300 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1930 



some of the solar radiation, since laboratory experiments tend to 

 show that it does. 



Dust in the air causes a decrease of the solar radiation reaching 

 the earth. When dust is blown to heights of 10 to 50 miles by vol- 

 canic eruptions — that is, into the isothermal region or stratosphere — 

 the direct solar radiation at high sun may be reduced as much as 

 20 per cent, as was noted following the Katmai eruption of June 6, 

 1912. Not all great volcanic eruptions decrease the surface tem- 

 peratures of the earth, but only those that drive a lot of dust into the 

 isothermal region of the atmosphere. Volcanic eruptions during his- 

 toric times and their influence on solar radiation and the weather 

 have been traced. Such eruptions may have accounted for many of 

 the great changes in the weather that the earth has experienced. 

 W. J. Humphreys has accepted this view and formed his vulcanism 

 theory of glaciation. 



Humphreys (1920) considers the average size of the volcanic dust 

 particles in the stratosphere to be 1.85 microns. He calculates that 

 it would require about one year for such particles to fall from an ele- 

 vation of 35 km. to the lower limit of the stratosphere, which has an 

 elevation above sea level of 17 km. in tropical latitudes, 11 km. in 

 middle latitudes, and 6 km. in polar regions. He considers the finest 

 volcanic dust, which may have reached an altitude of 40 to 80 km, 

 following the eruption of Krakatoa in 1883 and Katmai in 1912, and 

 which encircled the earth a number of times, to have taken two and 

 one-half to three years to reach the base of the stratosphere, or upper 

 cloud level. He estimates that the total quantity of volcanic dust 

 required to cut down the intensity of the direct solar radiation by 

 20 per cent to be only the 174th part of a cubic kilometer, or the 727th 

 part of a cubic mile, assuming that the particles are spherical. Since 

 the particles are more or less flat, it is probable that not more than 

 the 1,500th part of a cubic mile, or the 350th part of a cubic kilo- 

 meter, is needed to reduce the intensity of direct solar radiation 20 

 per cent. This amount, if indefinitely continued, he concludes, would 

 be capable of producing an ice age. 



With the particles all being 1.85 microns in size, he calculates that 

 the volcanic dust is some thirtyfold more effective in shutting out 

 solar radiation than it is in keeping terrestrial radiation in. This is 

 because radiation, both solar and terrestrial, is simply scattered by 

 such small particles and scattered in proportion to the inverse fourth 

 power of the wave length. Since the ratio of solar wave length to 

 terrestrial wave length is, roughly, 1 to 25, and the ratio to their 

 fourth powers as 1 to 39 x 10*, it follows that the interception of 

 outgoing radiation by the very finest, and therefore most persistent, 

 dust is wholly negligible in comparison with its interception of in- 

 comine: solar radiation. 



