Portal:Tropical cyclones

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Tropical Cyclones Portal

Typhoon tip peak.jpg

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rainfall. Tropical cyclones feed on the heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fuelled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows, leading to their classification as 'warm core' storm systems. Tropical cyclones originate in the doldrums near the Equator, approximately 10 degrees away.

The term 'tropical' refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term 'cyclone' refers to such storms' cyclonic nature, with anticlockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and intensity, a tropical cyclone can be referred to by names such as 'hurricane', 'typhoon', 'tropical storm', 'cyclonic storm', 'tropical depression', or simply 'cyclone'.

Pictured: Typhoon Tip

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Impact of a storm surge

A storm surge is an offshore rise of water associated with a low pressure weather system, typically a tropical cyclone. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry of the body of water. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. The term "storm surge" in casual (non-scientific) use is storm tide; that is, it refers to the rise of water associated with the storm, plus tide, wave run-up, and freshwater flooding. When referencing storm surge height, it is important to clarify the usage, as well as the reference point. NHC tropical storm reports reference storm surge as water height above predicted astronomical tide level, and storm tide as water height above NGVD-29.

In areas where there is a significant difference between low tide and high tide, storm surges are particularly damaging when they occur at the time of a high tide. In these cases, this increases the difficulty of predicting the magnitude of a storm surge since it requires weather forecasts to be accurate to within a few hours. Storm surges can be produced by non-tropical storms, such as the "Halloween Storm" of 1991 and the Storm of the Century (1993), but the most extreme storm surge events occur as a result of extreme weather systems, such as tropical cyclones. Factors that determine the surge heights for landfalling tropical cyclones include the speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline and the bathymetry of the water offshore. The SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model is used to simulate surge from tropical cyclones.

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Hurricane Emily.jpg

Hurricane Emily (2005) near peak intensity in July, 2005. This image was captured by MODIS on the Terra satellite on July 16, 2005.


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Tropical cyclone anniversaries

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  • October 6, 1973 - Typhoon Nora (pictured) reached its peak intensity with 295 km/h (185 mph) winds in the Philippine Sea. Nora made landfall in northeast Luzon a few days later, causing $2 billion of damage.
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  • October 8, 1976 - Hurricane Madeline (pictured) made landfall on the Mexican coast at its peak intensity. Extensive evacuations kept casualties to a minimum with a total of only seven deaths.
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