It's Raining Fish and Spiders (9 page)

Frank Picini

Did You Know Hurricanes Produce Tornadoes? It's True!

While both hurricanes and tornadoes are atmospheric “vortices” (counterclockwise rotation of air), they have little in common. Most tornadoes have diameters of hundreds of feet and are produced from a single convective storm (i.e., a thunderstorm or cumulonimbus). A hurricane, however, has a diameter of hundreds of miles and is comprised of several, sometimes dozens, of convective storms.

Tornadoes are produced in regions of large temperature gradient, meaning areas that are very hot at the surface but very cold high in the atmosphere. Hurricanes are generated in regions of near zero horizontal temperature gradient, meaning the temperature stays close to the same no matter how high you go into the atmosphere. Tornadoes primarily develop over land because the sun's heating of the land surface usually contributes to the development of the thunderstorm that spawns the vortex. In contrast, hurricanes are purely oceanic phenomena—they die out over land due to a loss of their moisture source. Last, hurricanes have a lifetime that is measured in days, while tornadoes typically last for a number of heart-pounding minutes.

However, hurricanes can spawn tornadoes when certain instability and vertical shear (change of winds with height) conditions are met. In hurricanes, most of the thermal instability is found near or below an altitude of 10,000 feet. Because the instability in hurricanes occurs at relatively low altitudes, the storm cells tend to be smaller and shallower than those usually found over land. But because the vertical shear in hurricanes is also very strong at low altitudes, small super cell storms can be produced inside the hurricane. These super cells are more likely to spawn tornadoes than an ordinary thunderstorm.

Which Hurricane Is the Biggest Tornado Producer?

Hurricane Ivan in 2004 caused a multiday outbreak of 127 tornadoes. Most of the tornadoes occurred on September 17 in the mid-Atlantic region of the United States, two days after Ivan's landfall in Alabama. State-by-state tornado counts from Ivan include 40 in Virginia, 25 in Georgia, 22 in Florida, 9 in both Maryland and Pennsylvania, 8 in Alabama, 7 in South Carolina, 4 in North Carolina, and 3 in West Virginia. At least seven people were killed and seventeen injured by these tornadoes.

Who's the Most Damaging Tornado Producer?

One of the tornadoes produced by Hurricane Allen in 1980 did about $50 million worth of damage (in 1980 dollars; about $127 million in 2005 dollars) in the Austin, Texas, area.

More recently, Hurricane Cindy in July 2005 spawned a strong tornado that damaged the Atlanta Motor Speedway and other nearby areas to the tune of some $71.5 million (in 2005 dollars).

Storm Surge—The Biggest Killer

The most dangerous part of the hurricane is the rapid rise in the water's sea level; this is called the
storm surge.
The storm surge is caused by the swirling ocean winds driving the water onto the shore. A hurricane's winds force energy into the ocean, causing the water to pile up. The storm's winds push the water inward, toward the right of the eye. When the hurricane is out at sea, that water spreads out harmlessly, but as the hurricane approaches land, this water becomes trapped between the shore and the hurricane itself. The sea level rises quickly: in just 2 to 3 hours, 20 feet or more of water can push inland. If the storm surge arrives at high tide, the water could rise even higher: up to 5 to 10 feet higher given the area where this occurs.

A storm surge during a hurricane
National Oceanic and Atmospheric Administration/Department of Commerce
NASA

Let's say the normal high tide on the shoreline is 5 feet above mean sea level. Add 20 feet of water from a hurricane and you have a surge of 25 feet. If the storm surge were to come during an astronomical high tide (a greater-than-normal high tide caused by the gravitational effects of the moon), the effects would be even greater. The Galveston Hurricane of 1900 came ashore with a devastating storm surge; between 6,000 and 12,000 lives were lost, making it the deadliest disaster in the history of the United States.

Some of the factors that determine the height of the surge are the speed, intensity, and size of the wind field of the hurricane; the angle at which the hurricane approaches the coast; and the physical characteristics of the coastline. The worst effects of storm surge occur where the ocean floor slopes gradually to the shore.

Frank Picini

Because water is so much heavier than air, the storm surge causes even more damage than the hurricane's fierce winds. Since coastlines today are so densely populated, with too many homes built too close to the ocean, even the smallest storm surges can cause very serious property loss and loss of life.

Stages of Hurricane Development

Tropical Disturbance

Often called the birth of a hurricane, tropical disturbances are areas of showers and thunderstorms with a little wind circulation around an area of low pressure. They may have some towering cumulonimbus clouds and might last for more than 24 hours. In this picture, you can see a tropical disturbance that looks like a blob of puffy clouds. Beneath those clouds are a number of thunderstorms. A circular wind pattern is starting to take place. When meteorologists see a satellite picture like this—either a still or an animated loop—they know that it's likely that the disturbance will grow stronger.

Tropical Depression

A tropical depression is the next stage of a growing tropical system. In this picture, the low-pressure area, or “depression,” is now surrounded by winds that have begun to blow in a circular pattern. At this point, the maximum wind speed will be 38 mph. The system will continue to draw in more warm, moist air, which will build more thunderstorms and feed existing ones. Meteorologists will plot the depression's path and watch it for signs that it is continuing to grow larger and stronger.

Tropical Storm