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How Hurricanes Work
Hurricanes are huge storms! They can be up to 600 miles across and have strong winds spiraling inward and upward at speeds of 75 to 200 mph. Each hurricane lasts for over a week, moving 10-20 miles per hour over the open ocean.
With warm air at its center, a hurricane is different from extratropical cyclones, which are the most common type of storm in the United States. The center of the storm is the calmest part. It is called the eye and has only light winds and fair weather. The low level storm winds blow counterclockwise around the eye in the Northern Hemisphere (clockwise in the Southern Hemisphere). Above 9 km, winds spiral outwards and clockwise in the Northern Hemisphere.
A storm goes through a series of stages before being classified as a hurricane.
| Tropical Disturbance | Thunderstorms with light cyclonic circulation | ||
| Tropical Depression | Wind speeds between 20 and 34 knots (23-39 mi/hr) | ||
| Tropical Storm | Wind speeds between 35 and 64 knots (40-73 mi/hr) | ||
| Hurricane | Wind speed greater than 64 knots (74 mi/hr) |
Where and When Do They Form?
Hurricanes do an important job for the Earth. They help move heat from warm tropical places to the cooler temperate zone. To do this, they typically form between 5 to 15 degrees latitude north and south of the equator. Then, they thunder across the warm oceans of the world such as the Atlantic, the Gulf of Mexico, the Caribbean, and the Western Pacific Ocean (where they are called typhoons), up to higher latitudes.
Hurricanes happen when the oceans have been warmed during summer months. In the North Atlantic, hurricane season is from June 1 to November 30, but most hurricanes happen during the fall.
Storm Surge
As a hurricane’s winds spiral around and around the storm, they push water into a mound at the storm’s center. This mound of water becomes dangerous when the storm reaches land because it causes flooding along the coast. The water piles up, unable to escape anywhere but on land as the storm carries it landward. A hurricane will cause more storm surge in areas where the ocean floor slopes gradually.
When high tide happens at the same time as a storm surge, the combination of the two is called storm tide. During a storm tide, the water level may be 20 feet or more above normal. This causes huge floods. Storm tide is especially dangerous for islands or coastal areas where even a few feet of surge may cause large areas of flooding.
There are computer
models that allow forecasters to predict the amount of storm surge that
will affect a coastal area. A model called SLOSH takes into account a
storm’s strength, its path, how the ocean shallows, and the shape
of the land and calculates how much storm surge a hurricane will probably
cause.
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