How a Hurricane Forms
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Source: NASA Observatorium
You never hear about hurricanes hitting Alaska. That's because hurricanes develop in warm, tropical regions where the water is at least 80 degrees Fahrenheit (27 degrees Celsius). The storms also require moist air and converging equatorial winds. Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, tropical ocean waters.
A hurricane's low-pressure center of relative calm is called the eye. The area surrounding the eye is called the eye wall, where the storm's most violent winds occur. The bands of thunderstorms that circulate outward from the eye are called rain bands. These storms play a key role in the evaporation/condensation cycle that feeds the hurricane.
The rotation of a hurricane is a product of the Coriolis force, a natural phenomenon that causes fluids and free-moving objects to veer to the right of their destination in the Northern Hemisphere and to the left in the Southern Hemisphere. Imagine flying a small plane directly south. While you're moving southward, the planet is rotating. If you plotted a flight from the North Pole to the equator on a map, the path will appear to curve to the right.
So in the Northern Hemisphere, winds deflect to the right. In the Southern Hemisphere, they deflect to the left. This wind deflection gets storms spinning. As a result, hurricanes in the Northern Hemisphere rotate counterclockwise and clockwise in the Southern Hemisphere. The force also affects the actual path of the hurricane, bending them to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) if you're south of the equator. If you can't remember, just move within five degrees of the equator; the Coriolis force is too weak there to help form hurricanes.
Hurricanes often begin their lives as clusters of clouds and thunderstorms called tropical disturbances. These low-pressure areas feature weak pressure gradients and little or no rotation. Most of these disturbances die out, but a few persevere down the path to hurricane status. In these cases, the thunderstorms in the disturbance release latent heat, which warms areas in the disturbance. This causes the air density inside the disturbance to lower, dropping the surface pressure. Wind speeds increase as cooler air rushes underneath the rising warm air. As this wind is subject to the Coriolis force, the disturbance begins to rotate. The incoming winds bring in more moisture, which condenses to form more cloud activity and releases latent heat in the process.
On the next page, we'll explore the brief, violent life of a hurricane.