JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. Ill, FEBRUARY 19, 1913 No. 4 



METEOROLOGY. — Violent uprushes in cumulus clouds. W. J. 

 Humphreys, Weather Bureau. 



Every observer of clouds is familiar with the peculiar boihng 

 and tumbling of large cumuli, their formation of new heads, and 

 the other evidences they give of rapid motions and confused turbu- 

 lence. And indeed the violence is much worse than appearances 

 would indicate, if we are to believe the emphatic statements of 

 the few balloonists who have survived the experience of such an 

 aerial maelstrom. 



As no such violent uprush is ever found in any other part of the 

 atmosphere, one naturally asks what is there peculiar to the large 

 cumulus cloud that produces this localized or exceptional result. 

 And the answer, tho not obvious, should be indicated by a dis- 

 cussion of the processes involved in the formation of the cloud 

 itself. 



Let us assume the temperature of the atmosphere near the sur- 

 face of the earth to be 30°C. and the relative humidity 42 per 

 cent, and let ordinary vertical convection, resulting from this 

 temperature, obtain up to the base of existing cumulus clouds — - 

 substantially the conditions that frequently obtain of summer 

 afternoons. 



Under the assumed conditions the temperature in the rising 

 column decreases approximately at the dry-air adiabatic rate of 

 10°C. per kilometer from the surface of the earth up to the satura- 

 tion level, 1.5 kilometers in the present case. At this level, how- 

 ever, because of the latent heat set free by condensation, there 



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