Undated (AP) _ The hurricane is a safety valve of nature, a fact of small consolation to those in the path of one of the most powerful forces on Earth.

A hurricane, swirling with awful violence, can move 3,600 million tons of air up to 200 mph, churn up 25-foot waves and dump torrential rains that bring flooding and death. The giant atmospheric disturbances can affect an area of 200,000 square miles.

They can move erratically, changing directions, making loops, slowing up, or temporarily stopping.

According to the National Hurricane Center in Miami, hurricanes are a necessary part of the atmosphere's dynamics, releasing heat built up in the Earth's tropics.

They rotate counter-clockwise around a moving low-pressure center, and have sustained wind speeds of at least 74 mph. If wind speed drops below 74 mph, the hurricane is assigned tropical storm status.

The high wind swirls around the calm of the eye, which averages about 25 miles in diameter but sometimes grows to 50 miles.

The most destructive part of a hurricane is the storm surge, a great dome of water often 50 miles wide that sweeps across the coastline near the area where the hurricane eye makes landfall.

The surge, aided by the hammering effect of breaking waves, acts like a giant bulldozer sweeping everything in its path. In the Bay of Bengal, hurricane storm surges that sweep across low-lying islands have caused some of mankind's largest disasters; one such storm killed up to 1 million people in 1970, according to the Guinness Book of Records.

A 1900 hurricane is responsible for the worst natural disaster in U.S. history. The storm hit Galveston, Texas, on Sept. 8, and 6,000 people died when the storm surge swept across the island.

The deadly circular storms are known as typhoons in the western Pacific Ocean, from the Chinese for ''great wind.'' In the Indian Ocean they are called cyclones, a term used in the United States for a more common, less violent weather system.