AEROGRAPHER'S MATE 3 & 2 



Pacific origin. Surface temperatures are colder, 

 and the vertical extent of the convective activity 

 is less. 



Maritime polar air also affects the south 

 Alaskan coast but seldom reaches the Alaskan 

 interior due to the mountain ranges which it 

 must cross. 



In western Europe, mP air masses predomi- 

 nate. They are quite varied in the weather they 

 cause, depending on whether their trajectories 

 are cyclonic or anticyclonic. The most unstable 

 conditions are the rule with a cyclonic tra- 

 jectory. 



In the Southern Hemisphere, mP air is the 

 most prevalent air mass and is quite similar 

 to its Northern Hemisphere counterpart in 

 characteristics and in the weather it causes. 



MARITIME TROPICAL (mTv. — The mT air 

 which is formed over the Gulf of Mexico is 

 usually conditionally unstable. This instability 

 may be released by frontal or orographic lifting. 

 When mT air is forced up in the eastern 

 mountains of the United States, practically the 

 same types of weather result as were discussed 

 under the Great Lakes effect. However, the 

 weather associated with the mT air is more 

 intense because of the greater quantity of 

 moisture involved, and extends to a higher 



level. If mT idr is forced over mountainous 

 terrain, as in the eastern part of the United 

 States, the conditional instability of the air 

 is released at higher levels. (See fig. 14-6.) 

 This might produce thunderstorms or at least 

 large cumuliform clouds. These clouds may 

 develop out of stratiform cloud systems and 

 therefore may be encountered without warning 

 when flying within the clouds. Icing may also 

 be present. Thus, as with the Great Lakes 

 effect, a combination of all three hazards (fog, 

 thunderstorms, and icing) is possible. 



Now consider the weather associated with 

 maritime tropical warm air (mTw) of Atlantic 

 origin. In winter, when the land surface is 

 relatively cold, the mT air moves northward 

 as mTw. It is cooled from below. This cooling 

 results in more stable conditions near the 

 surface. Due to the high moisture content, 

 condensation occurs (either as fog or as low 

 stratus) particularly at night when radiational 

 cooling plays an important part. (See fig, 

 14-7.) In the lower latitudes, the heating effect 

 of the sun usually causes sufficient convective 

 lift to produce cumuliform clouds in the 

 afternoon. 



Maritime tropical air of Pacific origin is 

 quite rare in winter, but it causes torrential 

 rains on those rare occasions when it invades 

 the California coast. 



Figure 14-6. — mT air moving northeastward. 

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