Hurricane Sandy, 10-28-2012
(Image courtesy of NASA Observatory Earth)
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 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 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.