KIMBALL: NORMAL ATMOSPHERIC TRANSPARENCY 25 



The twilight colors are intensified by the presence of fine dust in 

 the atmosphere, as was markedly the case after the eruption of 

 Krakatoa Volcano in 1883. In another paper ^ I have presented 

 evidence that during the fall of 1912 the red colors, especially, 

 were unusually brilliant, and perhaps, also, of unusual duration. 

 At the Mount Weather Observatory the brilliancy of the twi- 

 light colors has been the occasion of frequent remark during the 

 past few months, and especially during September, October, and 

 November. On some evenings the glow in the west has been 

 distinguishable for nearly an hour and a half after sunset. 



During the fall of 1912 a noticeable feature of twihghts was 

 the streaked appearance of the sky, as tho the dust or haze was 

 arranged in horizontal layers. This has not been the case during 

 the fall of 1913. 



Notes relative to twilight colors made by Weather Bureau Ob- 

 servers at the various stations thruout the United States are 

 rather inconclusive as to the intensity of these colors in 1913 as 

 compared with their intensity in 1912. It seems evident, how- 

 ever, that while the maximum of intensity culminated in the 

 month of November, in 1912, in 1913 the maximum was reached 

 as early as September. 



Summary. Pyrheliometric and polarimetric observations unite 

 in indicating a gradual increase in atmospheric transparency since 

 the marked minimum in August, 1912. This increase is prob- 

 ably due to the precipitation from the atmosphere of the dust 

 that was introduced into high levels by the eruption of Katmai 

 Volcano in June, 1912. 



At the same time, the solar and the anti-solar distances of the 

 neutral points of Babinet and Arago respectively, indicate that 

 there may still be traces of this dust in the upper atmosphere. 



The duration of the Katmai dust cloud appears to have been 

 less than the duration (2 years) of the dust cloud "that followed 

 the eruptions of 1902-03, and markedly less than the duration 

 (3 years) of the dust cloud that followed the eruption of Kra- 

 kotoa in 1883. 



6 Monthly Weather Review, 41, 153-159, 1913. 



