194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



est effect, on the average, just after daybreak. Hence, as the sun 

 ascends the temperatures of the warming surface of the earth and of 

 the lower air lag behind equilibrium with the incoming radiation, and 

 continue to do so until the intensity of the insolation has passed well 

 beyond its maximum. That is, the temperature continues to rise for 

 some time, generally two to four hours, after the sun has crossed the 

 meridian — as the sun begins to descend from its highest point the 

 temperature continues to ascend. 



THE ABSOLUTE MAXIMUM DIURNAL INSOLATION (HEAT SUPPLY) IS AT 



THE SOUTH POLE. 



If I is the solar constant, or quantity of solar energy per minute 

 per unit area, normal to the insolation at the limit of the atmosphere, 

 then the total amount Q of solar energy per any consecutive 24 hours 

 per unit area of a horizontal surface, also at the limit of the atmos- 

 phere, is given by the equation 



1440 

 Q = 7 (sin 4> sin 8H+ cos <j> cos 5 sin H) 



IT 



in which <p is the latitude of the place in question, 8 the declination 

 of the sun at the time, and H the hour angle, in radians, between noon 

 and sunrise or sunset. 



A great deal of interesting information is contained in this equa- 

 tion. The most interesting, perhaps, is the fact that if the value of 

 Q for the Equator at the time of the vernal equinox be represented 

 by 1,000, then that of the North Pole at summer solstice is 1,202, and 

 that of the South Pole at the corresponding solstice 1,284; each being 

 greater than the value of Q at that time for any other place in either 

 Hemisphere. The advantage in favor of the South Pole is owing to 

 the fact that the earth is then near perihelion and therefore closer to 

 the sun. 



Not only does the absolute maximum diurnal insolation at the limit 

 of the atmosphere occur at the South Pole, but, owing to the great 

 elevation of the South Polar region, the dryness of its atmosphere, 

 and its comparative freedom from dust, so also does the corresponding 

 maximum at the surface of the earth. 



The days, however, of abundant insolation at the Poles are com- 

 paratively few, nor is this insolation very effective in raising the tem- 

 perature, owing to the high reflecting power and great heat of fusion 

 of the always-prevalent ice and snow. And so it happens that 

 although for a time every year each Pole receives more diurnal insola- 

 tion than does any other place on the 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. 



