Hurricanes are powerful storms that form over warm ocean water, far from human settlements. Long before a storm makes landfall, modern storm-tracking technology provides the time and tools to anticipate the hurricane’s behavior and assess its potential threat to people and property. Even ships at sea are safer now than ever before. Captains make use of forecasts and satellite data to chart courses that will steer them away from the fiercest winds and high waves.
The Saffir-Simpson Scale is used to estimate the potential property damage and flooding expected from a hurricane.
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The strength of a hurricane is related to the air pressure in the eye of the storm. The strongest hurricanes generally have very low air pressure in their eye. Hurricane Wilma, which formed in 2005, had an extremely low air pressure of 882 millibars—the lowest recorded for an Atlantic hurricane. Wind speed varies within the regions of a hurricane. The strongest winds usually occur near the eye wall, where air pressure values change rapidly. The maximum intensity of winds in Wilma’s eye wall exceeded 275 km/h (171 mph). Wind speed gradually decreases outward from the eye wall. In the Northern Hemisphere, winds on the right side of a hurricane (with respect to its path) are usually stronger than winds on the left side of the storm. The strongest winds occur on the right side because in this region the horizontal winds that blow the storm forward are combining with the counterclockwise whirling winds within the storm.
If you listen to NOAA Weather Radio or other weather broadcasts, you’re probably aware that the
Saffir-Simpson Scale rates a hurricane’s intensity. Developed in 1969, this scale assigns a hurricane to one of five categories based on the hurricane’s wind speed.
Similar to the Enhanced-Fujita Scale, which rates tornadoes after they touch down, the Saffir-Simpson rating system began as a way to rate damage after a storm made landfall. Now, after many years of comparative storm observations, forecasters can categorize the hurricanes before they make landfall. In addition to wind speed, the Saffir-Simpson Scale predicts the rise in sea level caused by the arrival of the storm. But be prepared for changes! As hurricanes strengthen and weaken, their category rating is updated to reflect those changes in the storm’s intensity.
Although wind causes much damage during a hurricane, water can become an even greater threat to life and property. The rush of water driven to shore by the storm’s forceful winds is called a storm surge. As wind speeds increase, water within a surge can build to a height above the roof level of small dwellings. This wall of water, highest where the eye of the storm makes landfall, can impact 80–160 km (50–99 mi) of shoreline.
Storm surges are especially dangerous along the southern coast of the United States. In this area, the land rises only slightly above sea level, making the shoreline extremely vulnerable. As a result, it is possible for a moderate storm surge to spill over levees and floodwalls. With enough force, storm surges can even demolish the vertical barriers constructed to contain rising floodwaters.
Suppose that you knew a hurricane was on the way. What sort of storm surge should you expect? The approximate height of a storm surge can be inferred by the hurricane’s assigned category on the Saffir-Simpson Hurricane Scale. In general, a Category 1 storm will have a surge that is from 1.0–1.7 m (3–5 ft) above the normal level of the tide. A Category 2 storm will have a surge that can rise 1.8–2.6 m (6–8 ft) above normal. A Category 5 can produce storm surges of over 5.7 m (19 ft).
The final height to which the storm surge will rise is greatly influenced by the shape of the coastline.
For example, if the rising water is funneled into a narrow passageway, it piles up, creating an even higher surge. This is often observed in narrow inlets or bays. Another factor is the time of tide when the storm surge arrives. A surge will have less impact if it arrives at low tide than at high tide. The most destructive surges arrive during spring high tides, when the water is already well above the average tide level.
The high water, waves, and swift currents associated with a storm surge can erode barriers and rip structures apart. Buildings that remain standing sustain heavy water damage. A storm surge may also take many lives, because escaping from one can be quite difficult. Sometimes people take refuge in their homes. As the water level rises, they may eventually become trapped in the attic with no means of escape. Often, rescuers must cut through the roof to reach the trapped victims.
Most people don’t realize that just a few centimeters of swiftly moving water can knock them off their feet. A mere 15 cm (6 in.) of moving water can sweep a car off the road. Many drownings occur when people are trapped in cars caught in floodwaters. In fact, since 1970, more than half of all hurricane-related deaths have resulted from flooding.
A surge blown ashore by the high winds isn’t the only type of flooding that can occur during a hurricane. Heavy rain may cause rivers to overflow their banks, producing flood conditions many hundreds of miles inland. Like their coastal counterparts, these inland floods are destructive and dangerous.
Some hurricanes produce other violent storms—tornadoes. Tornadoes sometimes form in the outer edge of a hurricane as it makes landfall. Tornadoes can continue to develop for days after landfall and for long distances inland. Hurricane Beulah, which made landfall along the Texas coast in 1967, spawned an amazing 115 tornadoes!