CHAPTER 14 



AIR MASSES AND FRONTS 



Personnel studying for advancement to 

 Aerographer's Mate Third Class or Second 

 Class must know a great deal about air masses 

 and fronts. In this chapter a more complete 

 picture of the part that air masses and fronts 

 play in the overall weather story is given. It 

 is readily seen that air masses and fronts 

 are the keys to modern weather analysis and 

 forecasting. 



AIR MASSES 



The air-mass concept is one of the most 

 important developments in the history of 

 meteorology. An air mass is a large body 

 of air whose physical properties, particularly 

 temperature and moisture distribution, are 

 nearly homogeneous, level for level. Fore- 

 casting is largely a matter of recognizing the 

 various air masses in the weather picture, 

 determining their characteristics, and predicting 

 their behavior. 



CLASSIFICATION 



Air masses have been classified geographi- 

 cally and thermodynamic ally. The geographical 

 classification, which refers to the source region 

 of the air mass, divides air masses into four 

 basic categories. These are arctic/antarctic 

 (A), polar (P), tropical (T), and equatorial (E). 

 Some authors may include superior (S) or 

 monsoon (M) in their classification. The first 

 three are further divided into maritime (m) 

 and continental (c). Maritime arctic/antarctic 

 air masses are rare, since there is a pre- 

 dominance of land mass or icefields In the 

 polar regions. Virtually all equatorial air 

 masses are considered to be maritime in 

 origin. An air mass is considered to be mari- 

 time if its source of origin is over an oceanic 

 surface. If the air mass originates over a 

 land surface, it is considered continental. 



The less common air masses, superior and 

 monsoon, are limited in locale and extent. The 

 superior (S) air mass is generally found aloft 

 over the southwestern United States, but is 

 sometimes located at or near the surface. 

 Monsoon air (M), as the name implies, is 

 found over India and southeastern Asia, 



The types of air masses can easily be 

 remembered by using the letters in the word 

 "tape" to stand for the first letter of each 

 basic type of air mass. 



The thermodynamical classification applies 

 to the relative warmth or coldness of the air 

 mass. A warm air mass (w) is one which is 

 warmer than the underlying surface; a cold air 

 mass (k) is one which is colder than the under- 

 lying surface. For example, a continental polar 

 cold air mass is classified as cPk. An mTw 

 classification indicates that the air mass is 

 a maritime tropical warm air mass. 



Air masses can usually be identified by the 

 type of clouds within them. Cold air masses 

 usually show cumuliform clouds, whereas warm 

 air masses contain stratiform clouds. 



Sometimes, and with some air masses, the 

 thermodynamic classification may change from 

 night to day. A particular air mass may show 

 k characteristics during the day and w charac- 

 teristics at night and vice versa. 



SOURCE REGIONS 



The air-mass source region is the area 

 where the air mass originates. The condition 

 which is ideal for the production of an air 

 mass is the stagnation of air over a rather 

 uniform surface (water, land, or icecap) of 

 uniform temperature and humidity. The length 

 of time an air mass stagnates over its source 

 region depends on the surrounding pressures. 

 The air acquires definite properties and 



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