Tornado Spinning

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