Tornadoes(what)

Excerpt from a Le Monde 2011 article
translation: doxa-louise

How do tornadoes form?
Tornadoes are swirling wind tunnels of extreme violence, which form

at the base of a storm cloud (cumulonimbus) and sometimes reaching the

ground. They are the result of a meeting of the cold air of the high troposphere

and warm and humid air from lower atmospheric strata. The two air masses do not

mix but roll one around the other, thus creating strong winds. In cases of variations

in the strength and direction of winds, at different altitudes - what are called

wind shearings - , the storm disturbances experience strong rotation, creating a whirl.


What is their intensity?
In general, tornadoes last ten minutes, move at the speed of the storm, between 50

and 100 kms/hour. The diameter of the vortex vaccilates between some tens and some

hundreds of meters. But in extreme cases, winds can go up to 500kms/hour and the tuba

several kilometers in length. Tornadoes can then last for hours. They are measured on the

Fujita scale, established in 1971, as a function of the damage inflicted on human constructs

more than the strength of winds. Tornadoes are thus classified as F0, which induces light

damages, up to F5, where projected débris are extremely dangerous and where winds can

rise to 500 kms/hour, with dramatic consequences.


Why do these often occur in the United States?
The United States experience every year some 1 000 tornadoes. These are formed because of

the meeting in Spring of cold and dry currents from the North Pole at some ten kilometers in

altitude, and warm and moist currents from the Gulf of Mexico, at much lower altitudes.

The presence of the Rocky Mountains, in the west, produces as well an ondulation of the Jet

Stream - a very rapid current at ten kilometers of altitude - favorable to storm disturbances

on the Great Plains.