70 STORMS AND WEATHER FORECASTS 



about 50 miles per liour, and Lake Michigan would experience a 

 severe "northwester." 



The forecaster knows that high-pressure and low-pressure areas 

 drift across the countr}^ from the west toward the east at the rate 

 of about 600 miles daily, or about 37 miles per hour in winter 

 and 22 miles per hour in summer ; that the highs are attended 

 by dr}'-, clear, and cooler weather, and that they are drawing 

 down, by a vortical action of their centers, the cold air fi-om great 

 altitudes above the clouds and causing it to flow away laterally 

 along the surface of the earth in all directions from the center, 

 and that the high-pressure areas sometimes become so intense in 

 their vortical motion as to draw down such vast volumes of cold 

 air that we call them cold waves. 



In the downward movement of the air in cold waves we must 

 concede that the loss of heat by radiation through a cloudless 

 atmosphere is much greater than that dynamically gained by 

 compression, or else we must assume that the air possesses such 

 intense cold at the elevation from which it is drawn that not- 

 withstanding the heat gained by compression in its descent it is 

 still far below the normal temperature of the air near the surface 

 of the earth. 



The forecaster knows that although these intense high-pressure 

 areas first appear in the extreme northwest, they do not depend 

 on the land of their birth for the cold they bring to us, and that 

 cold waves are not simpl}^ immense rivers of air which have 

 been chilled by flowing over the great snow and ice fields of the 

 Arctic regions, as was once thought. He is also familiar with the 

 fact that in the low-i3ressure areas' the conditions of the air and 

 its various movements are exacth^ the reverse of what they are 

 in the high ; that the air is much warmer and moister, and that 

 it is drawn spirally inward from all directions instead of being 

 forced outward, as in the higli ; that it ascends as it approaches 

 the center of the depression, sometimes causing rain or snow as 

 it cools by expansion during its ascent, or as it encounters and 

 mixes with air strata of lower temperature than its own. 



We know that while our atmosphere expands upward to an 

 altitude probably of 50 miles, it is so elastic and its expansion is 

 so rapid as it recedes from the earth that half of its mass lies 

 below the 3-mile level, and that our storms and cold waves are 

 simply great swirls or eddies in the lower stratum of probably 

 not more than 5 miles in thickness ; that the air above the 6-mile 



