196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



A greater general prevalence of cirrus and cirrus haze during spot 

 maxima than during spot minima (indicated by certain observations) 

 would also account for this paradox; because such clouds, owing to 

 the size of their particles, shut out the short wave-length solar radia- 

 tion more effectively than they shut in the long wave-length earth 

 radiation. And perhaps these clouds really are generally most 

 prevalent during spot maxima, and, therefore, at least a contribut- 

 ing factor to the cause of the corresponding temperature minima. 

 At any rate the auroras are then most frequent, and they obviously 

 generate nitrous oxide and other hygroscopic compounds which, be- 

 cause of their density, slowly fall to the cirrus level Avhere they may 

 produce cloud particles in an atmosphere whose humidity is much 

 below that which otherwise would be essential to cloud formation. 



The maximum, then, of the cirrus screen and the minimum of the 

 ozone blanket, coincident with the highest temperature of the sun, 

 may very well account for the above paradox — the hotter the sun the 

 colder the earth. 



THE COOLER THE SUN THE WARMER THE EARTH. 



This paradox is practically included in the one just discussed. It 

 means that at times of sun-spot minima, when the solar constant seems 

 to be least, the average temperature of the earth is highest. 



At the times of spot minima the solar atmosphere is clearest ; the 

 extreme ultra-violet radiation presumably, therefore, at a maximum ; 

 the upper atmosphere richest in ozone, and the earth most conserva- 

 tive of its heat, and, because of the minimum (if that be the case) of 

 cirrus, also most receptive of solar radiation — so receptive and so 

 conservative, perhaps, as to gain slightly in temperature despite the 

 decrease in the heat supply. 



THE SUN RISES BEFORE IT IS UP. 



This paradox about the sun rising before it is up is equally true 

 of the moon and the stars, and is also one of the best known and 

 easiest explained of all meteorological paradoxes. 



Everyone is familiar with the fact that as light passes slantingly 

 from one medium to another, as from air to glass, for instance, it 

 does not continue on in the same straight line, but abruptly changes 

 direction at the interface according to well-known laws. And the 

 same thing is true of the rays of light that pass from space into and 

 through the atmosphere of the earth, except that, in this case, as the 

 density of the atmosphere gradually increases from zero at its outer 

 boundary to a maximum at the surface of the earth, so too the change 

 in direction of the entering light is equally gradual. The total 



