AEROGRAPHER'S MATE 3 & 2 



forms. Thus, evaporation may continue from 

 the warm water even after the cold air has 

 become saturated, resulting in condensation. 

 This condensation appears as fog, but due to the 

 instability of the air, the steam is lifted as 

 it forms, growing into clouds, After crossing 

 the lakes, this air again flows over a cold, 

 snow-covered surface. The surface cooling 

 increases the stability and may produce fog 

 at night. 



The air may be subjected to forced lift 

 in approaching the Appalachians. This upslope 

 flow causes cooling and condensation and may 

 result in the forming of towering cumulus or 

 cumulonimbus clouds with snow showers. On 

 the eastward side of the mountains the air 

 descends and warms adiabatically, causing the 

 clouds to be partially or completely dissipated. 

 (See fig. 14-3.) 



Consider how cPk air can become mT air. 

 In winter, when cPk air reaches the warm 

 waters off the southern coast of the United 

 States, its temperature is usually about 10° 

 lower than the water temperature. Tie air is 

 rather unstable when it reaches the water 

 surface. The same thing happens when the air 

 flows over the Great Lakes, except the Great 

 Lakes are relatively a small area, whereas 

 now the modification occurs over such a large 

 area that a new air mass is formed. Both the 

 temperature and moisture content of the air 

 rapidly increase, beginning in the lower levels 

 and quickly affecting the higher levels. Thus, 



rapid changes in the weather can be expected 

 as cPk air moves over an mT source region. 

 (See fig. 14-4.) 



After mP air crosses the Rocky Mountains 

 and stagnates in the Great Basin, it often 

 becomes cP air. By the same token, cP air 

 moving out over the Atlantic rapidly modifies 

 to become mP air. 



In Siberia, cP air is the coldest air mass 

 on record in the Northern Hemisphere; in the 

 Southern Hemisphere this air mass is unknown, 



MARITIME POLAR (mP). — Consider the 

 weather associated with maritime polar air 

 (mP). In its source region, maritime polar 

 air, in general, is characterized by surface 

 temperatures above the freezing point, moder- 

 ately steep lapse rates, and near saturation 

 up to rather high levels. However, since the 

 air aloft is cold, it has a low capacity for 

 water vapor; hence, the water content may be 

 small even though the relative humidity is high. 

 The weather is characterized by cumulus and 

 cumulonimbus clouds with showers and by good 

 visibility except in shower areas. 



North America is affected by maritime polar 

 air which comes primarily from a source region 

 in the North Pacific. Tliis air mass usually 

 results from the modification of cold continental 

 air which has moved from Asia or the frozen 

 arctic. The continental air is heated from below 

 and picks up additional moisture as it moves 

 over the ocean, resulting in a characteristically 



Temperature at 2000 ft -12°C Water temperature 1° C, 



_ Moisture and 



Temperature „ 



J-** -1 9 r ( war 



difference ■ z ^ } 



ai 



V/'/ , >/'^> 



Surface temperature -18° C 



Snow flurries 

 Surface temperature — 6° C 



NW 



SE 



Figure 14-3. — cP air moving over the Great Lakes. 



318 



210.79 



