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Date: Around 1993

Why do tornadoes turn in a counter-clockwise direction in the northern hemisphere and clockwise in the southern hemisphere? Is it magnetic?

Not to complicate things, but I am reasonably sure that tornadoes in the Northern Hemisphere have been observed to spin in both directions, although one is more prevalent...

Mark Fernau

At least the great majority of tornadoes rotate counterclockwise (as do all low-pressure systems) in the northern hemisphere, and clockwise in the southern hemisphere, for the reason (coriolis force) given by Eric Peterson in response #1. But occasionally, it would seem, northern hemisphere tornadoes do rotate clockwise: S. Flora's book "Tornadoes of the United States" cites an 1890 article in the American Meteorological Journal. Its author, a J.P. Finley, states that, of 550 American tornadoes he studied, 29 were deemed to have rotated clockwise. I have not been able to find any "modern" study of this question. But I believe it could be true. The region of swirling air that contracts to become the tornado is not itself large enough in extent to have its rotation dictated by the coriolis force; rather, it "inherits" this tendency from the great masses of air whose movement sets the stage for the storms and any associated tornadoes. If the study cited is correct and representative, on occasion the direction of rotation is set by some other factor, perhaps the topography in the area where the tornado forms, for example.

Ronald Winther

The previous responses were correct in that the coriolis force is the reason air circulates in a clockwise direction around high pressure and counterclockwise around low pressure in the northern hemisphere. The coriolis force can explain the rotation in large scale high and low pressure areas including hurricanes. However, the rotation of a tornado is much more complicated. Tornadoes in the northern hemisphere can rotate in either direction but counterclockwise rotation is much more common. The rotation is produced by wind shears and pressure forces in and near the parent thunderstorm. Thunderstorms form when warm moist air rises rapidly upward. This upward current of air within a thunderstorm is referred to as an updraft. If sufficient vertical wind shear exists, this updraft will rotate. Vertical wind shear is a change in wind speed and/or direction from the ground up through the atmosphere. If the wind speed increases rapidly with height and/or if the wind direction turns clockwise with height, air being drawn in toward the thunderstorm updraft will develop a spin about the horizontal axis. Think of an imaginary paddlewheel floating in the air. If winds blowing across the top of the paddlewheel are stronger than the winds at the bottom of the paddlewheel, it will spin. As the air rises into the updraft, the spin about the horizontal axis becomes a spin about the vertical axis. Imagine if you take a rope and roll it along the ground (it is spinning about the horizontal). Now pick up the middle of the rope but keep rolling it. Now the two dangling ends are spinning about the vertical. But one end is turning clockwise and the other counterclockwise. Strong winds blowing through the storm produce pressure forces within the storm that enhance or suppress the updraft. Most tornadoes in the United States occur in the warm humid air mass ahead of an approaching low pressure area. Because of the coriolis force, winds usually turn clockwise with height. This wind profile enhances the counterclockwise rotating updraft and suppresses the clockwise portion of the updraft. That is why most tornadoes turn counterclockwise. However if winds are from a nearly uniform direction throughout the depth of the storm, both circulations can be maintained. In this case the storm can split producing both a clockwise and anti-clockwise rotating tornadoes. This has been documented with radar.

Jim Allsopp

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