Barometers are one way to measure atmospheric pressure.

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Defining a Hurricane

To understand how a hurricane works, you have to understand the basic principles of atmospheric pressure. The gases that make up Earth's atmosphere are subject to the planet's gravity. In fact, the atmosphere weighs in at a combined 5.5 quadrillion tons (4.99 quadrillion metric tons). The gas molecules at the bottom, or those closest to the Earth's surface where we all live, are compressed by the weight of the air above them.

The air closest to us is also the warmest, as the atmosphere is mostly heated by the land and the sea, not by the sun. To understand this principle, think of a person frying an egg on the sidewalk on a hot, sunny day. The heat absorbed by the pavement actually fries the egg, not the heat coming down from the sun. When air heats up, its molecules move farther apart, making it less dense. This air then rises to higher altitudes where air molecules are less compressed by gravity. When warm, low-pressure air rises, cool, high-pressure air seizes the opportunity to move in underneath it. This movement is called a pressure gradient force.

These are some of the basic forces at work when a low-pressure center forms in the atmosphere -- a center that may turn into what people in the North Atlantic, North Pacific and Caribbean regions call a hurricane. What else is happening? Well, as we know, warm, moist air from the ocean's surface begins to rise rapidly. As it rises, its ­water vap­or condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air, causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. And the cycle continues, drawing more warm, moist air into the developing storm and moving heat from the surface to the atmosphere. This exchange of heat creates a pattern of win­d that circulates around a center, like water going down a drain.

But what about those signature ferocious winds? Converging winds at the surface are colliding and pushing warm, moist air upward. This rising air reinforces the air that's already ascending from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing the same speed at higher altitudes (up to 30,000 feet or 9,000 meters) help to remove the rising hot air from the storm's center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds don't blow at the same speed at all levels -- if wind shears are present -- the storm becomes disorganized and weakens.

­Even higher in the atmosphere (above 30,000 feet or 9,000 meters) high-pressure air over the storm's center also removes heat from the rising air, further driving the air cycle and the hurricane's growth. As high-pressure air is sucked into the low-pressure center of the storm, wind speeds increase. Then you have a hurricane to contend with.