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Hurricane Home • Hurricane Basics • Preparing for Hurricanes • Responding to Hurricanes
Tropical depressions, tropical storms and hurricanes are low pressure areas that develop in the tropical regions of the ocean (between 20 degrees N latitude and the equator). These storms are masses of thunderstorms that organize and begin to rotate. These systems, in order of intensity, are called depressions (winds between 25 and 38 mph), tropical storms (winds between 39 and 73 mph) and hurricanes (winds of 74 mph or greater).
The National Weather Service tracks these storms on charts during hurricane season, June 1 through Nov. 30, using the following symbols.
A tropical depression, the lowest intensity, is given a number once it has a counterclockwise spin and winds of 38 mph or less.
When wind speeds reach 39 mph and the storm is given a name from a pre-determined list, a tropical storm is born. While a tropical storm does not produce a high storm surge, its thunderstorms can still pack a dangerous and deadly punch. In 1972, Agnes was only a tropical storm when it dropped torrential rains that led to devastating floods in Pennsylvania, Maryland and Virginia. Sixteen people died in Virginia and damage was estimated at $222 million.
A hurricane is the most intense tropical event, with five categories and winds ranging from 74 mph to 155 mph or greater. Storm surge is a major concern with hurricanes. The extremely high winds cause ocean water to pile up, creating higher than normal sea levels with waves up to 40 feet in open water. High sea levels and shallow waters can devastate a coastline and bring ocean water miles inland.
A hurricane's bands of thunderstorms produce torrential rains and sometimes tornadoes. A foot or more of rain could fall in less than a day, causing flash floods and mudslides. Large rivers in the hurricane's path might still be flooding for days after the storm has passed. The storm's driving winds can topple trees, utility poles and damage buildings. Communication and electricity might be lost for days and roads are often impassable due to fallen trees and debris.