| Meteorological Sketches. 57 
or water-spout, or if, owing to any inequality of surface or other ac- 
cident, a depression is made upon the lower stratum, so as to enable 
the colder air to descend at this point, then an immediate gyration or 
convolution will take place in the two masses at this point, the warm 
air rising as it becomes displaced, and a copious condensation will 
immediately follow. It is movements of this character which pro- 
duce the dense and convoluted appearance known as a thunder- 
cloud, and the thunder and lightning, rain, and perhaps hail, follow 
as necessary results. 
The precipitation of the colder stratum thus commenced, is reg- 
larly continued and enlarged till an equilibrium is produced, and the 
thunder storm thus engendered, assumes, of course, the direction of ~ 
the upper current to which it is appended, and which, in the tem- 
perate latitudes, is commonly from the western quarter. The warm 
surface air which is thus displaced at the commencement of the pro- 
cess, rises immediately in front of the colder intruding mass, and by 
the gyratory action thus commenced, becomes convolved in de- 
tached masses or layers with the colder surrounding air, and by the 
reduction of temperature thus produced, furnishes the large supply 
of aqueous vapor which is first condensed in the thunder cloud, and 
then precipitated in a heavy fall of rain; and the electric phenomena 
which are induced by this sudden contact or intermingling of masses 
of air of different temperatures and hygrometric conditions, become 
highly vivid, and too often destructive. The active gyration which 
is commonly produced within the body of the thunder storm or gust, 
is in the direction of the advance of the storm and of the rising warm 
_ air which is forced upward, or in the direction of forward and up- 
_ ward at the lower front of the storm.* 
In consequence of this gyratory action, a storm which advances at 
the rate of fifteen or twenty miles an hour, is often known to ex- 
hibit a velocity of wind during the period of its greatest violence, 
of sixty or eighty miles an hour. If the axis of this gyration in a 
thunder storm assumes, from any cause, a vertical position, we then 
have a perfect whirlwind or tornado, which, if it be so situated as not 
to reach the earth by its direct action, will exhibit to us the phe- 
nomena of a heavy thunder storm accompanied by rumbling sounds 
and concussions, and a fall of hail in or near some portion of its path. 
But if the regular action of the whirlwind should reach the earth, 
* See also Prof. Mitchell on thunder storms: Am. Jour. of Science, Vol; XTX. 
p- 278—282. 
Vol. XX XIII.—No. 1. 8 
