How much stronger are hurricane winds aloft?

An entire hurricane category stronger for some tall Jax Beach condos.

Flooding in Jacksonville Monday, Sept. 11, 2017 (Photo: Kyle Caldabaugh and Lee Holt - Level Exposure)

A question from a viewer asked: If the winds of a hurricane are measured aloft, how much difference is there on the ground?

This is a good question, especially for people living in high-rise condos where winds can change from hurricane strength on the ground floor to major hurricane strength ot the top floor.

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Take for instance the 18-floor Pelican Point Condo, which is Duval’s tallest on the coast at 218 feet. If a strong category two hurricane with 110 mph winds blew through, winds at the top floor would reach major Category 3 strength at 126 mph.

The magnitude of winds would be exponentially higher for the 43-floor Bank of America Tower where a category two hurricane would pact category 4 hurricane winds atop its 617-foot roof; not to mention the force of wind would be 57 times stronger causing considerably more damage to upper story windows.

Hurricane winds are stronger aloft to a point

The winds in a hurricane are certainly intense enough at the ground level but they are actually about 10% faster aloft where aircraft reconnaissance planes measure wind speed observations.

This is because surface friction reduces the wind close to the ground. Winds over the ocean are faster because buildings trees or mountains do not block them.

Winds are stronger aloft and get stronger in a hurricane eyewall up to about 1600 feet before decreasing.

In the absence of an anemometer, the National Hurricane Center forecasters can assess the storm’s maximum sustained surface wind by estimating a reduction from winds measured by the plane.

Hurricane hunters typically measure winds in the core at 10,000 ft and reduce those winds by 10% for a surface speed estimate.

Reconnaissance planes fly through tropical storms at half the altitude and reduce the flight level winds by 20%.

A slightly higher 25% flight level wind reduction is estimated for very low level missions geared to spotting early stages of tropical cyclone circulation.

Researchers designed the guidelines after years of research deploying dropsonde canisters out of the plane’s belly that captures wind, pressure and moisture readings as it falls to the ocean.

This has implications for winds at the top of a 30-story building that will average about 20 mph (one Saffir-Simpson category) higher than at the surface.

Percentage of higher winds based on number of condo stories tall. (NOAA AMOL)

For example in Hurricane Georges, the surface winds at 33 feet are near the lower end of Category Three; yet at an altitude of 300 ft the winds are now in the middle of Category Four.

Dark solid vertical lines mark A Safir-Simpson Scale class. Curve line was vertical wind speed with height. (NOAA/AOML/Hurricane Research Division, Miami, Florida)

About the Author:

After covering the weather from every corner of Florida and doing marine research in the Gulf, Mark Collins settled in Jacksonville to forecast weather for The First Coast.