INTERCHANGE OF HEAT VON BEZOLD 399 



phere that in one part the insolation is in excess but in the other 

 part the radiation. 



If we indicate by the index a the quantities that relate to the 

 region where insolation is in excess, and by tbe index p the other 

 region then the equation (17) can be rewritten as 



where u £ is small with respect to the differences that enter both 

 sides of this equation. 



This theorem can be expressed as follows: 



"At any moment of the year the earth's surface is divided into 

 regions having insolation in excess and others having radiation in 

 excess." 



If we ignore the diurnal period, the equatorial zone always 

 belongs to the region of excess of insolation but the limiting neutral 

 lines are subject to important variations in the course of the year. 



The proof of this theorem consists simply in this, that at all 

 seasons of the year warm currents flow poleward from the tropics, 

 whereas insolation and radiation remain apparently uniform in 

 the tropics through the whole year, and therefore these currents 

 can be maintained to only a very slight extent by any energy that 

 may be stored up. 



The polar regions belong to the region of excess of insolation 

 although only during a small part of the year, since during mid- 

 summer of either hemisphere the corresponding polar region receives 

 during a limited interval of time more heat from the sun than the 

 regions of lower latitude or than those of the opposite hemisphere. 



Hence, while the two neutral lines depart in the same direction 

 from the average location that they occupy at the time of the 

 equinoxes (in the northern spring time both move northward but 

 in the southern spring southward), therefore the polar region 

 enclosed by one of these lines diminishes steadily until it entirely 

 disappears at midsummer. 



If then the convection of heat from lower latitudes continues 

 during the midsummers of the respective hemispheres, this can 

 only be explained by the fact that the whole of the heat gained by 

 excess of insolation and by convection is consumed in covering the 

 loss of energy that the polar region had suffered during the winter 

 half-year, and which had resulted in the formation of enormous 

 masses of ice and the diminution of the vapor contained in the 

 atmosphere. 



