DO \OLCAXIC KXPLOSIOXS AFFECT OUR CLIMATE? 185 



summer of 1912 produced a very marked 

 decrease in the direct solar radiation in 

 all parts of the spectrum,* and reached 

 nearly 20 per cent at high sun for the 

 total heat. 



There was, however, some compensa- 

 tion in the increased brightness of the 

 sky for this apparently very great loss in 

 1912. In order to understand this, think 

 for a moment what happens to the sun- 

 rays before they reach the earth's sur- 

 face. // ice could i:^o outside the earth's 

 atmosphere — to the moon, for instance — 

 the sky would look dark as it does at 

 night, studded zvith stars, except zchen ive 

 looked directly toward the brilliant sun. 

 zchich xi'ould shine wholly undimnied. It 

 is the earth's atmosphere which changes 

 all this, for in the passage of a sunbeam 

 through it, even on a cloudless day, two 

 kinds of losses occur — one imperceptible 

 to the eye, the other giving us the sky- 

 light. 



Firstly, some of the invisible rays of 

 the infra-red spectrum are totally ab- 

 sorbed by the water vapor, oxygen, and 

 carbon-dioxide of the earth's atmosphere. 

 and cease to exist as radiation long be- 

 fore the sunbeam reaches the earth's sur- 

 face. Secondly, the molecules of the air 

 and the fine dust suspended in it scatter 

 and diffusely reflect the sun-rays, and 

 make the sky bright, much as the motes 

 of dust in a sun-lit room reveal the path 

 of the sunbeam in it. 



Thus, of the sun-rays scattered in the 

 earth's atmosphere, some reach the ob- 

 server at the earth's surface, coming no 

 longer from the sun directly, but dif- 

 fusely reflected from every part of the 

 sky. The remainder are scattered away 

 into space and lost altogether for the 

 purpose of heating and lighting the 

 earth. 



IIKAT REFLECTED INTO SP.VCE INSTE.\D OF 

 REACHING THE EARTh'S .ATMOSPHERE 



It is this last-mentioned portion which 

 most interests us here, for we wish to 

 inquire how much more heat than is 

 usual was lost to the earth by rcflecti<Mi 

 of the atmosphere to space in 19 12, owing 



* This circumstance must have caused a de- 

 cided increase in the exposures required by 

 pliotoeraphers for solio prints. 



to the dust which came from Katmai 

 volcano. One can easily see that since 

 the light of the sky and the loss by re- 

 flection to space both depend on the j^res- 

 ence of the molecules and the dust of the 

 atmos])here, an increase (;f the dust (at 

 least up to a certain point) must make 

 the sky brighter and the loss to space 

 greater also. 



What, then, do we ordinarily receive 

 from the sun? 



(A) The direct solar beam. 



(B) The skylight. 



What else would we have received if 

 there were no atmosi)liere? 



(C) The rays absorbed by atmospheric 

 vapors. 



(D) The rays reflected away to space 

 from the upper atmosphere. 



The sum of these four quantities 

 should be approximately equal to the 

 heat of the solar beam outside the earth's 

 atmosphere, as, for instance, on the 

 moon. This we may call (E). As we 

 cannot measure (D) directly, we must 

 find it by subtracting A+B-j-C from E. 

 It is of course (D), the loss to space, 

 with which we are principally concerned. 



For we must ask ourselves: Jl'as 

 the earth's loss of heat by reflection of 

 the upper air to space made greater by 

 reason of the haze of IQI3? To answer 

 this we must know the value of the ex- 

 pression (D)HE— (A-|-B+C)}> as it 

 was in 191 2 and as it is ordinarily. 



Measurements of (A), the direct sun- 

 rays, and (C), the water vapor and other 

 absorption, we make every day. and I de- 

 vised and built with my own hands at 

 Bassour two pieces of apparatus for 

 measuring (B). the light of the sky. 

 From observations taken a little before 

 noon on September 5, 6, and 7. 191 2, we 

 found at Bassour the following results, 

 stated in calories per sq. cm. per minute: 



(.\) Heating effect of the direct beam of 



zenith sun 1 .250 



(R) Heating effect of the entire sky 0.245 



(C) Heating effect of the rays absorbed 



l)y water vapor from sun and sky 

 radiation 0. 175 



Total (.\ + B + C) I fi/O 



(R) Heating effect of total radiation 

 outside the earth's atmosphere 

 (from the moon for instance)... 1.05" 



(D) = (E)-[(.^) + (R) + (C)] 0.280 



