THE LITERARY 



OF THE LINN^AN ASSOCIATION OF PENNSYLVANIA COLLEGE. 



Vol. I. APRIL, 1845. No. 6. 



METEOROLOGY, NO. VI. 



INDIAN SUMMER. 

 BY PROF. JACOBS, OF PENNSYLVANIA. COLLEGE. 



3. Whence the haze or smoke., which so frequently destroys the per- 

 fect transparency of the atmosphere, and which is so characteristic of the 

 *■'■ Indian Summer P"* 



In reply, it may perhaps be best to state, first, what it is believed to 

 be, and secondly, under what conditions of the atmosphere it is found 

 to exist. 



That the cause of the obscuration of the atmosphere in the case in 

 question is due to moisture and not to actual smoke, will appear from the 

 consideration of a few facts. First, the morning fogs, especially those 

 of August, when not followed by immediate rain, instead of being dis- 

 solved in a pure clear sky, during the day, by the increasing solar heat, 

 aie mostly found to produce an obscurity like that of a dense smoke. 

 Secondly, both the cirrus and dense nimbus cloud, after having been 

 wafted overhead by upper currents, have frequently been found to be rap- 

 idly dissipated, rendering the sky obscured as by a dense smoke, instead 

 of leaving it nearly transparent as it was before. Thirdly, such states of 

 atmosphere are mostly broken up by copious rains, which are succeed- 

 ed by a clear sky. So constantly is this the result, during the summer 

 months, that a smoky sky is generally considered as a sure indication 

 of approaching rain. And fourthly, the quantity of moisture in the at- 

 mosphere, at such times, as indicated by the dew-point, is greater than 

 the average due to the prevailing temperature. These are some of the 

 facts which may be adduced to prove, that the obscurity of the sky, in 

 the cases cited, is due, not to real smoke, but to vapor; or in other 

 words, that it has no relation to the existence of extensive lires, as is 

 generally believed. The existence of extensive fires at such times is 

 purely accidental, being favored by the greater inflammability of combus- 

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