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Bangladesh after a cyclone.
Hurricanes (and their counterparts such as typhoons and tropical cyclones) are extremely powerful storms that originate in tropical regions and migrate into the middle latitudes. They bring enormous destruction and loss of life to many coastal regions of the world. The hurricane that hit Galveston Island, Texas, in 1900 was the greatest single natural disaster to hit North America, with a death toll of 6000. But this figure pales in comparison to the hundreds of thousands of fatalities associated with individual tropical cyclones in southern Asia.
Most hurricanes begin their life cycles as uneventful tropical disturbances, small clusters of thunderstorms. When they intensify and organize into a rotating band of cloud cover and thunderstorm activity, they are called tropical depressions. Further intensification results in their being classified as tropical storms, or hurricanes if their sustained wind speeds exceed 120 km/hr. Because strong tropical storms can form only over oceans having high surface temperatures, tropical depressions most often become tropical storms and eventually hurricanes over the western portions of the ocean basins.
Hurricanes are smaller than mid-latitude cyclones but very much larger than tornadoes. They can last for a week or more and travel thousands of kilometers before dissipating. The heaviest thunderstorm activity occurs within bands of thick cloud cover that spiral toward the center of the system in a pinwheel pattern. The intensity of the storm increases toward its center until reaching the eye wall, the concentric zone of maximum activity that surrounds the eye. The eye of a hurricane is strikingly different from the rest of the hurricane because it is marked by generally clear skies, light winds, and higher air temperatures. Often it is hard to discern the true structure of a hurricane from above because the anticyclonically rotating outflow in the upper troposphere creates a blanket of cirrostratus clouds overlying the thicker cumulus.
Hurricanes can produce damage in several ways. Copious amounts of rain can bring intense floods, and strong winds can bring down structures. But the most serious threat posed by a hurricane is the storm surge, the elevated rise in sea level due to low atmospheric pressure and the piling up of water by strong winds. When the storm surge coincides with a high tide, the flood waters (coupled with heavy surf) can penetrate considerable distances inland.
The National Hurricane Center of the National Weather Service uses a sophisticated network of satellites, research aircraft, and computer hardware and software to issue advisories on the likelihood of hurricane landfall. The erratic nature of hurricanes makes their prediction particularly difficult, but recent modernization at the National Weather Service has substantially increased forecast accuracy. In the next chapter we examine the process of forecasting everyday weather events.