562 F. H. KNOWLTOX EVOLUTION OF GEOLOGIC CLIMATES 



kilometers (36,000 feet). "This is, therefore, the upper limit of distinct 

 vertical convection and of cloud formation/^ 



All dust, from whatever source, is quickly washed out of the lower or 

 cloud region by rain or snow, but such as may happen to reach the upper 

 or isothermal region of the atmosphere continues to drift there until it is 

 brought down by gravity to the level of passing storms. In accordance 

 with recently made measurements on the terminal velocity of falling 

 globules, it appears that it may take from one to four or more years 

 before these dust particles return to the earth. In the meantime they 

 drift out from whatever source into a thin veil, "covering perhaps the 

 entire earth.^^ 



With these data it is possible to calculate the comparative action ol 

 volcanic dust on terrestrial and on solar radiations. According to 

 Humphreys, it is "roughly thirtyfold more effective in shutting solar 

 radiation out than it is in holding terrestrial radiation in. Therefore, a 

 veil of volcanic dust must produce in inverse greenhouse effect, and, if 

 long continued, should perceptibly lower an average temperature." Ho 

 concludes as follows : 



"From the above it appears quite certain that volcanic dust can lower the 

 average temperature of the earth by an amount that depends on the quantity 

 and duration of the dust, and that it repeatedly has lowered it certainly from 

 1° F. to 2° F. for periods of from a few months to fully three years." 



Humphreys gives a list of the most noted volcanic eruptions from 1750 

 to 1912, and, so far as can be gathered from the often meager historical 

 data, each, with a single exception, was followed by a markedly cold 

 period. 



In the summer of 1912 Abbot and Fowle ^^ conducted a series of ob- 

 servations at Bassour, Algeria, and at Mount Wilson, California, on the 

 effect on solar radiation of the volcanic dust believed to have been derived 

 from the eruption of Mount Katmai, Alaska, in the early part of June, 

 1912. The haziness due to this cause was first noted in Algeria on 

 June 19 and at Mount Wilson on June 21, and continued for more than 

 three months, when the observations terminated. These authors con- 

 clude that this veil of volcanic dust decreased the heat available to warm 

 the earth by about 10 per cent of the solar constant of radiation. They 

 write as follows : 



, "It might be expected that if so great a decrease as this should continue 

 indefinitely, the mean temperature recorded at meteorological stations would 



48 C. G. Abbot and F. E. Fowle : Volcanoes and climate. Smithsonian Misc. Coll., vol. 

 60, no. 29, 1913, pp. 1-24. 



