The meteorologist appears on your local news station with a concerned look on his face. “We are closely monitoring this area of low pressure for development, and will keep you informed as conditions change,” he says. Each year millions of people living in areas susceptible to hurricanes wait anxiously to see if the storm will get stronger or if it will make a turn toward their hometown.
But how does a hurricane come to be and what stages does it pass through as it develops?
Hurricanes are formed from simple thunderstorms, but they can only grow and become stronger with the right conditions in the ocean and the atmosphere. The official hurricane season in the Atlantic is June 1 through November 30, but hurricanes can and do occur outside these dates when conditions are right.
First, the ocean water must be warmer than 80 degrees Fahrenheit, and this temperature must exist throughout the water column down to a depth of about 150 feet. The heat and moisture from this ocean water is the hurricane’s source of energy.
Second, wind shear is an important factor. Wind shear is a change in speed and direction over a short distance. When the wind shear is weak, the storm grows vertically, and heat lost from condensation is released directly above the storm, encouraging development. When the wind shear is strong, the storm is less centered and the heat lost from condensation is spread over a much wider area, discouraging development.
Third, rotation is necessary. Rotation is started when the air within the storm is pulled inward toward the center. In conjunction with the Coriolis effect, the rotation causes the storm to begin a counterclockwise movement (in the Northern Hemisphere).
There are five measured stages of development as the storm system gradually becomes more organized and stronger.
The first stage of storm development is a Tropical Wave. This is a low pressure trough of persisting winds blowing in a westerly direction, lacking a closed circulation, with wind speeds less than 25 mph. Atlantic storms often form off the coast of Africa, leaving them thousands of miles for development as they move westward.
As warm, moist air over the ocean rises in the low pressure area, cold air from above replaces it. This produces the second stage, a Tropical Disturbance, which is a concentrated area of thunderstorms from 100 to 300 miles in diameter. It is accompanied by heavy rain and winds. At this point, the storm is still quite disorganized.
The third stage is a Tropical Depression. This is a more organized area of low pressure with a closed circulation and sustained winds of at least 25 mph.
The fourth stage is a Tropical Storm. At this stage, shower and thunderstorm activity moves over the closed circulation and sustained winds are at least 39 mph. This is the point at which storms are named.
And finally, the fifth stage is a Hurricane. An eye forms in the center of circulation and sustained winds are at least 74 mph. As air approaches the center of the storm, it rises rapidly and condenses into clouds and rain. The heat lost from condensation moves into the atmosphere, resulting in lower surface pressure and strengthening winds. This upward flow focuses the power of the storm in the center. A downdraft of air clears the clouds and creates the eye of the storm.
The growth of a storm from one stage to the next can occur within as little as half a day or take up to three or four days. Atmospheric and oceanic conditions are the major forces behind dissipation or development. Hurricanes can exist for weeks and travel thousands of miles. The longest Atlantic hurricane ever was San Ciriaco, which struck Puerto Rico in August 1899, and its total life cycle lasted 28 days.
Scientists measure the strength of hurricanes by the maximum sustained winds. Storm strength is rated on the Saffir-Simpson scale, which ranges from Category 1 storms with winds of 74-95 mph to Category 5 storms with winds greater than 155 mph. Examples of Category 5 storms are Hurricane Camille in 1969 and Hurricane Andrew in 1992. The categories were developed to help predict the likely damage from storms with different wind speeds, but they are not an absolute indicator. Hurricane Katrina was an incredibly damaging and costly storm and it was rated a Category 3.
The intensity of storms is rated based on the barometric pressure at the center. Although barometric pressure is always fluctuating, normal barometric pressure is considered to be near 1,000 millibars. Most storms have an average barometric pressure near 950 millibars. The lowest reading recorded in the Atlantic, and therefore the most intense storm, was 882 millibars for Hurricane Wilma in 2005.
Once a storm reaches hurricane strength, several factors influence whether it continues to grow or gradually falls apart. The longer it stays over warm tropical water, the best chance it has to strengthen. Any time a hurricane makes landfall, it is likely to lose intensity because it is no longer over warm water. Even after landfall, the huge accumulation of water in the clouds of a storm system can generate heavy rainfall far from the coast, as happened in 2004 with Hurricanes Frances and Jeanne.
If a storm encounters strong wind shear, it can actually be torn apart. In November 2008, Hurricane Paloma devastated Cayman Brac as a Category 4 storm and two days later was nothing more than a remnant low pressure area.
Hurricane movement is influenced by steering currents in the upper atmosphere and can result in odd storm tracks and storms looping back on themselves or making abrupt direction changes. Atlantic hurricanes tend to be driven westward by easterly trade winds in the tropics, which drive the storms toward the Caribbean and the Gulf of Mexico.
As a result of better understanding of the factors controlling storm development, hurricane forecasters continue to improve their ability to predict what storms will do. Sophisticated computer models aid predictions. However, weather conditions change constantly and it is always wise to be properly prepared if you live in a hurricane zone. For current hurricane information and storm tracking, visit the National Hurricane Center website at www.nhc.noaa.gov.