14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Sp 



The first of these modifying forces is the earth's rotation. The 

 efifect of this rotation is to cause a high pressure belt in middle lati- 

 tudes and a diminished pressure in the polar basin, although it can- 

 not entirely destroy the central high pressure at the pole due to 

 increased cold without stopping the circulation. Hence, any increase 

 in solar radiation should intensify the pressure belt in middle lati- 

 tudes and lower the pressure in the polar basin, and the reverse with 

 decreased solar radiation. This is exactly what happens. 



A second modifying force is the change in cloudiness caused by 

 increased or decreased atmospheric circulation. Clouds and water 

 vapor ' have an important influence on incoming and outgoing radia- 

 tion, so that the belts of cloudiness near the Equator and near 60° 

 of latitude have an important influence on the tetnperature and pres- 

 sure and thus should aid materially in maintaining the latitude efifects 

 of changes in solar radiation reaching the atmosphere of the earth. 



A third modifying force is the movement of ocean water under 

 the influence of wind. An increase in the general circulation should 

 cause an increased flow of ocean waters, with all the modifications in 

 weather which such an increase implies. 



A fourth modifying force is the distribution of land and water. 



The influence of all these modifying causes can be seen in the lati- 

 tude and seasonal efi"ects, with difi^erences in solar activity. 



III. SEASONAL INFLUENCES 



When the influences of solar changes on the pressure are worked 

 out separately for each month of the year for different places, it is 

 found that the effect is different at different seasons of the year. 



At continental stations in high latitudes, such as Dawson, the pres- 

 sure increases much more in mid-winter with increased solar radia- 

 tion than at other seasons, and at mid-summer the effect may even be 

 the reverse of that in mid-winter. Figure 9 shows the annual period 

 in the effect of increased solar activity at Dawson. At other stations 

 such as Stykkisholm in the North Atlantic and Nome in the North 

 Pacific there is a dominant semi-annual period in the solar influence. 

 (See fig. 9.) The dotted curves in figure 9 are sine curves derived 

 from the first and second terms of the harmonic formula in a 



' Simpson, G. C, Further studies in terrestrial radiation. Mem. Roy. Meteor. 

 Soc, Vol. 3, No. 21, 1928. Manson, M., The evolution of climates, Baltimore, 

 Md., 1922. Angstrom, A. K., On radiation and climate, Geogr. An., Vol. 7, p. 122, 

 Stockholm, 1925. Brooks, C. E. P., Climate through the ages, p. 138, London, 

 1926. Abbot, C. G., The radiation of the planet earth to space, Smithsonian Misc. 

 Coll., Vol. 82, No. 5, 1929. 



