Window On Nature
By Lowell and Kaye Christie, F47246
When the temperature heats up, coastal communities from Texas to North Carolina prepare for the possibility of large tropical storms.
A small storm forms over western Africa. It may turn out to be an average thunderstorm, or it could become a towering hurricane after it has made its westward journey across the Atlantic Ocean. What that small storm eventually becomes will depend upon the prevailing winds and the temperature of water along the way. Hurricane season in the Atlantic runs from June 1 to November 30, a time during which 97 percent of all hurricanes that threaten North America occur.
According to the National Oceanic and Atmospheric Administration (NOAA), the United States is the most severe-weather-prone country on the planet, with a yearly average of more than 10,000 thunderstorms, 2,500 floods, 1,000 tornadoes, and two hurricanes that actually make landfall.
While two hurricanes don’t seem like much, realize that it only takes one major hurricane to produce some of the most extensive damage to both life and property of any natural disaster. Last year, Hurricane Ike made landfall as a Category 2 storm along the coast of Texas. With 100 mile-per-hour winds and a 13-foot storm surge, it was the third most destructive hurricane in U.S. history, behind only Hurricane Andrew in 1992 and Hurricane Katrina in 2005. Although all three of those storms were financially costly, none of them exceeded the death toll of the Galveston, Texas, hurricane of 1900, which killed an estimated 8,000 residents.
During the next few months you’ll hear reports about storms brewing in the Atlantic Ocean, so understanding the terminology used by meteorologists and hurricane experts can be useful. These storms start out as tropical depressions; intensify into tropical storms; and then confusingly take on different names depending upon their location. In some areas, these storms are called cyclones or typhoons. In the Atlantic, they’re called hurricanes.
Wind speed determines the name. A storm with winds below 39 miles per hour is designated a tropical depression. When those winds increase to between 39 and 73 miles per hour, the system is elevated to tropical storm status and is given a name.
The World Meteorological Organization provides the names for tropical storms, using six lists that rotate so that every seventh year, they start over again. The lists are alphabetical, alternating male and female names, but leaving out the letters Q, U, X, Y, and Z. This year, if you hear of a storm named Fred, you’ll know that it is the sixth named storm of the season. After an extremely destructive storm, the name is permanently eliminated from the list. For example, the names Ike (2008), Katrina (2005), and Andrew (1992) have all been removed.
Once the sustained winds exceed 73 miles per hour, the storm is classified as a hurricane (or typhoon). As the hurricane winds gain speed, the classification will progress from Category 1 to Category 5 (sustained speeds greater than 155 miles per hour).
Circumstances causing a tropical storm to morph into a hurricane are still being studied, and more is discovered every year. One key factor is the temperature of the water. For a hurricane to develop, the ocean temperature must be above 80 degrees Fahrenheit down to a depth of approximately 160 feet. The heat disparity between the water and the air creates unstable conditions near the ocean surface, causing warm, moist air to form.
As the rising moist air in the developing system condenses, it releases more heat, making the environment inside the storm warmer than that outside. It’s the rotation of the Earth that causes the entire rising storm system to spin (called the Coriolis effect), and the end result may be a hurricane.
Strong winds in the early stages can prevent a hurricane from forming, but it recently has been discovered that lighter winds containing African desert dust may increase the chances that the tropical depression will grow. At present there is no way to predict which of the several hundred tropical depressions each year will actually become a hurricane.
By the time a storm’s winds reach hurricane speed, an eye often begins to form. The eye is a calm area in the center of the system, often measuring 20 to 40 miles across. Circling the central area is the hurricane eyewall, a series of strong thunderstorms that can stretch as much as 10 miles into the sky, containing the highest wind speeds, the tallest clouds, and the heaviest precipitation. The radius of a hurricane, measured from the center of the eye to its outside edge, can extend as much as 500 miles.
Even a small hurricane is extremely powerful. According to NOAA, the energy released by a mature hurricane is the equivalent of a 10-megaton nuclear bomb exploding every 20 minutes. There’s no effective way of diverting a hurricane, so most of the research efforts are concentrated on learning to predict the direction and intensity of these storms in order to give maximum warning to those in a hurricane’s path.
Until the 1960s and the era of weather satellites, the only way people received warning of approaching hurricanes was when a reconnaissance aircraft or ship encountered one, or if the storm hit inhabited land. Now NOAA has four geostationary operational environmental satellites (GOES) in orbit, although only two are being operated by the United States at the current time. (Of the remaining two, one is being used by Japan and the other is being stored in orbit as a replacement in case one of the others fails.) These satellites provide the information for our primary space-based weather monitoring systems.
Closer to earth we have the “Hurricane Hunters.” NOAA’s National Hurricane Center in Florida has 13 aircraft (10 provided by the Air Force Reserve) available for flights around and through tropical storms or hurricanes. The flight crews, including meteorologists, collect data directly or by dropping weather instruments attached to parachutes into the storm. With their highly sophisticated electronic equipment, these aircraft feed data back to the Hurricane Center, where experts run as many as 12 different computer models to predict the path of the storm.
Each year the predictions get more accurate, but those who live in hurricane regions must be ready to take cover or evacuate. It’s a little easier for those of us with motorhomes, since we can be ready to leave in very little time. And for those of us who are traveling in hurricane country for pleasure (many of our favorite places), the early warning systems can help us choose both a departure time and an evacuation route.
Coastal residents learn to live with the possibilities of unexpected wind and water. Most even learn to have a sense of humor about this serious subject. For example, a chain e-mail that periodically makes its way around Florida suggests three steps to take as hurricane season approaches:
STEP 1. Buy enough food and bottled water to last your family for at least three days.
STEP 2. Put these supplies into your car.
STEP 3. Drive to Nebraska and remain there until Thanksgiving!