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METEOROLOGY AND CLIMATOLOGY: 



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Icold 



warm I 



ADJACENT AIR MASSES 



FRONT 



CONVECTION 



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MOUNTAIN 



OCCLUDED FRONT 



Figure 2.9 Life history of a polar front. Involved in such fronts, or 

 mid-latitude cyclones, is the development and dissipation of distinct cold 

 and warm fronts. 



contacting the ground or some frozen object upon the 

 ground. 



Hail, ahhough it resembles sleet, is formed quite 

 differently. Hailstone formation involves vk^ater drop- 

 lets being carried upward by rising air currents. As 



LOW 



Figure 2.10 The primary causes of precipitation. Any cause involves 

 air rising and cooling, the cooling leading to otmospheric water vapor 

 condensing. 



these droplets are carried upward, they freeze and 

 may acquire an outer layer of snowfiakes. Finally, 

 they reach a mass that is no longer capable of being 

 supported by the updrafts. They fall, and, on the way 

 down, may "grow" by the addition of a coating of 

 water vapor and/or raindrops. The hailstones so 

 formed either continue toward the ground, or are 

 caught in new updrafts and repeat the cycle of ac- 

 cumulating more snow, falling, and adding more 

 raindrops. This up and down cycle may be repeated 

 many times until hailstones reach the size of base- 

 balls. The number of up and down cycles involved in 

 individual hailstone formation can be determined by 

 counting the number of clear (rain-formed) and 

 snowy layers. 



Two other forms of precipitation are dew and frost. 

 Dew is water vapor that has condensed upon cool sur- 

 face objects. It does not fall as does rain, but is the 



