TABLE OF CONTENTS A tornado is generated, according to the current interpretation of the observations made, when a warm air bubble formed in contact with the ground for various reasons and kept there by the presence of a thermal inversion layer, finds a way (e.g. because of the discontinuity of the inversion layer) to start its ascension in the atmosphere under the action of the buoyancy force due to the surrounding colder air mass. This rapid ascension of the air column, in the presence of strong translation winds at a certain elevation, is transformed in an upward translation motion and in a rotation around its axis. This phenomenon is similar to the generation of a vortex in the vertical motion of a water mass, which can be easily observed. As in the water vortices, the rotation is generally counter- clockwise in our hemisphere, for the action of the rotation of earth (Coriolis force). The ascension of the warm column is aided by the simultaneous condensation of the steam contained and by the consequent release of the corresponding condensation heat. This process originates at a height of 10 15 km and is characterized by cumulonimbus clouds.
The rotational speed may range between some metres per second to more than 100 m s ?1. The tornado also moves horizontally and its translational speed is usually rather low (up to a few tens of metres per second), which generally allows people who see it arriving to run away in time.
The tornado...
