JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV JANUARY 19, 1914 No. 2 



METEOROLOGY. — A return to normal atmospheric transpar- 

 ency. H. H. Kimball. Weather Bureau. 



Attention has already been invited in this Journal, 3, 269, to 

 the diminution in atmospheric transparency which was quite 

 generally observed during the latter part of 1912, and which was 

 attributed, in part at least, to the presence at high levels of great 

 quantities of dust derived from the eruption of Katmai Volcano 

 in Alaska, in June, 1912. It will doubless be recalled that the 

 first violent eruption occurred on June 6, that the dust cloud was 

 first observed at Madison, Wis., on June 8, at Mount Weather, 

 Va., on June 10, at various points in Europe between June 20 

 and June 27, and at Bassour, Algeria, on June 19. 



The purpose of the present paper is to present evidence de- 

 rived from observations made at the Mount Weather Observa- 

 tory, or under its supervision, relative to the gradual precipita- 

 tion of this dust from the atmosphere. 



Humphreys^ has computed that dust particles of the size that 

 are thrown to the greatest heights by the most violent volcanic 

 eruptions require from one to three years to fall to the under sur- 

 face of the isothermal layer. From this level they will quickly 

 be carried to the surface of the earth by the processes of condensa- 

 tion and precipitation of atmospheric moisture. These compu- 

 tations seem to be in accord with the duration of the various 

 optical phenomena that have been observed to follow volcanic 

 eruptions. 



1 Bull. Mount Weather Obs., 6, 11, 1913. 



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