they are areas of subsidence rainfall is unlikely. They may be classified as either cold or warm and each of these may be either permanent (long lasting) or temporary. Anticyclone

When the high pressure is brought about by the air a ir over an area being denser than that nearby a cold anticyclone is formed, cold air having a greater density than warm air. On the other hand a high pressure can be formed by larger than normal amounts of warm air over an area, in which case a warm anticyclone is formed. The cold anticyclone gives brilliant frosty weather in the centre but near the outer regions dull foggy weather is frequently experienced, as there is usually a temperature inversion close to the surface. The subsiding air warming adiabatically forms the inversion. The best known of the cold anticyclones is that over Siberia in winter, which is a permanent one. Cold anticyclones are

46

 

unlikely over oceans or over the land in summer, if they do form they rapidly become warm ones or disperse. Temporary cold anticyclones occur between the middle latitude depressions and bring a welcome though brief spell of very bright weather. Occasionally the ridge with which the temporary anticyclone is associated will intensify to build up an anticyclone composed entirely of cold air. If this occurs over the land in winter it may merge with the continental high and be kept as a cold anticyclone by continuous cooling of the land. If it occurs over land in summer or over the sea at any time it is h hardly ardly likely to persist as a cold anticyclone it may collapse or become a warm anticyclone. Warm anticyclones occur when the air in the troposphere is warmer than the surrounding air and an excessive depth of this warm air causes the high pressure. Permanent warm anticyclones are those, which are found over the oceans and are generally referred to as the subtropical highs. They are composed of warm dry air and the visibility is excellent. Temporary warm anticyclones may occur when there is an extension from the sub-tropical high or when a temporary cold anticyclone warms adiabatically. Over the land the air is very dry and by day fine hot weather will be experienced during the summer. By night there may be sufficient radiation cooling to give fog, this being this being particularly p articularly likely during the autumn. CHAPTER 10 Weather services for shipping

Weather Services for Shipping  World Meteorological Organization

The World Meteorological Organization is an intergovernmental organization with a membership of 187 Member States and Territories. It originated from the International Meteorological Organization (IMO), which was founded in 1873. Established in 1950, WMO became the specialized agency of the United Nations for meteorology (weather and climate), operational hydrology and related geophysical sciences. Since its establishment, WMO has played a unique and powerful role in contributing to the welfare of humanity. Under WMO leadership and within the framework of WMO programmes, National Meteorological and Hydrological Services have contributed substantially to the protection of life and property against natural disasters, to safeguarding the environment and to enhancing the economic and social well-being of all sectors of society in areas such as food security, water resources and transport. It has a unique role within the UN system it facilitates the free and unrestricted exchange of data and 47

 

information, products and services in real- or near-real time on matters relating to safety and security of society, economic well being and the prevention of the environment. As weather and climate know no n o national boundaries, international cooperation at a global scale is essential for the development of meteorology and operational hydrology as well as to reap the benefits from their applications. WMO provides the framework for such international cooperation.

In the specific case of weather natural disasters which account for nearly three-quarters of all are such events, WMO’s programmes provide the vital information for the advance

warnings that save many lives and reduce damage to property and the environment. Human induced disasters chemistry, nuclear and forest fire . Sources of weather information available to shipping  

Navtext – continuous however available only in coastal areas – not for ocean passages VHF – port information Weather FAX – all over the world Inmarsat C – Same broadcast as Navtext but for coverage area of Inmarsat High Frequency (HF) broadcasts – Ocean passages Information flow between merchant ships and some Meteorological Offices

Under the aegis of WMO, IMD has enlisted a cooperating fleet of Voluntary Observing Ships for collection of meteorological observations from ocean areas. The VOF consists of merchant ships of Indian registry, some foreign merchant vessels and a few ships of the Indian Navy, totalling 203. These ships have Meteorological instruments supplied by IMD. The observations are transmitted by the ships to the nearest coastal radio stations for onward transmission to the storm warning centres. Records of observations are passed on to the Department when the ships call at ports. IMD's Port Meteorological Offices provide asistance for callibration and maintenance maintenance of the equipment on board the ship 48

 

Services provided for shipping by Meteorological Offices

Meteorological Service Division, National Environment Agency is a pioneer in the provision of routine weather forecasts and storm warnings for shipping in the region. As a member of the World Meteorological Organisation, MSD participates actively in its Marine Meteorology Programme to enhance safety at sea and fufill our regional responsibility with WMO and the Safety of Life at Sea Convention (SOLAS, 1989 and 1974) 19 74) of the International Maritime Organisation (IMO). Services: Issue of storm warnings and routine weather bulletins for shipping transmitted directly to vessels at sea via High Frequency (HF) broadcasts Provision of port meteorological services for Singapore registered ships Collection and exchange of ship's weather reports relayed via Inmarsat from ships Provision of meteorological services for maritime search and rescue operations including oil spill incidents. Additional Services: Weather and seas conditions surveillance and issuance of storm warnings and advisories for shipping Forecasts for sea navigation, off-shore drilling and mining operations, cable and pipe- laying and the towing of vessels and platforms Weather watch and forecasts in and around harbour areas such as cargo handling, barge loading and navigation are Assessments of weather conditions for purposes of marine accident investigations and insurance claims Provision of climatologolical and historical weather & oceanic data for studies, site assessments and planning. Singapore MSD has the capability of providing the required weather information and forecasts in support of all of the mentioned activities. With its highly qualified internationally trained staff, advanced technology and quick access to the worldwide weather data, MSD is indeed well-poised to meet the demands of the region's maritime community. Weather bulletin and the contents of each of its sections

Weather Chart and the detailed message relating to the same. The message is divided in parts and the same is shown and what they contain.

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SINGAPORE WEATHER BULLETIN FOR SHIPPING (Regular updates at 0900 UTC and 2100 UTC) TTT Warning at 090600 A monsoon disturbance was located in the South China Sea, north of latitude zero five degrees North (05N) and east of PART ONE:

STORM

longitudes one zero five degrees East (105E). Sea

WARNINGS

moderate to rough and swell moderate to high from NE. Winds NE 20 to 25 Knots. This monsoon disturbance is expected to persist during the next 18 hours.

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PART

GENERAL

TWO:

SITUATION

PART THREE:

PHUKET

MALACCA

TIOMAN

Moderate NE Monsoon.

FORECAST VALID 12 HOURS FROM 09 Jan 2005 13:00 UTC ENE 10/20 Knots. Isolated showers. Sea slight to moderate. Swell low to moderate. NE/N 10 /15 Knots. Scattered showers/isolated thunderstorms. Sea slight. Swell low. NE/N 15/20 Knots. Scattered showers/isolated thunderstorms. Sea moderate . Swell moderate. NE/N 15/20 Knots. Scattered

BUNGURAN

showers/thunderstorms. Sea moderate . Swell moderate.

CONDORE

NE 25 Knots. Isolated showers. Sea rough. Swell high. NE 25 Knots in north, NE/N 15/20 Knots in south. Isolated showers in north and scattered

REEF

showers/isolated thunderstorms in south. Sea rough in north, moderate in south. Swell high in north, moderate in south.

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Types of information received by facsimile machine

All type of information is received by a weather FAX. In fact it is easy to understand since the notes form of weather message requires to be laid out on a chart, wheras the weather FAX already has a map printed on it.

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Services provided for storm warnings

Storm warnings are transmitted over various means of transmission. They include: Navtext – continuous however available only in coastal areas – not for ocean passages VHF – port information Weather FAX – all over the world Inmarsat C – Same broadcast as Navtext but for coverage area of Inmarsat High Frequency (HF) broadcasts – Ocean passages SAMPLE MESSAGE Issued on 09/01/2005 2312LT STORM WARNING: TROPICAL CYCLONE 01B Current position on 09/01/2005 12Z 6.4N 83.5E Max Winds/Gusts 25/ 35 (kt) Movement past six hrs: 015 deg 04 kt

Forecast position on 10/01/2005 00Z 6.5N 82.9E Max Winds/Gusts 25/ 35 (kt)

Forecast position on 10/01/2005 12Z 6.2N 81.9E Max Winds/Gusts 20/ 30 (kt)

Forecast position on 11/01/2005 00Z 5.9N 80.6E Max Winds/Gusts 20/ 30 (kt)

Forecast position on 11/01/2005 12Z 5.8N 78.9E Max Winds/Gusts 20/ 30 (kt) 

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Tropical Revolving Storms 

Terms associated with a TRS

The air circulation's of the Northern and Southern hemispheres are in opposite directions, so that disturbances, which form in one hemisphere, cannot cross into the other hemisphere. The area near the equator is one of convergence as the Northern Hemisphere N.E. Trade wind blows towards it, as does the S.E. Trade of the Southern Hemisphere. The general name for this area is the INTERTROPIC INTERTROPICAL AL CONVERGENCE ZONE (ITCZ), also known as the DOLDRUMS. When the ITCZ is well to the north or south of the equator the change in direction of the trades after crossing the equator will cause very strong convergence currents and it is possible that a cyclonic disturbance will form in this area. The possibility is increased when the ITCZ is in the vicinity of islands when local surface heating of air of high humidity gives rise to very unstable conditions. Low pressure areas frequently occur in the ITCZ but cyclonic circulation can only result if the geostropic force is sufficiently large (there is no geostropic force on the equator) and this is unlikely in latitudes less than 5°. The cyclonic disturbance once formed is known as a Tropical Revolving Storm whose diameter varies between 50 to 800 miles, 500 miles being an average. After formation between 5° to 10° of latitude the storm moves westwards at 10 - 12 knots, until reaching the tropic, where3 it slows down before re-curving eastwards and proceeding at 15 – 20 knots to the higher latitudes. TRS do not normally cross-land but when and if they do the supply of warm moist air necessary to their existence being cut off they tend to fill and thus dissipate. However if the TRS do cross over again from land to sea then they again deepen. Normally these occurrences are few and the land that is crossed is small, but sometimespowerful TRS can cross wide landmasses – as wide as South India. It is noticed that the T.R.S. is not found in the South Atlantic or the eastern side of the South Pacific. Reference to the charts will show that the ITCZ is at no time south of SOS in these areas. AREA

NAME 

SEASON 

North Atlantic Ocean Western

Hurricane

June to November

side

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North Pacific Ocean Eastern

Hurricane or

side

Cordonazo

North Pacific Ocean Western

Typhoon or Raguios

side South Pacific Ocean western

June to November

All the year but greatest frequency and intensity June to November.

Hurricane

December to April

Willy-willy

December to April

Cyclone

December to April

Cyclone

June and November but they may

side South Indian Ocean Eastern side South Indian Ocean Western side Bay of Bengal and Arabian Sea

occur during the S.W. monsoon season.

The following terms are in common use when reference is made to a T.R.S.: PATH: The direction in which the storm is moving. TRACK: The area, which the storm centre has traversed. STORM FIELD: The horizontal area covered by the cyclone conditions of the storm. SOURCE REGION: The region where the storm first forms. VERTEX: The furthest westerly point reached by the storm centre. EYE OF THE STORM: The storm centre. BAR OF THE STORM: The advancing edge of the storm field. ANGLE OF INDRAUGHT: The angle, which the wind makes with the isobars. VORTEX: The central calm of the storm. DANGEROUS SEMI-CIRCLE: SEMI-CIRCLE: The half of the storm, which lies to the right of the path in the Northern Hemisphere and to the left of the path in the Southern Hemisphere. DANGEROUS QUADRANT: QUADRANT: The leading portion of the dangerous semicircle where NAVIGABLE SEMI-CIRCLE: SEMI-CIRCLE: The half of the storm, which lies to the left of the path in the Northern Hemisphere and to the right of the path in the Southern Hemisphere. TROUGH LINE: A line through the centre of the storm at right angles to the path. The dividing line between falling and rising pressure.

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Oceanic depressions usually have one or more fronts extending from their centres, each

front representing a belt of bad weather, ac companied by a veer vee r (backing) of wind, which marks the change from the weather characteristic of one air mass to that of the other. During the first two or three days of its existence a depression has a warm and a cold front, the area between the two being known as the warm sector because the air has come from a warmer locality than that which is outside the sector. This is shown in the following diagram: Depression Northern Hemisphere:

Warm air is lighter than cold air and rises over the cold air ahead of the warm front as shown in the diagram:

These are intense depressions occurring in tropical latitudes accompanied by high winds and heavy seas. Although the pressure at the centre of a tropical storm is comparable to that of 56

 

an intense mid- latitude depression, the diameter of a tropical storm is much smaller (some 500 miles compared with 1,500 miles), and therefore the pressure gradients gradi ents and the wind speeds correspondingly greater. The wind blows round the centre of a tropical storm in a spiral movement inwards, anticlockwise in the N hemisphere and clockwise in the S hemisphere, he misphere, giving rise to the occasional alternative name Revolving Storm. Within 75 miles of the centre of a tropical storm, the wind is often very violent and the sea high and confused, causing considerable damage even to large and well-found ships. The danger is still greater when ships are caught in restricted waters without adequate room to manoeuvre. Due to torrential rain and sheets of almost continuous spray visibility near the storm centre (but outside the eye) is almost nil. Within 5 to 10 miles of the centre the wind is light or moderate and variable, the sky is clear or partially so, and there is a heavy, sometimes mountainous, confused swell; this area is known as the eye of the storm. The localities, seasons, average frequencies and local names of these storms are shown in the notes. The locating of tropical storms has greatly improved in recent years with the aid of weather satellites. Once identified by satellite, tropical circulations are carefully tracked and in some areas, e.g. the seas around the West Indies and the Philippines, weather reconnaissance aircraft fly into these circulations to measure characteristics such as wind speed and pressure.

Warnings of the position, intensity and expected movement of each circulation -are then broadcast at regular intervals (see Admiralty (see Admiralty List of Radio Signals). Signals).   Tropical storms generally originate between the latitudes of 7° and 15°, though some form nearer the equator. Those which affect the W part of the Pacific, South Indian and North Atlantic Oceans are usually first reported in the W parts of these oceans, though there are exceptions, such as in the North Atlantic during August and September when an occasional storm begins near Arquipelago de Cabo Verde. In the N hemisphere they move off in a direction between 275° and 350°, though most often within 30° of due W. When near the latitude of 25° they usually recurve away from the equator and, by the time they have reached a latitude of 30°, the track (or path as it is more usually called) is NE. In the S hemisphere they move off in a WSW to SSW direction (usually the former), recurve between latitudes of about 15° to 20°, and thereafter follow a SE path. Many storms, however, do not recurve but continue in a WNW (WSW) direction until they reach a large land mass where they fill quickly.

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The speed of the storms is usually about 10 knots in their early stages, increasing a little with latitude but seldom achieving 15 knots before recurving. A speed of 20 to 25 knots is usual after recurving though speeds of over 40 knots have been known. Storms occasionally move erratically, at times making a complete loop, but when this happens their speed is usually less than 10 knots. Winds of force 7 are likely up to. 200 miles from the centre of the storm and winds of gale force 8 up to 100 miles from the centre, at latitudes of less than 20°; but by a latitude of 35° these distances may be doubled though wind force near the centre may be diminished. Hurricane force winds are likely within 75 miles of the storm centre in the tropics and gusts exceeding 175 knots have been reported. As already stated, warning of the position, intensity and expected movement of a storm is given by radio at frequent intervals. Sometimes, however, there is insufficient evidence for an accurate warning, or even a general warning to be given and then ships must be guided by their own observations. The first of the following observations is by far the most reliable indication of the proximity of a storm, within 20° or so of the equator. It should be borne in mind, however, that th at very little warning of the approach of an intense storm of small diameter may be expected. Precursory signs of tropical storms

If a corrected barometer reading is 3 millibars or more below the mean for the time of the year, as shown in the climatic atlas or appropriate volume of the Sailing Directions, suspicion should be aroused and action taken to meet any development. The barometer reading must be corrected not only for height, latitude, temperature and index error (if mercurial) but also for diurnal variation, which is given in climatic atlases or appropriate volume of the Sailing Directions. If the corrected reading is 5 millibars or more below normal it is time to consider avoiding action for there can be little doubt that a tropical storm is in the vicinity. Because of the importance of pressure readings it is wise to take hourly barometric readings in areas affected by tropical storms. An appreciable change in the direction or strength of the wind. A long low swell is sometimes evident, proceeding from the approximate bearing of the centre of the storm. This indication may be apparent before the barometer begins to fall. Extensive cirrus cloud followed, as the storm approaches, by altostratus and then broken cumulus or scud.

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Radar may give warning of a storm within about 100 miles. At times the eye can be clearly seen. It is surrounded by a large area of moderate or heavy rain and outside this area the belts of rain are arranged in bands as shown. By the time the exact position of the storm is given g iven by radar, the ship is likely to be already experiencing high seas and strong to gale force winds. It should be in time, however, to enable the ship to avoid the eye and its vicinity where the worst conditions exist. To decide the best course of action if a storm is suspected in the vicinity, the following knowledge is necessary. 

The bearing of the centre of the storm The path of the storm If an observer faces the wind, the centre of the storm will be from 100° to 125° on his right hand side in the N hemisphere when the storm is about 200 miles away, i.e. when the barometer has fallen about 5 millibars and the wind has increased to about force 6. As a rule, the nearer he is to the centre the more nearly does the angle approach 90°. The path of the storm may be approximately determined by taking two such bearings separated by an interval of 2 to 3 hours, allowance being made for the movement of the ship during the interval. It can generally be assumed that the storm is not traveling towards the equator and, if in a lower latitude than 20° its path is most unlikely to have an E component. On the rare occasions when the storm is following an unusual path it is likely to be moving slowly. Diagrams below show typical paths of tropical storms and illustrates the terms dangerous and navigable semicircle. The former lies on the side of the path towards the usual direction of recurvature, i.e. the right hand semicircle in the N and the left hand semicircle in the S hemisphere.

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The advance quadrant of the dangerous semicircle(shown in red) is known as the dangerous quadrant as this quadrant lies ahead of the centre. The navigable semi- circle is that which lies on the other side of the path. A ship situated within this semicircle will tend to be blown away from the storm centre and re-curvature of the storm will increase her distance from the centre. Avoiding tropical storms.

In whatever situation a ship may find herself the matter of vital importance is to avoid passing within 50 miles or so of the centre of the storm. It is preferable but not always possible to keep outside a distance of 200 miles. If a ship has at least 20 knots, at her disposal and shapes a course that will take her most rapidly away from the storm before the wind has increased above the point at which her movement becomes restricted, it is seldom that she will come to any harm. Sometimes a tropical storm moves so slowly that a vessel, if ahead of it, can easily outpace it or, if astern of it, can overtake it. If a storm is suspected in the vicinity, the vessel, whilst observing her barometer, should continue on her course until the barometer has fallen 5 millibars (corrected for diurnal variation) below normal, or the wind has increased to force 6 when the barometer has fallen at least 3 millibars. Then she should act as recommended in the paragraphs below, until the barometer has risen above the limit just given and the wind has decreased below force 6. Should it be certain, however, that the vessel is behind the storm, or in the navigable semicircle, it will evidently be sufficient to alter course away from the centre.

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In the N hemisphere (ship initially moving Slowly). (a) If  the  the wind is veering the ship must be in the dangerous semicircle. The ship should proceed with all available speed with the wind 10° to 45°, depending on speed, on the starboard bow. As the wind veers the ship should turn to starboard, thereby tracing a course relative to the storm as shown in the above diagram. (b) If  the  the wind remains steady in direction, or if it backs, so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the starboard quarter and proceed with all available speed. As the wind backs the ship should turn to port as shown. In the S hemisphere (ship initially moving slowly). (a) If  the  the wind is backing the ship must be in the dangerous semicircle. The ship should proceed with all available speed with the wind 10° to 45° depending on speed, on the port bow. As the wind backs the ship should turn to port thereby tracing a course relative to the storm as shown. (b) If  the  the wind remains steady in direction, or if it veers, so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the port quarter and proceed with all available speed. As the wind veers the ship should turn to starboard as shown.

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If there is insufficient sea room to run, when in the navigable semicircle, and it is not practicable to seek shelter, the ship should heave to with the wind on her starboard bow in the N and on her port bow in the S hemisphere. If in harbour when a tropical storm approaches, it is preferable to put to sea if this can be done in time to avoid the worst of the storm. Riding out a tropical storm, the centre of which passes within 50 miles or so, in a harbour or anchorage, even if some shelter is offered, is an unpleasant and hazardous experience, especially if there are other ships in company. Even if berthed alongside, or if special moorings are used, a ship cannot feel entirely secure.

CHAPTER 11 Recording and reporting weather observations CHAPTER 12 Weather forecasting ===========================================================================

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