Formation of Precipitation

Formation of  Precipitation Engr. Dielene Fajutagana – De Guzman

NECESSARY CONDITIONS FOR THE OCCURRENCE OF PRECIPITATION

creation of saturation conditions  formation of water droplets or ice crystals growth of water droplets or ice crystals 

Creation of saturation conditions 

Cyclonic It is the lifting of the air converging into a low pressure area  Frontal cyclones are called extratropical because they  generally occur beyond the tropics while non-frontal cyclones generally occur in tropical regions of the  world.  Examples of non-frontal cyclones are the hurricanes (typhoons) 

Other names associated with the formation of non-frontal cyclones:

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Tropical depressions: Tropical storms: Hurricanes (typhoons): 

40 v < mph 40 ≤ v < 75 mph 75 v ≥ mph

schematic of the air flow for nonfrontal cyclones Convective. Lifting of the air is produced by uneven heating of the earth’s surface. Convective cooling generally produces thunderstorms

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schematic of the air flow for non-frontal cyclones Orographic. This may occur  when the air flow is deflected upward by orographic barriers such as mountains.

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Formation of Water Droplets or  Ice Crystals  When the air is close to saturation it requires the presence of certain nuclei for  water droplets or ice crystals to form. Nuclei are small particles of various substances of the order of 0.1 – 10 µm(106 m). They are called condensation nuclei  when they lead to the formation of water droplets and freezing nuclei when they  lead to ice crystals.

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Growth of water droplets or ice crystals 

Upon nucleation the water droplet or ice crystal grows to visible size in a fraction of a second through diffusion of water vapor into it, but growth thereafter is slow. Generally the growth of water droplets and ice crystals occur due to: diffusion of water vapor  collision of droplets or ice crystals  coalescence (molecular attraction of particles that are close to each other) 

General Characteristics of Storm Types 

Storms caused by frontal cyclones (movement of  extratropical cyclones generally at latitudes of 35 to 50 degrees North) • They generally cover large areas • Low to moderate intensity  • Highest intensity associated with cold fronts  because the steeper frontal surface gives a stronger lift • It travels in the direction west to east • It travels with a speed of about 30 mph

Storms caused by non-frontal cyclones (tropical cyclones) Cooling and lifting of the air caused by convergence in the central core.  • It originates with a weak disturbance along the thermal Equator  • The air movement towards disturbance is deflected by  the Coriolis force which forms a  vortex.  • The water vapor from a warm ocean is condensed by  rising air at the center  • The storm system generally moves westward due to the  westerly wind system 

Storms caused by non-frontal cyclones (tropical cyclones) The storm system decays when reaching land  because of the greater ground friction and the fact that the moisture (energy) supply is cutoff .  Also it dissipates when reaching colder oceans.  They can be design storms for 1,000 to 2,000 sq.mi areas in coastal regions such as southern and southeastern USA, Philippines, Taiwan, and Japan. 

Storms Caused by Thermal Convection (convective storms) Cooling and lifting caused by  thermal heating at the ground Generally occurs in the afternoon and early evening after the earth has been thoroughly heated by the sun. 

Typically covers small areas of the order of  5-10 sq-mi. although they may about 300 sq.mi. High intensity over a small area and a large spatial and temporal variation of  intensity. Sometimes accompanied by hail. They serve to construct design storms for small basins 

Storms Caused by Orographic Lifting Cooling and condensation caused by lifting of  the moist air mass over a topographic divide.  Precipitation is a function of elevation, rate of  rise and direction or moist air movement.  Generally widespread in a transverse direction along the orographic barrier. 

Storms Caused by Orographic Lifting Moderate to low intensity.  Duration of the storm persists as long as the moist air flow continues.  Orographic patterns often superimposed on other types of storms (e.g. a hurricane reaching an mountain range). 