THE ATMOSPHERE, WEATHER, AND CLIMATE 



21 



60° would be mighty short of air unless something 

 happened. Fortunately, there is a way that polar air 

 can get to us. a way that involves fronts. 



In our hemisphere, polar air gets south by form- 

 ing a Polar Front. This front forms when a "High" 

 polar air mass meets a "Low" tropical air mass. Be- 

 cause of the pile-up of air in the opposing masses, the 

 pressure in the "High" increases until the front can- 

 not be maintained. Therefore, polar air breaks 

 through. In this breakthrough cycle four kinds of 



COLD 



FRONT 



WARM 



FRONT 



...-^iiiiiiii 



OCCLUDED 



FRONT 



■ ■■■■■■■■■«iifiiiii<> *.'.'.'.', 



Figure 2.8 Kinds of fronts. The diagram shows the associated clouds, 

 direction of associated cold and worm air masses, and site of precipita- 

 tion (vertical lines). 



local fronts — cold, warm, occluded, and stationary — 

 are formed (Figure 2.8). A cold front is formed when 

 cool, swift-moving air overtakes and upthrusts warm 

 air. This usually results in violent winds and pre- 

 cipitation, often a cloudburst, but of short duration. 

 A warm front forms from a slowly moving, warm air 

 mass overtaking and overriding a cold air mass and 

 often leads to many days of rainfall over a wide area. 

 An occluded front is a combination of the first two 

 fronts, and forms when a warm front is overtaken and 

 pushed above by a cold front. An occluded front 

 has the weather characteristics of both a cold and a 

 warm front. The final type, the stationary front, is 

 no more than one of the above fronts that remains 

 within a locality. Under these stationary conditions, 

 the weather is unchanged until the front either moves 

 on or completes its cycle. 



An extratropical cyclone starts from the mixing of 

 polar and tropical air. The life history of such a 

 cyclone shows how cold and warm fronts combine to 

 become an occluded front and summarizes the de- 

 velopment and fate of a Polar Front. The life his- 

 tory is as follows: 



First, a warm and a cold air mass come into con- 

 tact. Second, a disturbance in the form of a "Low" 

 develops in the boundary between the air masses. 

 Third, a cyclone with its counterclockwise winds 

 develops and sweeps into the "Low." This inward 

 and circular movement of both polar and tropical air 

 creates a warm front in part of the "Low" and a cold 

 front in another part. With further development of 

 the cyclone, the two fronts approach one another. 

 Fourth, the cold front overtakes and thrusts the warm 

 front upward, forming an occluded front. Finally, the 

 cyclone uses up its energy and dissipates (Figure 

 2.9). 



PRECIPITATION 



Atmospheric moisture can be lost in many ways 

 (Figure 2.10). Rain is the loss of excess moisture in 

 the form of water droplets. It is the form of pre- 

 cipitation when the dew point is above freezing and 

 droplets fall immediately to the ground. When the 

 dew point is below freezing, excess water vapor 

 particles are transformed into crystals in the form of 

 snow. 



Rain can be changed into sleet or glaze. Sleet is the 

 term applied to raindrops that freeze on their way to 

 the ground; glaze, or icing, to rain that freezes upon 



