WEATHER — WOOLAED 



229 



January 22j /S3S 



St 



7A9/*y 



FiGUKB 4. — Vertical (north-south) cross section through the cold front shown in 

 figure 3, constructed from data obtained by airplane soundings in the early morning 

 of January 22, 1936. The numbers show centigrade temperature, relative humidity, 

 and moisture content in grams of water vapor per kilogram of air, at the points indicated, 

 arranged as shown at the bottom of the diagram. Note how the cold (Pc) air lies 

 In wedge form under the warmer air ; the vertical scale is greatly exaggerated (see scale 

 of slopes in lower right corner). 



of winds that attend barometric pressure formations. The elabora- 

 tion of a systematic technique for the delimitation of the individual 

 currents on the daily maps, and the study of the weather phenomena 

 produced by their interactions, have led to "air mass analysis," to 

 wliich so much prominence has been given in recent years. Over 

 large areas of the earth, especially in polar and in tropical regions, 

 the meteorological conditions are so nearly uniform and steady, and 

 the circulation so relatively weak or restricted for long periods that 

 the atmosphere up to great heights above these areas takes on dis- 

 tinctive properties of temperature, moisture, and stability that are 

 characteristic of the regions. For example, when air either stag- 

 nates, or else circulates for a long period, over the regions of the 

 Gulf of Mexico and Caribbean Sea, it becomes warm, moisture-laden 

 and unstable. Frequently an atmospheric current originates in one 

 of these areas and transports large quantities of air to distant parts of 

 the earth, as, for example, when a great body of intensely cold air 

 pours out from the Arctic and flows down over the globe to lower 

 latitudes, sometimes covering the larger part of the North American 

 continent with its frigid winds (figs. 1, 3). Moving masses of air 

 from different localities tend as they travel to retain many of the 

 initial physical properties characteristic of their respective source 



