tropical storm
News •
-
What is a tropical storm?
A tropical storm is an organized center of low pressure that originates over warm tropical oceans. With horizontal dimensions of about 160 km, its maximum sustained surface winds range from 63 to 118 km per hour.
Tropical storms are an intermediate stage between tropical depressions and more intense storms, like hurricanes, typhoons, or cyclones, depending on the region. They occur in any ocean basin where tropical cyclones are found. Once wind speeds in a tropical depression reach 63 km per hour, the storm is classified as a tropical storm and named.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does a tropical storm form?
Tropical storms are low-pressure centers that form over warm tropical oceans. They have sustained surface winds ranging from 63 to 118 km (39 to 73 miles) per hour and are an intermediate stage between tropical depressions and more intense storms such as hurricanes, typhoons, or cyclones.
Tropical storm formation requires specific conditions, including a warm ocean surface layer of at least 26.5 °C (80 °F) that is at least 50 meters deep, a pre-existing atmospheric circulation, a humid middle atmosphere, and a location at least 500 km from the Equator.
The storm begins as a tropical disturbance, which typically occurs when loosely organized cumulonimbus clouds in an easterly wave begin to show signs of a weak circulation.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where do tropical storms usually occur?
Tropical storms, a phase between tropical depressions and more intense cyclones, occur in any ocean basin where tropical cyclones are found, such as the North Atlantic, northeast Pacific, central Pacific, northwest and southwest Pacific, and Indian Oceans. Approximately 80 tropical storms form annually over tropical oceans, with about two-thirds becoming severe.
These storms predominantly develop within 20° north or south of the Equator, where sea surface temperatures are warm enough. The Pacific Ocean sees the highest number of tropical storms, while the Indian Ocean ranks second, and the Atlantic Ocean ranks third. They typically occur in the late summer to early fall in each hemisphere.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How are tropical storms different from hurricanes or typhoons?
Tropical storms, hurricanes, and typhoons are all types of tropical cyclones, but they are classified by their wind speeds and location.
A tropical storm has maximum sustained surface winds ranging from 63 to 118 km/h (39 to 73 mph). When a tropical storm's winds reach 119 km/h (74 mph), it is then classified as a hurricane, typhoon, or cyclone, depending on the region.
In the North Atlantic Ocean and the eastern North Pacific, they are called hurricanes, and in the western North Pacific around the Philippines, Japan, and China, the storms are referred to as typhoons. In the western South Pacific and Indian Ocean they are called severe tropical cyclones, tropical cyclones, or simply cyclones.
6 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What are the main dangers or effects of a tropical storm?
Tropical storms, which form over warm tropical oceans, can bring several dangers. High winds can cause considerable destruction to even solidly built structures. Torrential rains and storm surges lead to flooding, causing property damage and loss of life. Coastal regions are especially vulnerable to deadly storm surges, where the sea level rises significantly. Additionally, intense rainfall can overwhelm drainage systems, leading to severe flooding, particularly in low-lying and mountainous areas. Tornadoes and downbursts can also occur, adding to the potential for wind damage.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do meteorologists track and predict tropical storms?
Meteorologists track and predict tropical storms using various tools and techniques. Geostationary satellites provide continuous Earth surface displays, with infrared wavelengths tracking storm development by detecting cold, high clouds and deep convection. Aircraft, like the U.S. Air Force C-130, directly gather detailed data on a storm's strength and structure, using dropsondes to measure temperature, humidity, pressure, and wind speed.
National weather services and Regional Specialized Meteorological Centres (RSMCs) track storms across ocean basins. The National Hurricane Center uses satellite and aircraft data with computer models to forecast storm paths and intensity. These models range from statistical to dynamic, solving equations for atmospheric changes and cyclone interactions. Once landfall is probable, forecasters issue warnings and strike probabilities to enable timely evacuations.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
tropical storm, organized centre of low pressure that originates over warm tropical oceans. The maximum sustained surface winds of tropical storms range from 63 to 118 km (39 to 73 miles) per hour. These types of tropical cyclones represent an intermediate stage between loosely organized tropical depressions and more intense storms, which are known as hurricanes, typhoons, or cyclones in different parts of the globe (see also tropical cyclone). A tropical storm may occur in any of Earth’s ocean basins in which tropical cyclones are found (North Atlantic, northeast Pacific, central Pacific, northwest and southwest Pacific, and Indian). The size and structure of tropical storms are similar to those of the more intense and mature tropical cyclones; they possess horizontal dimensions of about 160 km (100 miles) and winds that are highest at the surface but decrease with altitude. The winds typically attain their maximum intensity at approximately 30–50 km (20–30 miles) away from the centre of the circulation, but the distinct eyewall that is a characteristic of mature tropical cyclones is usually absent.
The precursors of tropical storms in the Atlantic are easterly waves that form over Africa and propagate toward the west. The easterly waves are characterized by wind speeds of approximately 16 km (10 miles) per hour and convective clouds that are loosely organized around a central area of low pressure, or trough axis. The winds transfer heat and moisture from the sea surface to the atmosphere. If local atmospheric conditions support deep convection and low vertical wind shear, the system may become organized and begin to intensify. Intensification occurs as the air warmed at the surface begins to rise. The transfer of air away from the centre of the trough axis causes the surface pressure to fall, which in turn causes higher winds that increase the transfer of heat at the surface. The Coriolis force, which is a product of Earth’s rotation, causes the winds to rotate about the centre, thereby generating a closed and symmetric circulation pattern.
A similar process occurs in other ocean basins. In the western Pacific, tropical storms originate from loosely organized convection events in the monsoon trough, which is a large-scale area of low pressure that lies along the Equator. The exact mechanism that results in the intensification of the storm is not well understood, but surface pressure falls associated with tropical upper tropospheric troughs (TUTTs) likely play a role.
" height="20" alt="HEW Logo">
Once the surface wind speeds in a tropical depression reach 63 km (39 miles) per hour, the regional storm-warning centre assigns a name to the disturbance, and it is classified as a tropical storm. This tropical-storm classification is used until the wind speeds increase above 117 km (73 miles) per hour, in which case the storm is reclassified as a hurricane, typhoon, or cyclone. In the Atlantic and eastern Pacific, tropical cyclones are classified according to their intensity using the Saffir-Simpson scale (scaled from 1 to 5), a tool used to predict the extent of flooding from rainfall and storm surge and the level of property damage. A “category 1” storm possesses hurricane-force winds in excess of 119 km (74 miles) per hour. Australian forecasters have developed a similar scale, but a category 1 on the Australian scale corresponds to the tropical-storm range of wind speeds.
Since tropical storms are the precursors to the more intense tropical cyclones, they occur more often. The yearly average numbers of tropical storms occurring in the various ocean basins are as follows: North Atlantic 13, northeastern Pacific 16, northwestern Pacific 27, northern Indian 5, southwestern Indian 10, and Australian (that is, the southwestern Pacific and southeast Indian basins) 16. In all ocean basins, roughly 45 percent of the tropical storms continue to intensify to hurricane strength or greater.
A number of factors may result in the failure of a tropical storm to continue to intensify. In some cases, the storm moves into a region where the large-scale environment does not favour further growth. The sea surface temperature may be too low, the middle atmosphere too dry, or the winds at upper levels too high to support the continued vertical development of the storm. In other cases, the tropical storm makes landfall before reaching hurricane strength and begins to dissipate.
The extreme damage that often accompanies the landfall of hurricane-strength usually does not occur with tropical storms. The lower wind speeds result in a minimal storm surge of less than four feet (about one metre), and most damage is confined to plants, trees, and unanchored structures, such as mobile homes. Nevertheless, low-lying areas prone to flooding from prolonged periods of rain or mountainous regions subject to flash flooding may be severely impacted by tropical storms. In some regions, rains from tropical systems are an important part of the annual climate and contribute to the total hydrologic cycle.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
