274 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



these are the deposits of the times when the oceans have most widely transgressed the lands, 

 and therefore the times of greater humidity. The maximum of continental extension falls 

 in with red deposits and more or less arid climates. (See curve for aridity in figure 90.) 



VOLCANIC DUST AS A CLIMATIC FACTOR. 



As these pages are going through the press two interesting papers on the subject of 

 volcanic dust as a climatic factor have appeared. These articles, which are by W. J. 

 Humphreys,* should be read by every student of paleometeorology. The following 

 are the conclusions reached: 



[Volcanic dust in the upper atmosphere has been one of] ".several contrilniting causes of cli- 

 matic change, * * * a cause that during historic times has often l)een fitfully operative, and 

 concerning which we liave much definite information. * * * 



"At an elevation that in middle latitudes averages about 11 kilometers the temperature of the 

 atmosphere becomes substantially constant, or, in general, ceases appreciably to decrease wilti 

 increase of elevation, this is, therefore, the upper limit of distinct vertical convection and of cloud 

 formation. Hence, while volcanic or other dust in the lower or cloud region of the atmosphere is 

 fiuickly washed out by .snow or rain, that ^\'hich by any process happens to get into the upper or 

 isothermal region must continue to drift there until gravity can bring it do^\Ti to the level of passing 

 storm.?. In other words, while the lower atmosphere is cjuickly cleared of any given supply of dust, 

 the isothermal region retains such dust as it may have for a time that depends upon the size and 

 density of the individual dust pai tides themselves, or upon the rate of fall. * * * Volcanic dust 

 once in the upper atmosphere must remain in it for many months and he drifted out, from whatever 

 origin, into a thin veil covering perhaps the entire earth. * * * A veil of volcanic dust must produce 

 an inverse green-house effect, and if long continued, should perceptibly lower our average tem- 

 perature. Let us see then what observational evidence we have on the effect of volcanic dust on 

 insolation intensity and average temperatures. 



"Pyrheliometric records [show] that there was a marked decrease in the insolation intensity 

 from tiie latter part of 1883 (the year this kind of observation was begun) to and including 1S8G, 

 from 1888 to 1892, and during 1903. There has also been a similar decrease since about the middle 

 of 1912. Now all these decreases of insolation intensity, amounting at times to 20 per cent of the 

 average intensity, followed violent volcanic eruptions that filled the isothermal region with a great 

 quantity of dust. * * * 



"It appears quite certain that volcanic dust can lower the average temperature of the earth by 

 an amount that depends upon 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. Hence 

 it certainly has lieen a factor, in determining our past climates, and presumably may often be a 

 factor in the production of our future climates. Nor does it require any great volume of dust to 

 produce a marked effect. Thus it can be shown by a simple calculation that less than the one 

 thousandth part of a cubic mile of rock spread uniformly through the upper atmosphere as volcanic 

 dust would everywhere decrease the average intensity of insolation received at the surface of the 

 earth by at least 20 per cent and therefore would, presumably, if long continued, decrease our 

 average temperatures by several degrees. * * * This effect has been clearly traced back to 1750, or 

 to the time of the earliest reliable records. Hence it is safe to say that such a relation between 

 volcanic dust in the upper atmosphere and average temperatures of the lower atmosphere has 

 always obtained, and therefore that volcanic dust must have been a factor, possibly a very im- 

 portant one, in the production of many, perhaps all, past climatic changes" (a: 366-71). 



"The intensity of the solar radiation at the surface of the earth depends upon not only the dusti- 

 ness of the earth's atmosphere but also upon the dustiness, and of course the temperature, of the 

 solar atmosphere. Obviously dust in the sun's envelope must more or less shut in solar radiation 

 just as and in the same mamier that dust in the earth's envelope shuts it out. Hence it follows 

 that when this dust is greatest, other things being equal, the output of solar energy wlW be least, 



* A summary paper appeared first, entitled (a) " Volcanic Dust as a Factor in the Production of Climatic Changes," 

 Jour. Washington Acad. Sci., 3, 1913: 365-71. The complete article is (b) " Volcanic Dust and Other Factors 

 in the Production of Climatic Changes, and Their Possible Relation to Ice Ages," Bull. Mt. Weather Obscrv., 

 Washington, 6, Pt. I, 1913, 1-34. 



