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Maximum sustained wind
View on WikipediaThe 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 certain distance from the center, known 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 meters (33 ft) above mean sea level,[nb 1] 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.
Over the ocean, satellite imagery is often used to estimate 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 the damage potential of a tropical cyclone, through use of such scales as the Saffir–Simpson scale.
Definition
[edit]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.[2] 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 meters (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 meters (33 ft) height.[3] This is an important distinction, as the value of the highest one-minute sustained wind is about 14% greater than a ten-minute sustained wind over the same period.[4]
Estimation and measurement
[edit]In most tropical cyclone basins, use of the satellite-based Dvorak technique is the primary method used to estimate a tropical cyclone's maximum sustained winds.[5] 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.[6] Central pressure values for their centers of low pressure are approximate. The tracking of individual clouds on minutely satellite imagery could be used in the future in estimating surface winds speeds for tropical cyclones.[7]
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]
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| Tropical Storm Wilma at T3.0 | Tropical Storm Dennis at T4.0 | Hurricane Jeanne at T5.0 | Hurricane Emily at T6.0 |
Variation
[edit]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 especially, 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]
Relationship to tropical cyclone strength scales
[edit]In most basins, maximum sustained winds are used to define the category of a tropical cyclone on each basin's tropical cyclone scale. 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. 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 over one minute.
Notes
[edit]References
[edit]- ^ Simiu, Emil; Vickery, Peter; Kareem, Ahsan (July 2007). "Relation Between Saffir-Simpson Hurricane Scale Wind Speeds and Peak 3-s Gust Speeds Over Open Terrain". Journal of Structural Engineering. Technical Notes. 133 (7). Reston, Virginia: 1043. doi:10.1061/(ASCE)0733-9445(2007)133:7(1043).
- ^ Brian W. Blanchard and S. A. Hsu. ON THE RADIAL VARIATION OF THE TANGENTIAL WIND SPEED OUTSIDE THE RADIUS OF MAXIMUM WIND DURING HURRICANE WILMA (2005). Archived 2012-09-05 at the Wayback Machine Retrieved on 2008-07-04.
- ^ Tropical Cyclone Weather Services Program (June 1, 2006). "Tropical cyclone definitions" (PDF). National Weather Service. Retrieved 2006-11-30.
- ^ United States Navy: "SECTION 2. INTENSITY OBSERVATION AND FORECAST ERRORS". Archived from the original on 2007-09-16. Retrieved 2008-07-04.
{{cite web}}: CS1 maint: bot: original URL status unknown (link) Retrieved on 2018-10-07. - ^ "Objective Dvorak Technique". University of Wisconsin–Madison. Retrieved 2006-05-29.
- ^ Chris Landsea (June 8, 2010). Subject: H1) What is the Dvorak technique and how is it used? Atlantic Oceanographic and Meteorological Laboratory. Retrieved on 2011-01-14.
- ^ A. F. Hasler, K. Palaniappan, C. Kambhammetu, P. Black, E. Uhlhorn, and D. Chesters. High-Resolution Wind Fields within the Inner Core and Eye of a Mature Tropical Cyclone from GOES 1-min Images. Retrieved on 2008-07-04.
- ^ a b Franklin, James L., Michael L. Black, and Krystal Valde. GPS dropwindsonde wind profiles in hurricanes and their operational implications. Retrieved on 2008-07-04.
- ^ J. TUTTLE and R. GALL. A single-radar technique for estimating the winds in tropical cyclones. Retrieved on 2008-06-12.
- ^ Haby, Jeff. "The Importance of Friction". theweatherprediction.com.
- ^ Mapping of Topographic Effects on Maximum Sustained Surface Wind Speeds in Landfalling Hurricanes. Retrieved on 2008-07-04.
- ^ Peter Black. Subject: Re: Offshore vs nearshore sonde composite. Retrieved on 2008-07-04.
- ^ Harrison, Roy (1999). Understanding Our Environment. Cambridge: Royal Society of Chemistry. pp. 11. ISBN 0-85404-584-8.
- ^ Thompson, Russell (1998). Atmospheric Processes and Systems. New York: Routledge. pp. 102–103. ISBN 0-415-17145-8.
Maximum sustained wind
View on GrokipediaDefinition and Fundamentals
Definition
Maximum sustained wind refers to the highest average wind speed over a specified averaging period, typically 1 minute in the United States, measured or estimated at a height of 10 meters above the surface in open terrain or over water, and it represents the standard metric for assessing the intensity of a tropical cyclone.[2] This measurement is taken near the storm's center, often within the eyewall where the strongest winds are concentrated.[9] In other regions, such as those following World Meteorological Organization standards, a 10-minute averaging period may be used instead, though the core concept remains the average speed over the defined interval to capture persistent wind strength. Unlike instantaneous gusts, which are brief peaks in wind speed lasting only a few seconds and can exceed sustained values by 20-50% in tropical cyclones due to turbulence, maximum sustained wind filters out these short-term fluctuations to provide a more representative measure of the storm's overall power.[10] This distinction is crucial because gusts reflect momentary extremes that may cause localized damage, whereas sustained winds indicate the enduring force capable of widespread structural impacts.[1] The maximum sustained wind typically occurs at the radius of maximum wind (RMW), the distance from the storm center where these peak speeds are found, serving as a key indicator of tropical cyclone intensity and aiding in forecasts of potential devastation.[9] It plays a central role in classifying storms, such as defining hurricanes under the Saffir-Simpson scale when speeds reach 74 mph or higher.[4] The concept of maximum sustained wind originated in mid-20th century tropical cyclone monitoring, building on early anemometer records from the 1930s and 1940s that often used 5-minute or hourly averages to document storm winds, as seen in reanalyses of events like the 1938 New England hurricane.[11] By the 1970s, the term had evolved into a standardized metric with the development of the Atlantic hurricane database (HURDAT), adopting the 1-minute averaging period for consistent intensity tracking across U.S. agencies.[12]Radius of Maximum Wind
The radius of maximum wind (RMW) is defined as the distance from the center of a tropical cyclone to the annular region where the highest sustained wind speeds occur. In intense tropical cyclones, this distance typically ranges from 20 to 50 km, reflecting the compact structure of stronger storms.[13] The peak winds at the RMW are concentrated in the eyewall, a cylindrical band of deep convection encircling the calm eye, where intense updrafts driven by latent heat release accelerate tangential flow. Moisture convergence in the boundary layer funnels humid air into this region, enhancing convective activity and angular momentum conservation, which culminates in the maximum wind speeds typically exceeding 74 mph (119 km/h).[9][14] The RMW varies considerably throughout a tropical cyclone's evolution, often contracting during periods of intensification as diabatic heating strengthens low-level inflow and reduces the radial extent of peak vorticity flux. This inward contraction can occur rapidly, sometimes preceding rapid intensification by enhancing the efficiency of energy transfer to the winds. In contrast, the RMW tends to expand during weakening phases or eyewall replacement cycles, when outer rainbands organize and the inner eyewall dissipates, redistributing the convective maximum outward.[14][15] Representative examples illustrate this range: Hurricane Wilma (2005) exhibited an exceptionally small RMW of about 10 km during its peak intensity in the northwestern Caribbean Sea, contributing to its record-low central pressure. Weaker systems, by comparison, often feature larger RMWs exceeding 100 km, resulting in broader but less intense wind fields.[16][17]Measurement and Estimation
Direct Measurement Methods
Direct measurement methods for maximum sustained winds in tropical cyclones primarily involve in-situ observations from aircraft reconnaissance and surface-based instruments, providing the most accurate data for operational intensity assessment, particularly in the Atlantic basin where routine missions occur.[18] Aircraft reconnaissance has evolved significantly since the 1940s, when the U.S. military's 53rd Weather Reconnaissance Squadron began flying manned missions into hurricanes to collect basic pressure and wind data using rudimentary instruments.[18] By the 1990s, advancements introduced GPS dropsondes, parachute-borne sensors deployed from NOAA and U.S. Air Force aircraft that measure high-resolution vertical wind profiles from flight levels down to the surface, achieving accuracies of 1-4 mph with 15 ft resolution.[19][20] Additionally, the Stepped Frequency Microwave Radiometer (SFMR) on board these aircraft directly estimates surface wind speeds by measuring microwave emissions from the ocean surface at multiple frequencies, calibrated to account for rain attenuation, providing real-time surface wind data along flight tracks essential for intensity determination.[21] In modern operations, primarily conducted by NOAA in the Atlantic basin, flight-level winds are measured at approximately 10,000 ft (700 mb) using onboard sensors, then adjusted to estimate 10-meter surface winds by applying a 10% reduction factor, as surface winds are typically 90% of flight-level values over water.[19][22] Dropsondes complement this by directly sampling near-surface winds, with over 350 profiles collected by 1999 to refine eyewall wind structures.[20] Surface observations rely on anemometers mounted at 10 meters on buoys, ships, and coastal stations, capturing 1-minute sustained winds during storm passages.[23] For instance, during Hurricane Ike's 2008 landfall, a research automated weather station deployed by the Texas Tech Hurricane Research Team near the eyewall recorded sustained winds of 74 mph, while moored buoys in the Gulf of Mexico provided offshore data.[24] Similarly, in Hurricane Andrew (1992), ship and coastal anemometer readings helped map wind fields after exposure adjustments reduced variance from 40-50% to about 10%.[23] These methods face key limitations, including sparse spatial coverage outside reconnaissance zones like the Atlantic, where global tropical cyclone monitoring depends on fewer direct samples.[19] Additionally, anemometer exposure errors arise in gusty conditions, as instruments on land or ships may underestimate peak sustained winds due to shielding or platform motion, necessitating post-event site validations.[23]Indirect Estimation Techniques
Indirect estimation techniques for maximum sustained wind in tropical cyclones rely on remote sensing and modeling approaches when in-situ measurements are impractical, providing global coverage through satellite and radar observations. These methods infer wind speeds from cloud patterns, radar reflectivity, and ocean surface signatures, often calibrated against historical data to estimate the maximum 1-minute sustained wind at 10 meters above the surface. Developed primarily in the late 20th century and refined with modern sensors, these techniques have become essential for real-time intensity assessment in data-sparse regions.[25] The Dvorak technique, introduced in the 1970s, uses visible and infrared satellite imagery to estimate tropical cyclone intensity via pattern recognition of cloud features such as curved bands and the central dense overcast. Developed by Vernon Dvorak, it assigns a Current Intensity (CI) number on a scale from 1.0 to 8.0 based on the storm's developmental stage and cloud organization, empirically correlated to maximum sustained winds. The basic satellite wind estimation follows , where (often denoted as the T-number or CI) represents the pattern-based intensity, and is an empirical function derived from historical correlations between satellite-observed cloud patterns and verified wind speeds from reconnaissance aircraft and surface observations. This relation was initially formulated in 1975 and refined in 1984 through statistical analysis of over 200 cases, yielding a lookup table for conversion; for example, a CI of 4.0 corresponds to approximately 65 knots (75 mph), while a CI of 6.5 indicates 127 knots (146 mph). The table below summarizes the standard conversions for Atlantic and Northwest Pacific basins:[25][26]| Current Intensity (CI) | Maximum Sustained Wind (knots) | Maximum Sustained Wind (mph) | Central Pressure (Atlantic, mb) | Central Pressure (NW Pacific, mb) |
|---|---|---|---|---|
| 1.0 | 25 | 29 | - | - |
| 1.5 | 25 | 29 | - | - |
| 2.0 | 30 | 35 | 1009 | 1000 |
| 2.5 | 35 | 40 | 1005 | 997 |
| 3.0 | 45 | 52 | 1000 | 991 |
| 3.5 | 55 | 63 | 994 | 984 |
| 4.0 | 65 | 75 | 987 | 976 |
| 4.5 | 77 | 89 | 979 | 966 |
| 5.0 | 90 | 104 | 970 | 954 |
| 5.5 | 102 | 117 | 960 | 941 |
| 6.0 | 115 | 132 | 948 | 927 |
| 6.5 | 127 | 146 | 935 | 914 |
| 7.0 | 140 | 161 | 921 | 898 |
| 7.5 | 155 | 178 | 906 | 879 |
| 8.0 | 170 | 196 | 890 | 858 |




