Tropical cyclone, an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water, a tropical cyclone generates winds that exceed 119 km (74 miles) per hour. In extreme cases winds may exceed 240 km (150 miles) per hour, and gusts may surpass 320 km (200 miles) per hour. Accompanying these strong winds are torrential rains and a devastating phenomenon known as the storm surge, an elevation of the sea surface that can reach 6 metres (20 feet) above normal levels. Such a combination of high winds and water makes cyclones a serious hazard for coastal areas in tropical and subtropical areas of the world.
Every year during the late summer months (July–September in the Northern Hemisphere and January–March in the Southern Hemisphere), cyclones strike regions as far apart as the Gulf Coast of North America, north-western Australia, and eastern India and Bangladesh. 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 north western 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". All these different names refer to the same type of storm.
Tropical cyclones are compact, circular storms, generally some 320 km (200 miles) in diameter, whose winds swirl around a central region of low atmospheric pressure. The winds are driven by this low-pressure core and by the rotation of Earth, which deflects the path of the wind through a phenomenon known as the Coriolis force. As a result, tropical cyclones rotate in a counter clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. The wind field of a tropical cyclone may be divided into three regions.
First is a ring-shaped outer region, typically having an outer radius of about 160 km (100 miles) and an inner radius of about 30 to 50 km (20 to 30 miles). In this region the winds increase uniformly in speed toward the centre. Wind speeds attain their maximum value at the second region, the eyewall, which is typically 15 to 30 km (10 to 20 miles) from the centre of the storm. The eyewall in turn surrounds the interior region, called the eye, where wind speeds decrease rapidly and the air is often calm.
The fuel for a tropical cyclone is provided by a transfer of water vapour and heat from the warm ocean to the overlying air, primarily by evaporation from the sea surface. As the warm, moist air rises, it expands and cools, quickly becoming saturated and releasing latent heat through the condensation of water vapour. The column of air in the core of the developing disturbance is warmed and moistened by this process. The temperature difference between the warm, rising air and the cooler environment causes the rising air to become buoyant, further enhancing its upward movement. If the sea surface is too cool, there will not be enough heat available, and the evaporation rates will be too low to provide the tropical cyclone enough fuel.
Energy supplies will also be cut off if the warm surface water layer is not deep enough, because the developing tropical system will modify the underlying ocean. Rain falling from the deep convective clouds will cool the sea surface, and the strong winds in the centre of the storm will create turbulence. If the resulting mixing brings cool water from below the surface layer to the surface, the fuel supply for the tropical system will be removed. The vertical motion of warm air is by itself inadequate to initiate the formation of a tropical system.
However, if the warm, moist air flows into a pre-existing atmospheric disturbance, further development will occur. As the rising air warms the core of the disturbance by both release of latent heat and direct heat transfer from the sea surface, the atmospheric pressure in the centre of the disturbance becomes lower. The decreasing pressure causes the surface winds to increase, which in turn increases the vapour and heat transfer and contributes to further rising of air. The warming of the core and the increased surface winds thus reinforce each other in a positive feedback mechanism.
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 replenishment 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. Tropical cyclones use warm, moist air as their source of energy or fuel. As climate change is warming ocean temperature, there is potentially more of this fuel available.
Tropical cyclones typically bring large amounts of water into the areas they affect. Much of the water is due to rainfall associated with the deep convective clouds of the eyewall and with the rain bands of the outer edges of the storm. Rainfall rates are typically on the order of several centimetres per hour with shorter bursts of much higher rates. It is not uncommon for totals of 500 to 1,000 mm (20 to 40 inches) of rain to be reported over some regions. Rainfall rates such as these may overwhelm the capacity of storm drains, resulting in local flooding.
Flooding may be particularly severe in low-lying regions such as in Bangladesh and the Gulf Coast of the United States. It is also a problem in areas where mountains and canyons concentrate the rainfall, as occurred in 1998 when floods caused by rains from Hurricane Mitch washed away entire towns in Honduras. In the intervening night of May 27 & 28, 2024, heavy downpour occurred in Manipur due to Cyclone Remal which caused heavy flood all over Manipur (both hill & valley) causing extensive havoc.
Another source of high Precipitation may be provided by the migration of moist air from the clouds of the mature tropical cyclone. When this moisture moves into areas of low pressure at higher latitudes, significant precipitation may result. An example of this occurred in 1983, when the remnants of the eastern Pacific Hurricane Octave moved into a Pacific cold front that had stalled over the south-western United States, drenching the Arizona desert with 200 mm (8 inches) of rain in a three-day period. On average, that region receives 280 mm (11 inches) of rain in an entire year.
