Maximum sustained wind

Last updated
Saffir–Simpson scale
CategoryWind speeds
(for 1-minute maximum sustained winds)
m/s knots (kn) mph km/h
Five ≥ 70 m/s   ≥ 137 kn   ≥ 157 mph   ≥ 252 km/h  
Four  58–70 m/s    113–136 kn    130–156 mph    209–251 km/h  
Three  50–58 m/s    96–112 kn    111–129 mph    178–208 km/h  
Two  43–49 m/s    83–95 kn    96–110 mph    154–177 km/h  
One  33–42 m/s    64–82 kn    74–95 mph    119–153 km/h  
Related classifications
(for 1-minute maximum sustained winds)
Tropical storm  18–32 m/s    34–63 kn    39–73 mph    63–118 km/h  
Tropical depression  ≤ 17 m/s    ≤ 33 kn    ≤ 38 mph    ≤ 62 km/h  

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span (see the definition, below), anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and 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".

Radius of maximum wind

The radius of maximum wind (RMW) is the distance between the center of a cyclone and its band of strongest winds. It is a parameter in atmospheric dynamics and tropical cyclone forecasting. The highest rainfall rates occur near the RMW of tropical cyclones. The extent of a cyclone's storm surge and its maximum potential intensity can be determined using the RMW. As maximum sustained winds increase, the RMW decreases. Recently, RMW has been used in descriptions of tornadoes. When designing buildings to prevent against failure from atmospheric pressure change, RMW can be used in the calculations.

Contents

Over the ocean, satellite imagery determines the value of the maximum sustained winds within a tropical cyclone. Land, ship, aircraft reconnaissance observations, and radar imagery can also estimate this quantity, when available. This value helps determine damage expected from a tropical cyclone, through use of such scales as the Saffir–Simpson scale.

Weather satellite type of satellite

The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting, covering the entire Earth asynchronously, or geostationary, hovering over the same spot on the equator.

Satellite imagery imagery of the Earth or another astronomical object taken from an artificial satellite

Satellite imagery are images of Earth or other planets collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell images by licensing them to governments and businesses such as Apple Maps and Google Maps.

Weather radar radar used to locate and monitor meteorological conditions

Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.

Definition

The maximum sustained wind normally occurs at a distance from the center known as the radius of maximum wind, within a mature tropical cyclone's eyewall, before winds decrease at farther distances away from a tropical cyclone's center. [1] Most weather agencies use the definition for sustained winds recommended by the World Meteorological Organization (WMO), which specifies measuring winds at a height of 10 metres (33 ft) for 10 minutes, and then taking the average. However, the United States National Weather Service defines sustained winds within tropical cyclones by averaging winds over a period of one minute, measured at the same 10 metres (33 ft) height. [2] This is an important distinctions, as the value of the highest one-minute sustained wind is about 14% greater than a ten-minute sustained wind over the same period. [3]

World Meteorological Organization Specialised agency of the United Nations

The World Meteorological Organization (WMO) is an intergovernmental organization with a membership of 192 Member States and Territories. Its current Secretary-General is Petteri Taalas and the President of the World Meteorological Congress, its supreme body, is David Grimes. The Organization is headquartered in Geneva, Switzerland.

National Weather Service United States weather agency

The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the purposes of protection, safety, and general information. It is a part of the National Oceanic and Atmospheric Administration (NOAA) branch of the Department of Commerce, and is headquartered in Silver Spring, Maryland, within the Washington metropolitan area. The agency was known as the United States Weather Bureau from 1890 until it adopted its current name in 1970.

Determination of value

In most tropical cyclone basins, use of the satellite-based Dvorak technique is the primary method used to determine a tropical cyclone's maximum sustained winds. [4] The extent of spiral banding and difference in temperature between the eye and eyewall is used within the technique to assign a maximum sustained wind and pressure. [5] Central pressure values for their centers of low pressure are approximate. The intensity of example hurricanes is derived from both the time of landfall and the maximum intensity. [6] The tracking of individual clouds on minutely satellite imagery could be used in the future in estimating surface winds speeds for tropical cyclones. [7]

Dvorak technique

The Dvorak technique is a widely used system to estimate tropical cyclone intensity based solely on visible and infrared satellite images. Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The primary patterns used are curved band pattern (T1.0-T4.5), shear pattern (T1.5–T3.5), central dense overcast (CDO) pattern (T2.5–T5.0), central cold cover (CCC) pattern, banding eye pattern (T4.0–T4.5), and eye pattern (T4.5–T8.0).

Eye (cyclone) region of mostly calm weather at the center of strong tropical cyclones

The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 km (20–40 miles) in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.

Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 1013.25 mbar (101325 Pa), equivalent to 760 mm Hg (torr), 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

Ship and land observations are also used, when available. In the Atlantic as well as the Central and Eastern Pacific basins, reconnaissance aircraft are still utilized to fly through tropical cyclones to determine flight level winds, which can then be adjusted to provide a fairly reliable estimate of maximum sustained winds. A reduction of 10 percent of the winds sampled at flight level is used to estimate the maximum sustained winds near the surface, which has been determined during the past decade through the use of GPS dropwindsondes. [8] Doppler weather radar can be used in the same manner to determine surface winds with tropical cyclones near land. [9]

Dropsonde

A dropsonde is an expendable weather reconnaissance device created by the National Center for Atmospheric Research (NCAR), designed to be dropped from an aircraft at altitude over water to measure storm conditions as the device falls to the surface. The sonde contains a GPS receiver, along with pressure, temperature, and humidity (PTH) sensors to capture atmospheric profiles and thermodynamic data. It typically relays these data to a computer in the aircraft by radio transmission.

Satellite Images of Selected Tropical Cyclones and Associated T-Number from Dvorak technique
Wilma-17-1315z-T30-discussion1500z.png Dennis-06-1445z-T40-discussion1500z.png Jeanne-22-1945z-T50-discussion2100z.png Emily-14-1915z-T60-discussion15-0300z.png
Tropical Storm Wilma at T3.0 Tropical Storm Dennis at T4.0 Hurricane Jeanne at T5.0 Hurricane Emily at T6.0

Variation

Friction between the atmosphere and the Earth's surface causes a 20% reduction in the wind at the surface of the Earth. [10] Surface roughness also leads to significant variation of wind speeds. Over land, winds maximize at hill or mountain crests, while sheltering leads to lower wind speeds in valleys and lee slopes. [11] Compared to over water, maximum sustained winds over land average 8% lower. [12] More specifically, over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the geostrophic wind speed aloft; while over open water or ice, the reduction is between 10% and 30%. [8] [13] [14]

The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis force and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars. This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation. Geostrophic flow in air or water is a zero-frequency inertial wave.

Relationship to tropical cyclone strength scales

In most basins, maximum sustained winds are used to define their category. In the Atlantic and northeast Pacific oceans, the Saffir–Simpson scale is used. This scale can be used to determine possible storm surge and damage impact on land. [15] In most basins, the category of the tropical cyclone (for example, tropical depression, tropical storm, hurricane/typhoon, super typhoon, depression, deep depression, intense tropical cyclone) is determined from the cyclone's maximum sustained wind. Only in Australia is this quantity not used to define the tropical cyclone's category; in their basin, wind gusts are used. [16]

See also

Related Research Articles

Wind speed

Wind speed, or wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Note that wind direction is usually almost parallel to isobars, due to Earth's rotation.

The Saffir–Simpson hurricane wind scale (SSHWS), formerly the Saffir–Simpson hurricane scale (SSHS), classifies hurricanes – Western Hemisphere tropical cyclones – that exceed the intensities of tropical depressions and tropical storms – into five categories distinguished by the intensities of their sustained winds.

Rainband

A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.

Tropical cyclones are ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds and which tropical cyclone basin(s) they are located in. Only a few scales of classifications are used officially by the meteorological agencies monitoring the tropical cyclones, but some alternative scales also exist, such as accumulated cyclone energy, the Power Dissipation Index, the Integrated Kinetic Energy Index, and the Hurricane Severity Index.

Central dense overcast

The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity. Locating the center within the CDO can be a problem for strong tropical storms and with systems of minimal hurricane strength as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery.

Tropical cyclone windspeed climatology

Tropical cyclone windspeed climatology is the study wind distribution amongst tropical cyclones, a significant threat to land and people. Since records began in 1851, winds from hurricanes, typhoons and cyclones have been responsible for fatalities and damage in every basin. Major hurricanes usually cause the most wind damage. Hurricane Andrew for example caused $45 billion in damage, most of it wind damage.

Tropical cyclone observation

Tropical cyclone observation has been carried out over the past couple of centuries in various ways. The passage of typhoons, hurricanes, as well as other tropical cyclones have been detected by word of mouth from sailors recently coming to port or by radio transmissions from ships at sea, from sediment deposits in near shore estuaries, to the wiping out of cities near the coastline. Since World War II, advances in technology have included using planes to survey the ocean basins, satellites to monitor the world's oceans from outer space using a variety of methods, radars to monitor their progress near the coastline, and recently the introduction of unmanned aerial vehicles to penetrate storms. Recent studies have concentrated on studying hurricane impacts lying within rocks or near shore lake sediments, which are branches of a new field known as paleotempestology. This article details the various methods employed in the creation of the hurricane database, as well as reconstructions necessary for reanalysis of past storms used in projects such as the Atlantic hurricane reanalysis.

Hurricane Adolph Category 4 Pacific hurricane in 2001

Hurricane Adolph of the 2001 Pacific hurricane season was the first and one of only two East Pacific hurricanes in May to reach Category 4 strength on the Saffir-Simpson Hurricane Scale since record keeping began in the East Pacific. Adolph was the first depression of the season, forming on May 25; it became a hurricane three days later. After rapidly intensifying, Adolph became the most powerful storm in terms of maximum sustained winds this season, along with Hurricane Juliette. The storm briefly threatened land before dissipating on June 1, after moving over colder waters.

An Australian region tropical cyclone is a non-frontal, low pressure system that has developed, within an environment of warm sea surface temperatures and little vertical wind shear aloft in either the Southern Indian Ocean or the South Pacific Ocean. Within the Southern Hemisphere there are officially three areas where tropical cyclones develop on a regular basis, these areas are the South-West Indian Ocean between Africa and 90°E, the Australian region between 90°E and 160°E and the South Pacific basin between 160°E and 120°W. The Australian region between 90°E and 160°E is officially monitored by the Australian Bureau of Meteorology, the Papua New Guinea National Weather Service and the Indonesian Agency for Meteorology, Climatology and Geophysics, while others like the Fiji Meteorological Service and the United States National Oceanic and Atmospheric Administration also monitor the basin. Each tropical cyclone year within this basin starts on 1 July and runs throughout the year, encompassing the tropical cyclone season which runs from 1 November and lasts until 30 April each season. Within the basin, most tropical cyclones have their origins within the South Pacific Convergence Zone or within the Northern Australian monsoon trough, both of which form an extensive area of cloudiness and are dominant features of the season. Within this region a tropical disturbance is classified as a tropical cyclone, when it has 10-minute sustained wind speeds of more than 65 km/h (35 mph), that wrap halfway around the low level circulation centre, while a severe tropical cyclone is classified when the maximum 10-minute sustained wind speeds are greater than 120 km/h (75 mph).

South-West Indian Ocean tropical cyclone type of tropical cyclone located in South West Indian Ocean and measured by Météo-France La Reunion scale

In the south-west Indian Ocean, tropical cyclones form south of the equator and west of 90° E to the coast of Africa.

Meteorological history of Hurricane Andrew

The meteorological history of Hurricane Andrew, the strongest tropical cyclone of the 1992 Atlantic hurricane season, lasted from mid to late August 1992. The hurricane developed from a tropical wave that moved off the coast of Africa on August 14. Tracking westward due to a ridge, favorable conditions allowed it to develop into Tropical Depression Three on August 16 in the deep tropical Atlantic Ocean. The cyclone gradually intensified, becoming a tropical storm on August 17. However, wind shear soon impacted the storm, causing significant increases in barometric pressure and nearly destroying its low-level circulation by August 20. Wind shear sharply decreased starting on August 21, and with warm sea surface temperatures, Andrew began rapid deepening, starting on the following day. By August 23, Andrew peaked as a Category 5 hurricane on the Saffir–Simpson hurricane wind scale while approaching The Bahamas.

Glossary of tropical cyclone terms

The following is a glossary of tropical cyclone terms.

Meteorological history of Typhoon Haiyan

Typhoon Haiyan's meteorological history began with its origins as a tropical disturbance east-southeast of Pohnpei and lasted until its degeneration as a tropical cyclone over Southern China. The thirteenth typhoon of the 2013 Pacific typhoon season, Haiyan originated from an area of low pressure several hundred kilometers east-southeast of Pohnpei in the Federated States of Micronesia on November 2. Tracking generally westward, environmental conditions favored tropical cyclogenesis and the system developed into a tropical depression the following day. After becoming a tropical storm and attaining the name Haiyan at 0000 UTC on November 4, the system began a period of rapid intensification that brought it to typhoon intensity by 1800 UTC on November 5. By November 6, the Joint Typhoon Warning Center (JTWC) assessed the system as a Category 5-equivalent super typhoon on the Saffir-Simpson hurricane wind scale; the storm passed over the island of Kayangel in Palau shortly after attaining this strength.

Cyclone Ernie Category 5 Australian region cyclone in 2017

Severe Tropical Cyclone Ernie was one of the quickest strengthening tropical cyclones on record. Ernie was the first Category 5 severe tropical cyclone in the Australian region since Cyclone Marcia in 2015, and also the strongest tropical cyclone in the Australian region since Cyclone George in 2007. Ernie developed from a tropical low into a cyclone south of Indonesia in the northeast Indian Ocean on 6 April 2017, and proceeded to intensify extremely rapidly to a Category 5 severe tropical cyclone. A few days later, on 10 April, the system was downgraded below cyclone intensity following a period of rapid weakening, located southwest of its original position. Ernie had no known impacts on any land areas.

2019 North Indian Ocean cyclone season North Indian Ocean Ocean cyclone season in 2019

The 2019 North Indian Ocean cyclone season is an ongoing event in the annual cycle of tropical cyclone formation. The North Indian Ocean cyclone season has no official bounds, but cyclones tend to form between April and December, with the two peaks in May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northern Indian Ocean. The season's first named storm, Pabuk, entered the basin on January 4, becoming the earliest-forming cyclonic storm of the North Indian Ocean on record. The second cyclone of the season, Fani, was the strongest tropical cyclone in the Bay of Bengal by 3-minute maximum sustained wind speed and minimum barometric pressure since the 1999 Odisha cyclone.

Tropical cyclones in 2004 were spread out across seven different areas called basins; the strongest of these tropical cyclones was Cyclone Gafilo, which strengthened to a minimum barometric pressure of 895 mbar becomes the most intense tropical cyclone ever recorded in the South-West Indian Ocean before striking the east coast of Madagascar. 130 tropical cyclones had formed this year to date. 81 tropical cyclones had been named by either a Regional Specialized Meteorological Center (RSMC) or a Tropical Cyclone Warning Center (TCWC). The most active basin in 2004 was the Western Pacific, which documented 29 named systems, while the North Atlantic, despite only amounting to 15 named systems, was its basin's hyperactive season since 1995. Conversely, both the Eastern Pacific hurricane and North Indian Ocean cyclone seasons experienced the least number of cyclones reaching tropical storm intensity in recorded history, numbering 12 and 4, respectively. Activity across the southern hemisphere's three basins—South-West Indian, Australian, and South Pacific—was spread evenly, with each region recording seven named storms apiece.

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