154 ANlSrUAL REPORT SMITHSONIAN INSTITUTION, 1942 



of about 22 kilometers, but which probably occupies a region extend- 

 ing from 15 to 35 kilometers. If all the ozone in this region were 

 to be brought to the standard conditions of temperature and pressure 

 of our atmosphere at the earth's surface, it would represent a layer 

 of only 2 to 3 millimeters in thickness. Yet this small amount of 

 ozone is the defense between us and extremely dangerous radiations 

 in the ultraviolet region of the sun's light. Were the absorption, 

 however, of this region of the solar spectrum even a little greater 

 than it is, we should be deprived of that small amount of ultraviolet 

 light filtering through our atmosphere that is so essential for health 

 and the production of our sunshine vitamin D. Whether or not 

 variations in the sun's radiation are suflSciently great or changes in 

 the absorption of the earth's atmosphere suflBciently large to bring 

 about dangerous variations in the production of vitamin D in living 

 organisms at the earth's surface is an interesting question for specula- 

 tion and for investigation. 



We can be confident, however, that it is a fortunate combination 

 of the sun and our atmosphere that makes life on the earth possible. 

 The sun not only radiates its health-giving sunshine, but it also emits 

 literally death-dealing rays. Were it not for the protecting shield of 

 the earth's atmosphere, the sun would be the annihilator of us all. 

 The atmosphere provides on the one hand oxygen for maintaining 

 life, and on the other hand protects us from the highly penetrating 

 rays. It is a sort of a buffer state, the very top of which receives a 

 violent bombardment of high-frequency radiations from the sun. and 

 the lower layers of which form a blanket that enables the earth to 

 retain during the night much of the warmth generated by the sun- 

 shine that has penetrated through it, thus mitigating the extremes of 

 temperature between night and day to which the earth would other- 

 wise be subjected. 



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 strato- 

 sphere 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 strato- 

 sphere would have a relatively simple task, for day after day, year 



