THE SUN AND THE ATMOSPHERE STETSON 153 



are dangerous in the extreme. The atmosphere is a sort of buffer 

 state, the very top of which receives a violent bombardment of high 

 frequency radiations from the sun that is prevented from reaching 

 the surface of the earth by the absorbing power of the molecules of 

 gases contained in it. Up where tins bombardment occurs, we witness 

 the auroral displays, such as the brilliant occurrences of last January. 

 These displays are in reality electrical discharges in the thin atmos- 

 phere occurring at the active front where are received the heavy 

 artillery of corpuscles, electrons, and radiations which fall upon it 

 from space outside and for which the sun appears to be chiefly 

 responsible. 



It is this blanket of atmosphere that enables the earth to retain 

 during the night much of the warmth generated by the sunshine 

 falling upon it during the day, thus preventing the temperature of 

 the earth from falling to extremely low temperatures during the 

 hours of darkness. 



Though the effective atmosphere is many miles thick, when com- 

 pared to the size of the earth itself it is but a thin shell, hardly more 

 than the thickness of the paper upon which the map is printed on a 

 desk globe compared to the size of the globe itself. 



If we look at a cross-section of the earth's atmosphere, it may for 

 convenience be divided into three zones or layers in which the strato- 

 sphere occupies the middle ground. The region below the stratosphere 

 is that which contacts our immediate surroundings and provides the 

 winds and atmospheric currents, giving rise to all our weather. We 

 call this lower region comprising perhaps the first 5 or 6 miles the 

 troposphere. The region above the stratosphere is the ionosphere. 

 If we send a recording thermometer aloft, we find that while passing 

 through the troposphere the temperature steadily falls until a height 

 of 10 or 12 kilometers is reached, when the temperature reaches the 

 extremely low value of —55° C, or some 68° below zero Fahrenheit. 

 Strangely enough, for the next 30 miles or so there appears to be little 

 change in temperature. This is the region of the stratosphere. The 

 weather forecaster for the stratosphere would have a relatively simple 

 task, for day after day, year in and year out, his prognostications 

 would be "clear and cool," and his forecasts would be 100 percent 

 correct. At a height of 60 kilometers or some 40 miles, the tempera- 

 ture would begin to rise again. Recent investigations give some 

 evidence that at extreme heights, up where the auroral fires play, 

 temperatures of 1,000° C. have to be postulated to account for the 

 presence of the oxygen that is there. The extremely rarefied condi- 

 tion of this upper atmosphere, however, calls for perhaps a quite 

 different interpretation of temperature than that to which we are 

 ordinarily accustomed when determining temperatures by the ther- 

 mometer at the earth's surface. 



