Tropical cyclone
A tropical cyclone is a rapidly rotating storm system with a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane (/ˈhʌrɪkən, -keɪn/), typhoon (/taɪˈfuːn/), tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn (120 km/h; 75 mph) or more.[1] Tropical cyclones carry heat away from the tropics and transport it towards temperate latitudes. This plays an important role in regulating global climate. Tropical cyclones can be very harmful for people.
"Hurricane" redirects here. For a strong tropical cyclone that occurs in the Atlantic Ocean, see Atlantic hurricane. For other uses, see Hurricane (disambiguation).
Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms. Those storms are powered primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter.
The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, cyclones rarely form within 5° of the equator, although there have been some cases. Tropical cyclones are very rare in the South Atlantic (although occasional examples do occur) due to consistently strong wind shear and a weak Intertropical Convergence Zone. In contrast, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea.
The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and they weaken quite rapidly over land. This causes inland regions to be much less vulnerable to cyclones than coastal regions, with residents of tropical islands facing the greatest threat of all, although tidal flooding is often worse on continental coasts than on islands. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes.
Tropical cyclones draw in air from a large area and concentrate the water content of that air into precipitation over a much smaller area. This replenishing of moisture-bearing air after rain may cause multi-hour or multi-day extremely heavy rain up to 40 km (25 mi) from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local water control structures across a large area.
Climate change can affect tropical cyclones in different ways due to its effects on the water cycle. It is possible that climate change can lead to making rain and wind stronger, making the most severe storms more common, causing cyclones to reach farther north or south, but also reducing how often they happen.[2] Tropical cyclones use warm, moist air as their source of energy or fuel. As climate change is warming ocean temperatures, there is potentially more of this fuel available.[3]
Definition and terminology
A tropical cyclone is the generic term for a warm-cored, non-frontal synoptic-scale low-pressure system over tropical or subtropical waters around the world.[4][5] The systems generally have a well-defined center which is surrounded by deep atmospheric convection and a closed wind circulation at the surface.[4] A tropical cyclone is generally deemed to have formed once mean surface winds in excess of 35 kn (65 km/h; 40 mph) are observed.[1] It is assumed at this stage that a tropical cyclone has become self-sustaining and can continue to intensify without any help from its environment.[1]
Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones", and such storms in the Indian Ocean can also be called "severe cyclonic storms".
Tropical refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Cyclone refers to their winds moving in a circle, whirling round their central clear eye, with their surface winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect.