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Characteristics of the Gust Factor Measured by Coastal-Marine Automated Network (C-MAN) Stations During Hurricane Georges in 1998

Professor S.A. Hsu
Louisiana State University

The gust factor is the ratio of peak gust to sustained wind speed. Two questions often raised are: "Does the gust factor increase with wind speed?" and "Does it increase with height?" In order to respond, simultaneous measurements from a large number of stations are needed. Such an opportunity arose during Hurricane Georges in September 1998. The measurements are listed in Table 1, along with the anemometer height for each station. It can be seen from this table that the gust factor does not increase with either height or sustained speed within approximately 20 to 160 ft and 24 to 81 kts.

Between 1400 and 1500 UTC on 25 September 1998, four stations along the Florida Keys provided an interesting sub-data set. These four C-MAN stations are: Molasses Reef (MLRF1), Long Key (LONF1), Sombrero Key (SMKF1), and Sand Key (SANF1). When the wind speed increased from 46 kts at MLRF1 to 81 kts at SMKF1, the gust factor remained virtually the same at both locations, even though the anemometer height at MLRF1 is 52 ft versus 159 ft at SMKF1. We therefore conclude from the data provided in Table 1 that the gust factor does not increase with either height or speed. Certainly, more data are needed to substantiate this conclusion.

Location
(Ranked by
anemometer ht)
Sustained
Wind
(kts)
Peak
Gust
(kts)

Gust
Factor
Anemometer
Height
(m)
Anemometer
Height
(ft)
Dry Tortugas, FL 59 68 1.15 5.7 19
Long Key, FL 47 58 1.23 7.0 23
Cape San Blas, FL 38 43 1.13 9.8 32
Keaton Beach, FL 30 37 1.23 10.0 33
Cedar Key, FL 29 34 1.17 10.0 33
Venice, LA 24 27 1.13 11.6 38
Sand Key, FL 56 71 1.27 13.1 43
Lake Worth, FL 30 35 1.17 13.7 45
Molasses Reef, FL 46 53 1.15 15.8 52
Grand Isle, LA 40 50 1.25 15.8 52
Dauphin Island, AL 59 71 1.20 17.4 57
Southwest Pass, LA 54 63 1.17 30.5 100
Fowey Rocks, FL 45 52 1.16 43.9 144
Sombrero Key, FL 81 92 1.14 48.5 159
Location
(Ranked by sustained
wind speed)
Sustained
Wind
(kts)
Peak
Gust
(kts)

Gust
Factor


Day


Time
Venice, FL 24 27 1.13 30 1800
Cedar Key, FL 29 34 1.17 30 0500
Lake Worth, FL 30 35 1.17 25 1400
Keaton Beach, FL 30 37 1.23 29 2300
Cape San Blas, FL 38 43 1.13 29 1900
Grand Isle, LA 40 50 1.25 27 2000
Fowey Rocks, FL 45 52 1.16 25 1000
Molasses Reef, FL 46 53 1.15 25 1400
Long Key, FL 47 58 1.23 25 1400
Southwest Pass, LA 54 63 1.17 27 2200
Sand Key, FL 56 71 1.27 25 1400
Dry Tortugas, FL 59 68 1.15 26 0000
Dauphin Island, AL 59 71 1.20 28 0600
Sombrero Key, FL 81 92 1.14 25 1500

Data Source: Pasch, R. J., L. A. Avila, and J. L. Guiney, 2001: Atlantic hurricane season of 1998. Monthly Weather Review, 129, 3085 - 3123.

Table 1. Variations of the gust factor with height and sustained wind speed as measured by NDBC's C-MAN stations during Hurricane Georges in September 1998.

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This article previously appeared in the Mariners Weather Log Fall/Winter 2002 without the accompanying table. The article is reprinted in this issue to include the table.

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Page last modified: December 19, 2003