ICE AGES — SIMPSON 



297 



Let us fix our attention on a region where at present the summer 

 temperature does not rise above the freezing point, so that there is 

 little or no summer melting of the ice — the Antarctic is such a locality. 

 Now let the solar radiation increase. In consequence, the precipita- 

 tion and the mean annual temperature both increase, as indicated by 

 curve I of figure 3. At first all the precipitation is in the form of 

 snow, but when the summer temperature approaches the freezing 

 point — say, at A — some of the summer precipitation falls as rain. 

 Curve II represents the annual snowfall, and as the radiation increases 

 beyond the point marked A the curve for snowfall falls increasingly 



RADIATION 

 Figure 3.— Effect of increasing radiation on precipitation and accumulation of snow. 



below the curve for the total precipitation. Curve III represents the 

 evaporation and summer melting combined. At first there is no 

 melting, only evaporation, but after A melting becomes more im- 

 portant, and at B the loss of ice through evaporation and melting 

 equals the total snowfall. Curve IV in the lower half of the diagram 

 represents the annual accumulation of snow ; it is obviously the differ- 

 ence between curves II and III. At first, the annual accumulation 

 of snow is small; as the solar radiation increases, the accumulation of 

 snow increases until it reaches a maximum, after which it rapidly 

 decreases, owing to the rapid increase in the amount of melting. 



