As a result of intensive study by the U.S. Weather Bureau the following meteorological conditions have been known to exist in association with the formation of a tornado:-
(1) Near the surface of the earth, there exists a layer of warm and moist air. An inversion layer marks the upper boundary of the tropical maritime air adjoining the land. In the thick layer of dry air aloft, the lapse rate is very steep.
(2) The column of moisture rises to great heights.
(3) There exists a narrow band of strong winds with velocities up to 64 km per hour between 3000 to 6000 meters.
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(4) In the moist layer of air at the surface, moisture must be concentrated in a definite narrow belt.
(5) The axis of the aforesaid band of strong winds which projects downward must intersect the axis of the wedge of moist air.
(6) The level of free convection in all the tornadoes is rather low. The entire length of the air column must be characterized by convective instability.
(7) There must exist some mechanism whereby the air column is given considerable uplift.