296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



half the solar radiation which reaches the earth is intercepted and 

 returned by the clouds without taking any part in warming up the 

 earth. 



We will now consider what happens to the radiation which reaches 

 the earth. Because of the shape of the earth, more solar radiation 

 falls per square foot on the surface at the Equator than at the poles. 

 The Equator is therefore maintained at a higher temperature than 

 the poles. This difference of temperature causes differences of pres- 

 sure in the atmosphere, and a circulation of air is set up between the 

 Equator and the poles. This is called the general circulation of the 

 atmosphere ; and all winds ultimately can be traced back to differences 

 of temperature, mainly those between the Equator and the poles. 



In the course of time the atmospheric conditions reach a steady 

 state with a certain temperature distribution, a certain amount of 

 air motion, and a certain amount of cloud, which combined are just 

 sufficient to cause a balance between the incoming and outgoing 

 radiation. 



Now let us consider what would be the effect of increasing the solar 

 radiation by a specified amount. At first the increased radiation 

 would reach the earth's surface and warm it up, but on account of 

 the shape of the earth the Equator would be warmed up more than 

 the poles. Hence, the difference in temperature between the Equator 

 and the poles would be increased and this would lead to an increase 

 in the general circulation of the atmosphere. The increased tempera- 

 ture and the increased wind cause an iD crease in the amount of water 

 evaporated from the oceans. This increased water in the atmos- 

 phere leads to greater formation of cloud and, as the precipitation 

 must equal the evaporation, to an increase in precipitation also. 



The increased cloud, however, reacts on the solar radiation and 

 reflects an increased proportion of it. The final state is a new balance 

 between the incoming and outgoing radiation produced mainly by 

 the increased amount of cloud, the only temperature change being 

 that necessary to produce the greater amount of cloud. 



Different parts of the world will be affected differently by these 

 changes. In equatorial and temperate regions where the precipita- 

 tion all falls as rain, the effects of increasing the solar radiation are 

 simple: there is a slight increase of temperature and a general in- 

 crease of rainfall. Thus the most important effect of increasing and 

 decreasing the solar radiation is to increase and decrease the pre- 

 cipitation, and the two keep step with one another; a period of in- 

 creased solar radiation is always a period of increased rainfall and 

 vice versa. 



When, however, we approach the poles, where the precipitation 

 may be either in the form of snow or rain, the conditions are much 

 more complicated and must be studied in more detail. 



