Lori Watkins, age 13, of Austin, Tex., for her question:
WHAT CAUSES HURRICANES?
A hurricane is a powerful, whirling storm that can measure several hundred miles in diameter. Hurricanes develop over warm ocean water and weaken after moving over land areas. A hurricane is caused by an area of low pressure that forms over oceans in tropical regions.
A hurricane has two main parts: an eye and wall clouds.
The winds swirl around the eye, a calm area in the center of the storm. The eye of a hurricane usually measures about 20 miles in diameter and has few winds or clouds.
Storm clouds called wall clouds surround the eye. The strongest winds and heaviest rain of a hurricane occur within its wall clouds.
Such a hurricane in the North Pacific Ocean is called a typhoon and one in the South Pacific or Indian ocean is called a cyclone.
In the United States, most hurricanes affect areas near the Atlantic Ocean or the Gulf of Mexico. Hurricanes occur in the North Atlantic Ocean from June to November, with most of them striking in September. Atlantic hurricanes form in an area north of the equator from the Cape Verde Islands to the western Caribbean Sea.
In the Northern Hemisphere, hurricane winds blow around the eye 1n a counterclockwise direction. They blow clockwise in the Southern Hemisphere.
Hurricane eyes travel at speeds of 10 to 30 miles per hour. Most hurricanes move westward at first and become larger and stronger as they travel. Then they turn from the equator and pickup speed. Most hurricanes turn east after they reach temperate latitudes and many end as week storm centers over cool oceans.
In many hurricanes, winds in the wall cloud area blow at speeds of 130 to 150 miles per hour. The winds and rain, combined with the force of the sea, produce huge waves. These waves, called a storm surge, rise several feet above normal and cause floods over land.
A storm surge can be especially destructive if it occurs at high tide.
When a hurricane moves over land, strong winds and heavy rain hit the area for several hours. As the eye reaches the area, the rain stops and the air becomes calm. Less than an hour later, the eye passes and the rain and wind return.
The hurricane weakens as it moves over land because it needs the warm sea to supply energy by evaporation. Friction between the land and wind also helps use up the storm's energy. Heavy rain continues even after the winds decrease.
The National Hurricane Reserach Laboratory often conducts cloud seeding experiments during hurricanes. In these experiments, scientists try to slow the winds of the hurricane by producing rain and changing the air pressure and temperature in the storm.
During the hurricane season in the United States, meteorologists of the National Weather Service keep a close watch on the Atlantic Ocean and Caribbean Sea. They use extensive data to forecast where a hurricane will hit and how strong it will be.