METEOROLOGICAL, PARADOXES HUMPHREYS. 195 



THE HOTTER THE SUN, THE COLDER THE EARTH. 



It is not yet universally conceded that this paradox, "the hotter 

 the sun, the colder the earth," really is true, but the evidence in favor 

 of it is already very strong. It is known, for instance, that several 

 extensive studies of the temperature records of the earth have all 

 shown that on the average it is a little colder during the years of 

 sun-spot maxima than during the years of sun-spot minima. Further- 

 more, numerous careful measurements of the solar radiation made 

 during the past dozen years or more seem to compel the assumption, 

 at least tentatively, that the eifective temperature of the sun is greater 

 during spot maxima than during spot minima. If, then, both these 

 conclusions are true — if the temperature of the earth is lowest during 

 spot maxima and the solar constant highest — it follows that the above 

 paradox is also true. 



But by what possible process can the earth get colder when the sun 

 grows warmer? It has been suggested that the increase of the solar 

 constant causes a corresponding increase in the atmospheric circula- 

 tion, and therefore a decrease in the surface temperature, owing to 

 the greater flow of cold air from the higher toward the lower latitudes. 

 But the very great mixing of the convective portion of the atmosphere, 

 and the consequent prevention of the formation of over- and under- 

 flowing strata, seem to render this suggested explanation untenable. 



The key to this paradox may perhaps be found in the greater extent 

 and density of the solar corona at the times of spot maxima than 

 at the times of spot minima. The corona — since in large measure it is 

 only so much dust about the sun — obviously must interfere with the 

 passage of radiation through it, and to a far greater extent with the 

 ultra-violet radiation than with the visible and infra-red. Hence, 

 during spot maxima, or when the solar atmosphere is dustiest, the 

 solar energy must, it would seem, be poorest in ultra-violet radiation. 



Now when cold dry oxygen, such as exists in the upper atmosphere, 

 is acted upon by certain regions, at least, of the ultra-violet spectrum, 

 some of it is converted into ozone, a substance known to be in the 

 upper atmosphere to a far greater extent than in the lower. Hence 

 when sun spots are most numerous the upper air should contain a 

 minimum amount of ozone. But ozone is intensely absorptive of 

 earth radiation and that, too, in the spectral region of its greatest in- 

 tensity, and where water vapor is least absorptive and carbon dioxide 

 not at all. That is, at the time of spot maxima when the solar con- 

 stant is (apparently) greatest, the earth's blanket of ozone is (pre- 

 sumably) least. Even, therefore, if the earth should be receiving 

 an increased amount of heat at this time it might, nevertheless, grow 

 slightly colder because of the coincident depletion of the heat-con- 

 serving blanket of ozone. 



