Weather: The Triple Threat

The Triple Threat

Hurricanes are responsible for delivering, in a huge way, the three Ws: wind, water, and waves. Wind is often thought to be the real destructive element within a hurricane, even though the vast majority of damage and destruction will be related to water and waves. But the wind is not without its problems.

As the wind increases to hurricane intensity, the system seems to cross an important threshold. Damage becomes widespread. Because the force of the wind increases with the square of the velocity, a doubling of wind will cause a four-fold increase in the damage potential. But in addition to the normal hurricane winds, these storms frequently spawn tornadoes. Even after a storm moves inland and weakens, with winds dropping off, tornadoes can continue to spin for hours or even days. These tornadoes are not the huge, Midwest-variety, but they are strong enough to rip roofs off and uproot trees. Typically the tornadoes form in the right-front quadrant of a hurricane. If the storm is heading northward, the tornadoes form in the northeast sector. In general, that sector contains the strongest wind.

Deadly right quadrant.

When a hurricane is moving northward, that northward trajectory combines with the northward flowing air on the storm's eastern side to enhance the strength of the wind. On the western side, the northward trajectory opposes the southward flowing air, so the net wind is lower. The following figure shows how the storm path affects the wind. In New England, one of the great concerns is for a hurricane making landfall in western portions of the region and moving onshore at a rapid forward speed. That combination is exactly what happened during the great hurricane of 1938, when 600 people were killed.

Torrential rains accompany hurricanes and, like the wind, can persist long after the storm has moved inland. Some of the most destructive storms have delivered flash flooding. In the United States, inland flooding has been responsible for more than 70 percent of the lives lost from hurricanes and tropical storms. For example, Tropical Storm Claudette in 1979 brought 45 inches of rain to Alvin, Texas. Damage ran in the hundreds of millions of dollars. Hurricanes Connie and Diane teamed up to deliver about 20 inches of rain to New England in August 1955. The flooding created one of the greatest natural disasters known to that region with damage at 4.2 billion dollars, adjusted to 1990s dollars. Two hundred people were killed. Then there was Hurricane Agnes in 1972, which moved onshore and lingered for days as a weakened but soaking-wet tropical storm. In terms of adjusted dollars, damage came to 6.4 billion dollars. The death toll reached 122. In June 2001, Tropical Storm Allison weakened to a depression when it moved into the Houston area, but it generated 30 to 40 inches of rain. More than 40 deaths and 5.0 billion dollars of damage occurred as the storm slowly moved from Texas to Florida, and northeastward into Virginia. In 1998, Hurricane Mitch became the most deadly storm to have struck the Atlantic basin since 1780. Its torrential rains set off mudslides in Nicaragua, Honduras, and El Salvador. At least 9,000 people were killed. The only other hurricane that claimed more lives in the Atlantic basin was a storm that struck from Barbados to Martinique in October 1780. That storm claimed 22,000 lives.

The massive rains associated with these storms occur because the storms have a huge mass of moisture and are quite unstable. If the terrain becomes rough with lots of hills, the air will be pushed upward, and that will put the big squeeze on the atmosphere. Most inland areas fear the rains from hurricanes far more than the wind.

There are also the waves and surges of water that occur at the shore. The strong winds pile up the water along coastal areas that look into the wind. This creates a surge, which can measure 10 to 20 feet above the normal tide level. Additionally, waves of 20 or even 30 feet will combine with the surge. Also the low pressure in the center of the storm will draw the water to even greater heights, as though it were drawn through a straw. When the storm arrives during a normal high tide, the situation is even worse. The Galveston Hurricane of 1900 took the lives of 6,000 people, most by the surging tide. Much of Hurricane Camille's damage in 1969 was related to a 25-foot storm tide when the storm moved into coastal Mississippi.

Taming the Tiger

For decades, seeding experiments were performed on hurricanes with the hope of taming their fury. The project was called Stormfury and was a joint effort of the National Weather Service and the U.S. Navy. The concept was to diffuse the strong wind from the center of the hurricane. Silver iodide was released in different locations within the hurricane circulation, allowing for forced condensation and a forced heat release in zones other than the eye wall. The hope was for this procedure to deliver a less defined, single center of concentrated heat release and lowered pressure. The pressure field would become less organized, and the wind field would become more confused. At least that was the theory.

In 1963, when Hurricane Beulah was seeded, surface pressure in the eye began to rise, and the highest winds were no longer concentrated at the center. In 1969, Hurricane Debbie diminished in intensity after seeding, but the question always remained whether these changes would have taken place without the seeding. Also the number of hurricanes that have been available for seeding have been relatively few. By 1990, because of mixed results, Project Stormfury was abandoned.

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.