Chapter 14 — AIR MASSES AND FRONTS 



FOG OR LOW STRATUS 



WARM OCEAN 



COLO CONTINENT 



209.369 

 Figure 14-7. — mT (Gulf or Atlantic) air of 

 winter moving northward over cold continent. 



In western and southern Europe, the invasion 

 of mT air brings milder weather than in the 

 United States. Low clouds, fog, and drizzle are 

 the normal results with mT, because the air 

 is usually quite stable. Tiis air mass seldom 

 invades eastern Europe. 



Maritime tropical air masses of the Southern 

 Hemisphere are very similar to those of the 

 Northern Hemisphere in both characteristics 

 and weather. 



In Asia, mT air is rare. The continent 

 is dominated by the outflow from the Asiatic 

 high which pushes southward over the Himalayas 

 to India and southeast Asia and arrives in a 

 highly modified form (the air of winter 

 monsoon) . 



CONTINENTAL TROPICAL (cT). — Continen- 

 tal tropical air is entirely absent from the 

 North American Continent in winter. The 

 primary regions with cT air in winter are 

 north Africa and Asia Minor. This air is 

 unstable but quite dry and thus causes little 

 weather, and it does not reach southern 

 Europe in winter. 



The only other source of true cT air in 

 winter is the interior of Australia. In the 

 Southern Hemisphere winter, this air is 

 unstable but dry and causes little weather. 



SUPERIOR AIR (S). — Superior air is a 

 high-level air mass found over the Southwestern 

 United States. It sometimes reaches the 

 surface; and due to subsidence effects, it is 

 the warmest air mass on record in the North 

 American Continent in both seasons. It is 

 extremely dry and provides cloud-free skies 

 and excellent visibility. 



Summer Air Masses 



In summer the arctic front retreats to 

 northern Canada and disappears in the arctic 

 over the North American Continent. In Europe 

 and northern Asia, an outbreak of arctic air 

 is rare, and little needs to be said about this 

 air. In the Southern Hemisphere, the arctic 

 air which leaves the Antarctic Continent is 

 rapidly modified to mP and therefore is of 

 little interest to us here. 



MARITIME POLAR. — Over the North Ameri- 

 can Continent, the whole Pacific coast is 

 usually under the influence of mP air. It is 

 much milder in summer than in winter, and 

 seldom extends east of the Rockies. The coastal 

 weather is generally clear with scattered 

 cumulus. 



Along the Atlantic coast the occasional 

 influx of mP brings relief from heat waves, 

 but causes little weather. 



In Europe, mP air is predominant. It is 

 quite unstable, but usually too dry to cause 

 much shower activity. Maritime polar air is 

 rare in Asia, but in the Southern Hemisphere 

 it is the predominant air mass, encircling 

 the whole Southern Hemisphere. 



CONTINENTAL POLAR. — The source region 

 is confined roughly to the northern two-thirds 

 of Canada, but polar air of Canadian origin 

 occasionally invades the United States in 

 summer. When it does, it preserves to a large 

 extent its temperature characteristics. Con- 

 vective activity in the United States is 

 extensive but mild, confined in height to 700 mb 

 (10,000 ft) or less. 



Continental polar air is infrequent over 

 Europe in summer. Occasional invasions of 

 this air come from Russia or the Balkans, 

 and these invasions have a southerly trajectory 

 and, consequently, are warmer than cP to the 

 east; but the weather is relatively stable. 



MARITIME TROPICAL. — On the west coast, 

 mT air of Pacific origin has little or no influence 

 on the weather, but on the east coast mT air 

 is the most extensive air mass in summer. 

 The mT air is quite warm and quite moist 

 with a dewpoint near or in excess of 70° F 

 at the surface. Low stratiform clouds are the 

 rule in the mornings, especially along the east 

 coast, becoming convective clouds during the 



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