Joseph loves Science, and they have been talking about Ida, the tropical cyclone that has devastated some cities and towns in the United States.
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure centre, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and/or squalls.
Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, tropical storm, cyclonic storm, tropical depression, or simply cyclone.
A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as tropical cyclones or 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 winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect.
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 recondenses 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 North Easters and European wind storms, which are fuelled primarily by horizontal temperature contrasts.
Tropical cyclones are typically between 100 and 2,000 km in diameter. Every year tropical cyclones impact various regions of the globe including the Gulf Coast of North America, Australia, India and Bangladesh.
More information: National Hurricane Center
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, they rarely form within 5° of the equator.
Tropical cyclones are almost unknown in the South Atlantic due to a consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.
The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and weaken quite rapidly over land. This causes coastal regions to be particularly vulnerable to tropical cyclones, compared to inland regions. 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, from atmospheric moisture and moisture evaporated from water, 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 kilometres 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.
Although their effects on human populations can be devastating, tropical cyclones may play a role in relieving drought conditions, though this claim is disputed. They also carry heat and energy away from the tropics and transport it towards temperate latitudes, which plays an important role in regulating global climate.
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.
The systems generally have a well-defined centre, which is surrounded by deep atmospheric convection and a closed wind circulation at the surface.
Historically, tropical cyclones have occurred around the world for thousands of years, with one of the earliest tropical cyclones on record estimated to have occurred in Western Australia in around 6000 BC. However, before satellite imagery became available during the 20th century, many of these systems went undetected unless it impacted land or a ship encountered it by chance.
These days, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 120 km/h or more.
Around the world, a tropical cyclone is generally deemed to have formed, once mean surface winds in excess of 65 km/h are observed. 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.
Tropical cyclones on either side of the Equator generally have their origins in the Intertropical Convergence Zone, where winds blow from either the northeast or southeast. Within this broad area of low-pressure, air is heated over the warm tropical ocean and rises in discrete parcels, which causes thundery showers to form. These showers dissipate quite quickly, however, they can group together into large clusters of thunderstorms. This creates a flow of warm, moist, rapidly rising air, which starts to rotate cyclonically as it interacts with the rotation of the earth.
More information: National Weather Service
There are several factors required for these thunderstorms to develop further, including sea surface temperatures of around 27 °C and low vertical wind shear surrounding the system.
In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be staged a tropical cyclone passes through during its formation or dissipation.
An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes.
A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses; although not as frequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone.
From space, extratropical storms have a characteristic comma-shaped cloud pattern. Extratropical cyclones can also be dangerous when their low-pressure centres cause powerful winds and high seas.
A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wideband of latitudes, from the equator to 50°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.
More information: BBC
despite our technologies,
most of the nature remains unpredictable.
Diane Ackerman
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