How are hurricane and typhoon categories defined?

The Saffir–Simpson scale uses the 1-minute sustained wind: 33–42 m/s is Category 1, 43–49 Category 2, 50–58 Category 3, 58–70 Category 4 and above 70 m/s Category 5. China's GB/T 19201 uses the 2-minute wind: tropical depression from 10.8 m/s, tropical storm 17.2, severe tropical storm 24.5, typhoon 32.7, severe typhoon 41.5 and super typhoon from 51.0 m/s. Because the averaging periods differ, the same storm can get slightly different numbers.

Why does lower central pressure mean stronger wind?

Wind is driven by the pressure difference. Near the radius of maximum winds the inward pressure-gradient force roughly balances the centrifugal and Coriolis forces (gradient-wind balance). The Holland model puts the peak wind near √(BΔp/ρe), so the wind scales roughly with the square root of the pressure deficit. A larger B concentrates the winds near the eyewall and gives a higher peak for the same pressure drop.

Why do tropical cyclones need water warmer than 26.5 °C?

A tropical cyclone is a heat engine fuelled by evaporation. The near-surface air must be warm and moist enough for its equivalent potential temperature to exceed the saturated value in the mid-troposphere, otherwise deep convection cannot reach the tropopause. The tropical mid-troposphere is set by the warmest oceans, which puts the threshold near 26–27 °C. The warm layer also needs to be about 50 m deep, or the storm stirs up cold water and starves itself. The tool also gives Emanuel's potential intensity: warmer seas and a colder outflow raise the theoretical maximum wind.

Why do hurricanes spin counter-clockwise in the Northern Hemisphere?

As air flows in toward the low, the Coriolis effect deflects it to the right in the Northern Hemisphere, which sets up a counter-clockwise circulation; in the Southern Hemisphere it deflects left and the storm turns clockwise. Near the equator the Coriolis parameter f = 2Ω sinφ is nearly zero, so air cannot organise into a closed vortex and storms almost never form within about 5° of the equator.

How reliable are the storm surge and wind load numbers?

They are order-of-magnitude estimates. The surge includes only the inverse barometer effect (about 1 cm per hPa) and steady wind setup over the shelf; tides, wave setup, bay funnelling and storm motion are ignored, so real surges can be much higher or lower, and official warnings always take priority. Wind loads use q = ½ρv² with typical pressure coefficients; real designs follow building codes that account for height, terrain and gusts.

🌀

🌀Hurricane & Typhoon Physics Simulator - Wind Field, Category, Potential Intensity, Storm Surge

Enter the central and environmental pressure, the radius of maximum winds and the latitude. The Holland parametric profile gives surface wind and pressure versus radius, the gale-force radii, and an automatic category on both the Saffir–Simpson scale and China's national typhoon grades. In the top-down animation, particles spiral in at true relative speeds with an adjustable inflow angle, counter-clockwise in the north and clockwise in the south. Extra tabs cover Emanuel potential intensity versus sea temperature, a rough storm surge estimate and wind loads on a building facade.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

Loading tool...

How to use Typhoon & Hurricane Physics

1Pick a famous storm (Mangkhut, Haiyan, Katrina, Idai) or set the central pressure, radius of maximum winds, latitude and B
2Under Wind field, read the peak wind, categories, wind radii and rotation sense, and change the inflow angle to watch the spiral
3Under Intensity, change the sea surface and outflow temperatures to see the potential intensity and why 26.5 °C water is needed
4Under Storm surge and Wind load, set the shelf width and depth or the facade size to estimate the surge and the force on walls

FAQ

How are hurricane and typhoon categories defined?: The Saffir–Simpson scale uses the 1-minute sustained wind: 33–42 m/s is Category 1, 43–49 Category 2, 50–58 Category 3, 58–70 Category 4 and above 70 m/s Category 5. China's GB/T 19201 uses the 2-minute wind: tropical depression from 10.8 m/s, tropical storm 17.2, severe tropical storm 24.5, typhoon 32.7, severe typhoon 41.5 and super typhoon from 51.0 m/s. Because the averaging periods differ, the same storm can get slightly different numbers.
Why does lower central pressure mean stronger wind?: Wind is driven by the pressure difference. Near the radius of maximum winds the inward pressure-gradient force roughly balances the centrifugal and Coriolis forces (gradient-wind balance). The Holland model puts the peak wind near √(BΔp/ρe), so the wind scales roughly with the square root of the pressure deficit. A larger B concentrates the winds near the eyewall and gives a higher peak for the same pressure drop.
Why do tropical cyclones need water warmer than 26.5 °C?: A tropical cyclone is a heat engine fuelled by evaporation. The near-surface air must be warm and moist enough for its equivalent potential temperature to exceed the saturated value in the mid-troposphere, otherwise deep convection cannot reach the tropopause. The tropical mid-troposphere is set by the warmest oceans, which puts the threshold near 26–27 °C. The warm layer also needs to be about 50 m deep, or the storm stirs up cold water and starves itself. The tool also gives Emanuel's potential intensity: warmer seas and a colder outflow raise the theoretical maximum wind.
Why do hurricanes spin counter-clockwise in the Northern Hemisphere?: As air flows in toward the low, the Coriolis effect deflects it to the right in the Northern Hemisphere, which sets up a counter-clockwise circulation; in the Southern Hemisphere it deflects left and the storm turns clockwise. Near the equator the Coriolis parameter f = 2Ω sinφ is nearly zero, so air cannot organise into a closed vortex and storms almost never form within about 5° of the equator.
How reliable are the storm surge and wind load numbers?: They are order-of-magnitude estimates. The surge includes only the inverse barometer effect (about 1 cm per hPa) and steady wind setup over the shelf; tides, wave setup, bay funnelling and storm motion are ignored, so real surges can be much higher or lower, and official warnings always take priority. Wind loads use q = ½ρv² with typical pressure coefficients; real designs follow building codes that account for height, terrain and gusts.

Features

Holland 1980 gradient-wind and Holland 2010 surface-wind profiles, with B set by hand or by empirical fits
Categories on China's GB/T 19201 scale, the Saffir–Simpson scale and the Beaufort scale
Animated top-down particles at true relative speeds, adjustable inflow, rotation set by hemisphere
Emanuel potential intensity and near-surface equivalent potential temperature against the 26.5 °C threshold
Rough storm surge from the inverse barometer effect plus wind setup across the shelf
Facade and window loads from the dynamic pressure q = ½ρv²

Related tools

View all 196 Physics Tools →