R-CWM 



- 2 - 



5-16-30 



the difference. To the untrained eye; the sky just looked a little whiter. 



As he exiDlains it, what happens is thatj these tremendous volcanic ex- 

 plosions hurl very fine ashes fifty miles or more above the earth. Far above 

 the highest clouds and above the sconns these billions of very light glassy 

 particles of microscopic size settle very, very slowlyi They hang as a fog in 

 the upper air. This excess of very fine particles in the uoper air increases 

 the intensity of the sunset colors. 



Too rauch dust in the lower air, however, cuts do',vn the briglitness of 

 sunsets, and even blots th^cm out. The smoke and dust particles down here near 

 earth don't stay up as long as the very light volcanic ash particles of the 

 upper air. Once about every ten years we get that pronounced fogging of the 

 up-per a3 r. Every day we get those bigger particles thrown from our chimneys 

 into the lower air. That dust and anoke nearer earth here also cuts off from 

 UP Fome of che sun's rays, including the invisible ultra-violet rays doctors te. 

 us are so ir.portant for our health. 



Dr. Kimball and his division of the U. S. Weather Bureau are now measur- 

 ing the sun's rays of all kinds as they filter through our city smoke. From 

 measurements already made, it is clear that this smoke screen blocks out much 

 of sunlight, and other rays, especially the ultra-violet. In the winter time, 

 Chicago gets little more than lialf the amount of radiation received at Madison, 

 Wisconsin, a short distance to the north. Even in the suriiner months, fifteen 

 per cent less radiation filters through the Chicago air than reaches the earth 

 at Madison. 



All this time we've been talking about all the sun's rays, those you 

 can see and those you can't. Most of us talking about the sun's rays think of 

 those you can see, those we call "li^t". Dr. Kimball and his men have made 

 important studies in that narrower range of wave-lengths, 



"What size windows should we have? How many windows does a school-room 

 need? Just ..ow much light can we get in north windows, in east or west or 

 south windows from the sky alone? Archi oects and illuminating enginoc-j "xOw 

 have tables to readily determine those things. They got them from the calcu- 

 lations made from the sunlight and skylight measurements of the U. S. Weather 

 Bureau, That is just another one of those services credited to our weather 

 experts. 



We get a great deal of light from an average cloudy sky. But we get 

 nearly four t.ij^ies as much light from the sun and sky combined when there are 

 no clouds, and when it rains, the li^ht is only about half as strong as it is 

 frqn a clo^^dy sky from which no rain is falling. 



Moisture in the air even on a clear day seems to have a decided effect 

 on the amount of ultra-violet radiation which reaches us. But the ultra-violet 

 rays in sunlight increase fast the higher we get above sea-level. The '.Tater- 

 va^or in the lower air layers seems to screen out much of that health-promoting 

 part of the s^onshine. It is on high mountains that we get the greatest effect 

 from the ^XLtra- violet, Dr, Kimball's measurements show. 



