1 68 Humphreys: meteorological paradoxes 



earth, it is always cold; and the south pole, though having- the 

 greater maximum diurnal insolation, is the colder of the two, 

 owing to its elevation and greater distance from open water. 



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 evi- 

 dence in favor of it is already very strong. It is known, for in- 

 stance, 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 sunspot maxima than during the years 

 of sunspot minima. Furthermore, numerous careful measure- 

 ments of the solar radiation made during the past dozen years 

 or more, seem to compel the assumption, at least tentatively, 

 that the effective temperature of the sun is greater during spot 

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

 clusions 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 atmos- 

 pheric circulation, and, therefore, a decrease in the surface tem- 

 perature, owing to the greater flow of cold air from the higher 

 towards the lower latitudes. But the very great mixing of the 

 convective portion of the atmosphere, and the consequent pre- 

 vention of the formation of over- and under-flowing strata, seems 

 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 

 srreater extent with the ultraviolet radiation than with the visible 

 and infrared. Hence, during spot maxima, or when the solar 

 atmosphere is dustiest, the solar energy must, it would seem, 

 be poorest in ultraviolet radiation. 



