How do hurricanes form?

In response to a question from blog reader Craig Odem, here is a fairly short explanation of how tropical storms and hurricanes form, strengthen, and weaken. This is not my strongest subject in meteorology, but this is my understanding. Please excuse the artwork in my drawings.

The storms that make it to the Gulf typically begin as tropical waves, or other areas of thunderstorms over the open water of an ocean, sea, or gulf. Sometimes, if there is no wind shear to tilt the thunderstorms or break them up, the latent heat released by the condensing water in the storms causes the air to be lighter, lowering the pressure at the surface under the storms. This low is called a tropical depression. In the pictures below, the view from the top is shown on the left, and the view from the side along the line AB is shown on the right.

Once an area of low pressure forms at the surface, air begins to flow towards the low pressure. This process causes convergence of air near the center of the low, pushing the air upward and causing more storms to form near the low. Also, when the temperature of the ocean is warm (above 80 normally), and the wind begins to blow over it forming waves and causing sea spray, this raises the humidity of the air further, causing the storms to be able to be stronger and release more latent heat, further lowering the pressure. In a tropical storm, this feedback of air converging toward the low and rising, forming storms, and the storms forming and lowering the pressure, helps the storm to intensify further. In addition, the Coriolis force, caused by the rotation of the earth and turning winds to the right in the northern hemisphere, causes the winds to start blowing counterclockwise as they approach the low.

If the water is warm enough, there is little or no wind shear, and air can diverge aloft (carrying the air away from the center of the storm allowing the pressure to drop), the system may become a hurricane. A hurricane often has thunderstorms right around the center, allowing for the lowering of the pressure to be maximized there. The gradient of pressure is very high, so the air is being pushed toward the low. But, the Coriolis force and centrifugal force roughly balance that, keeping the air going almost in a circle. There is little friction over the open ocean, so air rotates around the hurricane, moving only slowly toward the center.

So, for a hurricane to be maintained or to strengthen, it needs: warm ocean water, no wind shear, upper-level divergence, and low surface friction. Once a storm goes over land, friction increases, slowing wind down and allowing it to flow toward the center, filling in the low pressure area and raising the pressure. Also, the source of warm water is gone.

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