Environmental Hazards: Hurricanes

Hurricanes are tropical cyclones that occur in the Atlantic and Eastern Pacific Oceans, with wind speeds exceeding 73 miles per hour (mph). Hurricanes are multifaceted weather systems known to cause significant losses. As of 2011, the 20 costliest hurricanes in the United States ranged from $5 billion to over $100 billion in economic loss, adjusted to 2005. An understanding of the characteristics of hurricanes and the associated specific hazards is becoming more important as populations continue to migrate to coastal areas. The ability to withstand environmental hazards, such as hurricanes, protects human life and property and results in more sustainable communities.

Tropical cyclones are weather systems that develop in or near tropical latitudes, characterized by rotation about a center of low pressure (also referred to as closed circulation). In the Northern Hemisphere, tropical cyclones rotate in a counterclockwise (also called anti-clockwise) direction, [Page 188]while in the Southern Hemisphere, this rotation is in the clockwise direction. The direction of rotation is caused by the Coriolis effect, a phenomenon attributed to the motion of Earth's rotation. The intensity of a tropical cyclone is calculated on the basis of its maximum sustained wind speed, measured at a height of 10 meters (33 feet) above the open sea surface. Sustained wind speed refers to wind speed that is continuously measured and averaged over a period of one minute. All tropical cyclones are similar events, consisting of a cyclonic weather system rotating around a low pressure center; however, these systems have different names depending on the water bodies affected. Tropical cyclones are referred to as typhoons in the western North Pacific Ocean and as tropical cyclones in the Indian Ocean and the western South Pacific Ocean.

In the Atlantic and eastern Pacific oceans (storms affecting the United States), tropical cyclones with sustained wind speeds less than 39 mph are called tropical depressions; those with sustained wind speeds ranging from 39 mph to 73 mph are called tropical storms; those with sustained wind speeds exceeding 73 mph are called hurricanes. In the United States, both tropical storms and hurricanes are named events, while tropical depressions are numbered. The naming convention starts each year with a name beginning with the letter A and proceeds through 21 preassigned names. If more than 21 named events occur in a particular hurricane season, 24 Greek alphabet letters ranging from alpha to omega are then used. A 6-year rotation is used for naming tropical cyclones, with storm names being repeated every 6 years unless a particular storm is extremely deadly or costly. An example of a retired storm name is Katrina. In 2005, Hurricane Katrina caused over 1,000 deaths and resulted in approximately $100 billion in losses (2005 dollars). Naming conventions for tropical cyclones around the world are established by the World Meteorological Organization.