Hurricanes are among the strongest storms on earth, taking lives and causing an average of $3 billion in damage every time they make landfall. They cause massive floods, rip homes from their foundations and scatter boats on dry land like a child's forgotten toys. But what exactly is a hurricane and what causes them?

The simple answer is that hurricanes are tropical storms prodded to fury by conflicting trade winds and other factors in a lifecycle that plays out over the Atlantic Ocean between June 1 and November 30 of each year. This lifecycle sometimes ends with the storm plowing ashore on the eastern and gulf coasts of the United States, Mexico, Central America and the Caribbean.

It is that dangerous time -- when a hurricane makes landfall -- that strong winds, heavy rains and a powerful storm surge can devastate coastal areas. Hurricane Katrina, the costliest natural disaster in U.S. history, killed 1,836 people and did more than $81 billion in damage in 2005.

The only defense against a hurricane is preparation, and our ability to track them and predict landfall continues to grow with the help of the National Weather Service and the National Hurricane Center, agencies that have had success in warning of storms well in advance of their arrival. But to understand how these storms are tracked, you have to understand how they develop.

HURRICANE LIFECYCLE

At its beginning, a would-be hurricane may not be much as it drifts out over the Atlantic from the Sahara. For a while, it is just a minor weather system: Warm air gathering atop a body of warm water, cumulus clouds developing into thunderheads, nothing to get too concerned about. It's merely a trough of low pressure drifting west.

But the chance of that weather system becoming a threat increases exponentially during hurricane season, a time of year when the southeast trade winds migrate north of the equator and smack into the northeast trades. Blame it on the battle between these two trade winds: the southeasterly winds pushing one way and the north easterlies pushing back. It's not long before a storm system has formed and begins to rotate counterclockwise.

What follows is a chain reaction: moisture is condensed from the clouds, heat is released and the air begins to rise. Storms require heat and moisture to grow. It is there, in the tropical ocean, five to 20 degrees north of the equator, that weather systems find plenty of both.

As the storm grows, air flows into the rotating weather system faster and faster, and the system draws air and strength from farther and farther away. Measuring 20 to 40 miles in diameter, the storm will develop an "eye" which is marked by relative calm (hence the saying, "the eye of the storm"), and meteorologists will announce the presence of a Tropical Depression.

In time, spiral bands of clouds will develop, gradually becoming stronger as they flow. Soon the storm will grow as large as 100 miles in diameter with winds reaching 33 knots, and it will be upgraded to a Tropical Storm. The storm will keep drifting, sometimes on an erratic track, but always westward. Its speed may be leisurely, perhaps only 15 knots, but warm water will continue to feed it.

If its coverage area becomes larger and the winds feeding it reach 35 knots, it will be given a formal name. When wind speeds reach 64 knots, the storm will be officially designated as a hurricane, which can be rated on the Saffir-Simpson Scale (Category 1-5) according to wind speed and other factors.

HURRICANE PATH

Hurricanes travel around the edge of the Azores-Bermuda High, a high-pressure system that lies between the Azores, a chain of islands off Portugal, and the Bermuda Islands in the western Atlantic.

In effect that high creates a bubble of pressure that sits in the middle of the Atlantic. The hurricane can't move into the bubble, so it goes around it. Some hurricanes stay largely at sea, sliding up between the high pressure zone and the east coast. Others swing wide and plow right into North and Central America.

Hurricane Katrina did more than $81 million in damage and killed 1,836 people.

The longer a hurricane dawdles in warm tropical waters, the bigger it gets. That means the winds will continue to increase and create big waves. These waves tend to move faster than the winds; as they move away from the storm area, they smooth out and stretch into long swells. These swells can cover 1,000 miles in a day, whereas a hurricane usually travels only about 330 miles. That means shore-dwellers often get a two-day warning that something ominous is headed in.

Because winds spiral counterclockwise, and a hurricane moves forward, the storms right side packs its most powerful punch. This is known as the hurricane's "dangerous semicircle." Meanwhile, winds on the left side of the storm are reduced by the speed of its advance, producing the so-called "navigable semicircle." That term sounds benign, but it was Katrina's navigable semicircle that plowed through New Orleans, permanently disfiguring one of the nation's most historic cities.

Hurricanes are born on the east side of the North Atlantic and eventually die on the west side. If a hurricane stays out to sea, cold water from the north will eventually kill it. Similarly, as a hurricane crosses onto land and moves inland, it loses its source of energy and begins to decay. In time the storm's winds slowly decrease. But even rainfall from a dying hurricane can be prodigious–enough to wash the topsoil off mountains and turn them into mud slides. While people living on the coast suffer the most wind, it's those living inland who tend to face danger from floods.

Similar storms in the Pacific are known as typhoons.