Chapter 14 — AIR MASSES AND FRONTS 



of the cold front, the warmer air is forced 

 away and replaced by the colder air mass. 

 As the cold air descends the lee side of 

 the mountain, the air warms adiabatically (fig. 

 14-19) and clearing occurs within it. However, 

 since the cold air is displacing warm air, 

 typical cold frontal clouds and precipitation may 

 occur within the warm air if the warm air is 

 sufficiently moist and conditionally unstable. In 

 some cases maritime polar air which has 

 crossed the Rockies is less dense than mari- 

 time tropical air from the Gulf of Mexico which 

 may lie just east of the mountains. If the 

 maritime polar air is moving with a strong 

 westerly wind current and the maritime tropical 

 air is moving with a strong southerly wind 

 current, the maritime polar air may overrun 

 the maritime tropical air. This results in 

 extremely heavy showers and violent thunder- 

 storms and is one of the conditions under which 

 tornadoes occur. 



If colder stagnant air lies to the lee side 

 of the mountain range, „ the cold front on passing 

 over the range does not reach the surface and 

 travels as an upper cold front. Under this 

 condition frontal activity is at a minimum. This 



situation does not continue indefinitely; either 

 the stagnant air mixes with the air above and 

 the surface of separation becomes spread out, 

 or the upper cold front breaks through to the 

 ground with the development of thunderstorms 

 and squalls. 



As a cold front passes a mountain range, 

 it may develop a bulge or a wave as a portion 

 of the front is retarded. In the case of an 

 occlusion, a new and separate cyclone circu- 

 lation may occur at the peak of the warm 

 sector as the occluded front is retarded by a 

 mountain range. 



In general, it may be said that the area 

 of precipitation is widened as the front 

 approaches the range and that there is increased 

 intensity of the precipitation area and cloud 

 system on the windward side of the range and 

 a decrease on the leeward side, (See fig. 14-20.) 



Consider the effect of a mountain range on 

 a WARM FRONT. When a warm front 

 approaches a mountain range, the upper section 

 of the frontal surface is above the effects of 

 the mountain range and does not come under 



dissipating Clouds 



Clear and Dry 



Figure 14-19.— Effect of adiabatic heating. 

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