Why do hurricanes spin?

Hurricane Sandy is probably the biggest piece of weather news we've had all year (at least that people paid much attention to).  It is a massive storm that now (10-30-2012) has affected millions of people and caused billions of dollars worth of damage.  It's so big that its weather effects are being felt as far inland as Ohio and Indiana.

Hurricane Sandy, 10-28-2012 

(Image courtesy of NASA Observatory Earth)

This is NASA satellite photo (above) shows some of Hurricane Sandy, and the video below shows how the entire storm spins as air spirals toward the eye of the storm.



If you've ever wondered why hurricanes move like this, then this is your lucky day.  I'll do my best to explain why this happens.

First of all, a hurricane grows out of a tropical depression (tropical low pressure system).  A tropical depression is a weather system where sea surface temperatures are high and the air is loaded with moisture due to sea surface evaporation.  The resulting warm, moist air is extremely unstable and less dense than the air around it, so it rises into the upper atmosphere (troposphere, actually - the layer of the atmosphere right next to the Earth's surface, 3-10 miles thick).

As warm, moist air continues to rise a low pressure region forms.  This means that as air moves into the upper atmosphere it has to be replaced by air from neighboring air masses.  You can imagine a low pressure system to act like a valley or depression that neighboring air flows into.

The top image above shows a side view of a low pressure system (at least one way to imagine it).  Warm moist air rises, and that air is replaced by air from surrounding areas.  The larger and stronger a low pressure system is, the farther away it can pull air in.

The lower image shows a top view of a low pressure cell.  Imagine air in the center of the low pressure area moving up toward you and air from nearby areas flowing toward the low pressure area to replace the air that rose and moved into the upper troposphere.  Well, those arrows show what air would do if the Earth didn't spin.

Because the Earth rotates and moving air is not physically attached the surface, the Earth rotates under moving air.  Resulting physical effects, collectively called the Coriolis Effect, causes the path of air or water currents to deflect to the right in the northern hemisphere, and to the left in the southern hemisphere.

The upper image shows Coriolis Effect on air moving toward the low pressure region.  Coriolis Effect deflects the moving air to the right as it moves, in this case approaching the center of the low pressure cell.  The blue arrows in the upper figure shows how air would move if the Earth did not rotate, but the peach colored arrows show the movement of air under the influence of the Coriolis Effect.

The map below shows that most of the air moving toward the storm center ends up moving more or less parallel to the eye of the storm.  This is why there is usually little air movement at all in the eye, except upward.  This deflection to the right occurs at all distances from the eye.  The stronger the winds are, and the farther they blow, and the larger the Coriolis Effect.  This ends up making an entire hurricane spin in a counterclockwise direction (in the northern hemisphere - it's opposite in the southern hemisphere).

(Image courtesy of NASA)

The low pressure cell at the center of a hurricane is extremely powerful and pulls air in from hundreds of miles away.  Wind blowing over these long distances toward the strong low pressure cell at the eye of a hurricane deflects significantly and create a significant spiraling wind pattern.

That's it.