182 HH. A. Hazen— Tornadoes. 
air flowing northward. At numerous spots in this region this 
acquires an upward motion, thereby giving rise to local upward 
currents of air which cool rapidly as they rise. The cooling is 
a mechanical result of the expansion of the rising air and very 
soon a temperature is reached low enough to condense clouds 
from the hitherto invisible moisture. With the formation of 
clouds, the tendency to rise increases, so that in fact an upward ~ 
suction is experienced under the clo ud and more air is drawn 
in from all sides to feed this suction.” 
“ Whatever causes a sudden uprush of moist air ota 
to the formation of the cloud or the tornado. Hills or low 
mountains are very effective. But it is equally aR to 
consider the cool dry air that flows from the north toward a 
low center and becomes a west wind as it turns around the low, 
runs into the mass of warm moist air coming from the south, 
and being denser, underruns and lifts up this warm air and is 
_ in many eases more effective than a mountain in starting the 
formation of a cloud and local storm 
and often more than one Boassd feet in ace begins a 
rotary motion, rises at the center and passes a 
It is probable that theories upon the effect peg the got ’s ies fs 
upon the lower atmosphere have been at fault. It 
assumed that “ this heat is developed in certain central ae 
of greatest heat.” It is ordinarily considered also that the heat 
The effect of the sun’s heat may be considered as follows: 
the stratum of air nearest the earth will first be heated an 
