Each year, the ocean waters from the Gulf of Mexico to the eastern Atlantic begin to warm up. By late spring they reach a temperature of 27 degrees C (80 degrees Fahrenheit) to a depth of 125 feet or more below the surface. This is the temperature at which tropical storms can form.
The combination of warm water and warm air causes high levels of evaporation, and the air becomes laden with moisture. Warm air tends to rise and if the air becomes unstable updrafts form, carrying the moist air high into the atmosphere. At high altitude, the air is cold, and the moisture in the air condenses, forming clouds. Soon, a thunderstorm is born. If conditions are right, this will be the start of a tropical depression. Any tropical storm is big and needs plenty of room, so they will only rarely form less than 300 miles (500 kilometers) from a major land mass. In addition, as it develops, any cross winds must be light. The complex spiral structure of a tropical storm is unstable in its early stages, and a strong wind will disrupt it.
In an area with enough thunderstorm activity and unstable atmospheric conditions, a region of lowered barometric pressure is created. Winds from surrounding regions of higher pressure begin to flow into the growing storm, bringing with them still more warm, humid air to feed the process. As these winds start moving inward, they curve in response to the earth’s rotation (the “Coriolis Effect”) and the whole system begins to rotate. A tropical depression has formed. This is the first phase of any cyclonic storm, whether a tropical storm or a hurricane (or, in the Pacific, a cyclone or typhoon).
As the storm grows stronger, it becomes better organized, assuming the distinctive shape of the cyclonic storm, with rain bands spiraling out from the center, an eyewall where the inflowing warm water-laden air is pushed upward to feed the storm, and the calm eye at the center. If the winds of a tropical depression reach a sustained speed of 39 miles per hour, it is classified as a Tropical Storm. This is also the point when meteorologists name a storm.
A tropical storm, once formed, may continue to grow in strength and will become a hurricane if its winds reach a sustained velocity of 754 miles per hour. However, even if it never reaches hurricane strength a tropical storm is not something to take lightly if it makes landfall. Winds of 39 to 74 miles an hour (with gusts at higher speeds) are capable of causing damage. A tropical storm also produces torrential rain. For example, in 1994, Tropical Storm Alberto dumped so much rain on Florida that the resulting floods caused $500 million in damage. Once over land, however, Alberto, like any cyclonic storm, lost its source of moisture and heat energy and quickly disintegrated.