CIRCULATION OF THE ATMOSPHERE 437 



THE CIRCULATIONS OF THE ATMOSPHERES OF THE 

 EARTH AND OF THE SUN 



By Professor FKANK H. BIGELOW 



U. S. WEATHER BUREAU 



The Two Causes of Circulation on a Non-Rotating Earth 

 Gravity, Temperature and Pressure 



IT must seem rather ambitious to attempt to treat so great a subject 

 as that of the circulation of the atmospheres of the earth and the 

 sun in a single lecture. It is true that if it should be discussed fully, 

 in the technical way, it would require a great many lectures, but of 

 course there are at the same time certain fundamental principles which 

 are common to all circulations that can easily be studied, and then 

 illustrated by the known facts of the circulation in these two atmos- 

 pheres. All circulation depends upon two primaiy causes, the first 

 being the attraction of gravitation, by the laws of the action of the 

 earth upon its atmosphere or the great body of the sun upon its atmos- 

 phere; and, secondly, the difference of temperature which exists in differ- 

 ent parts of a given atmosphere. If we had an earth standing still in space 

 without rotation upon its axis and the sun were withdrawn for a consider- 

 able time, the atmosphere of the earth would gradually settle down into a 

 quiescent state, which may be described as consisting of a series 

 of concentric shells, each shell having a certain fixed temperature pass- 

 ing around the earth at the same distance from a center, as if a balloon 

 were floating at the same height above the surface, where will be found 

 the same barometric pressure and the same temperature in all latitudes 

 and longitudes. If the balloon falls from one shell to another it would 

 pass into layers of greater density, and if it rises, into layers of less 

 density. The boundary of each shell may be conceived as a surface 

 having the same force of gravity acting upon it, and this is called the 

 gravity level. In this case the surfaces of equal pressure or the isobars, 

 and the surfaces of equal temperatures, isotherms, both coincide with 

 their own gravity levels. Everything is quiescent and there is no cir- 

 culation. It is quite important to secure a clear idea of the fact that the 

 isobars, isotherms and gravity levels coincide wherever the layers in 

 the atmosphere have the same temperature. As a matter of fact the 

 earth is not without rotation, and the sun is shining upon it, sending 

 enormous masses of heat which fall upon the tropics, and it is our 

 problem to study the effect of this heat, at certain layers in the earth's 

 atmosphere, upon the circulation of the entire mass. 



