Posted by: pyeager | May 5, 2009

Wind Shear and Tornadoes

Tornadoes are the most intense storms on the planet, and they’re never discussed without at least some mention of the term wind shear. Many of us sitting at home, though, have no idea what wind shear is, or if we do, how it affects tornado production.

What is Wind Shear
Wind shear, although it might sound complex, is a simple concept. Wind shear is merely the change in wind with height, in terms of wind direction and speed. I think that we all understand that the wind is generally stronger in the atmosphere over our heads than it is here on the ground, and if we think of the atmosphere in terms of the three dimensions that it has, it should not be surprising that the wind above us might also be blowing from a different direction than the wind at the ground. When that happens–the wind speed and direction vary with height–wind shear is occurring.

Wind Shear and Supercell Thunderstorms
This wind shear is an important part of the process in the development of a supercell thunderstorm, from which the vast majority of strong tornadoes form.

All thunderstorms are produced by a powerful updraft–a surge of air that rises from the ground into the upper levels of the atmosphere, and when this updraft forms in an area where wind shear is present, the updraft is influence by this speed and different direction of the wind above, pushing the column of air in the updraft into a more vertical alignment.

Rain’s Influence on Tornado Production
Needless to say, thunderstorms typically produce very heavy rain, and rain-cooled air is much heavier than the warm air of the updraft, so the rain-cooled air, produces a compensating downdraft (what comes up, must come down). This downdraft pushes the part of the rotating air that was forced in its direction by the stronger wind aloft downward, and the result is a horizontal column of rotating air.

NOAA image showing supercell thunderstorm production

NOAA image showing supercell thunderstorm production

That’s Not a Tornado!
I know what you’re thinking that you’ve seen enough TLC or Discovery Channel shows to know that a horizontal column of air is NOT a tornado; you need a vertical column of air.

This Can Be a Tornado

You’re right, but remember the updraft that is driving the thunderstorm is still working, and it’s able to pull the horizontal, spinning column of air into the thunderstorm, resulting in a vertical column of spinning air.tornado_2

(NOAA image showing vertical column of air in a supercell thunderstorm)

The result is a rotating thunderstorm capable of producing a tornado, and it would not be possible without wind shear.

tornado_3

(NOAA image showing tornado formation in supercell thunderstorm)

–Paul Yeager

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