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Wind Force Scales Beaufort Wind Scale, Saffir-Simpson Wind Scale, Fujita Wind Scale

Fluids Engineering

Wind Force Scales: Beaufort Wind Scale, Saffir-Simpson Wind Scale, Fujita Wind Scale

Beaufort Scale: A common way to describe wind and sea state is with the Beaufort Scale. In the early nineteenth century, Admiral Sir Francis Beaufort developed a scale for wind force based on the behavior of sailing ships. The British Admiralty later adopted this scale, and Bowditch (1977, p. 1059) helped popularize it.

In effect, the Beaufort number or force B is related to the wind speed (in m s^-1) at a standard reference height of 10 m, U10, through (List 1984, p. 119; Strangeways 2001)

U₁₀ = 0.836B^3/2 Eq. 1

But the key feature of the Beaufort Scale is that it associates U₁₀ and B with a description of wind and sea state and, thus, provides an estimate of U₁₀ from visual observations alone.

Subsequently, the Beaufort Scale was adapted to use over land (e.g., Bowditch 1977, p. 1059; List 1984, p. 119). Table 1 shows the Beaufort Scale and includes descriptions of conditions for a given Beaufort force over both land and sea.

Saffir-Simpson Scale: The Beaufort Scale classifies all ocean storms with surface-level winds above 32.7 m s^-1 as hurricanes. But conditions at sea and when the storm comes ashore vary widely depending on the wind speed. The Saffir-Simpson Scale, developed by Herbert Saffir and Bob Simpson, further divides hurricanes into five categories. Table 2 shows the Saffir-Simpson Scale.

At sea, storms are assigned to a Saffir-Simpson category on the basis of their maximum surface-level wind speed and minimum central pressure. Storms may change category during their lifetime as they intensify or degrade. The “Storm Surge” listed in Table 7 is the height of the ocean wave that comes ashore ahead of the storm. The values shown give a typical range; the actual storm surge will depend on the slope of the continental shelf.

The main relevance of the Saffir-Simpson Scale is that it attempts to forecast flooding and damage if the storm does move onshore. The “Effects” column in the table lists these predictions. Effects range from minor for a Category 1 storm to catastrophic, as they were with Mitch, a 1998 Category 5 hurricane that killed over 800 people in Honduras and Nicaragua.

Table 1, Beaufort Scale, with the associated wind speed ranges for each Beaufort force in meters per second, knots, and miles per hour. H_1/3 is the significant wave height, the average height of the highest one-third of all waves occurring during a period (Kinsman 1965, p. 302f, 390f.).

Force	Wind description	U10			H1/3 (m)	Over the sea	Over land
		(m s-1)	(knots)	(mph)
0	Calm	0.0–0.2	<1	<1	0	Sea like a mirror	Calm; smoke rises vertically
1	Light air	0.3–1.5	1–3	1–3	0.1–0.2	Ripples with appearance of scales; no foam crests	Smoke drift indicates wind direction; vanes do not move
2	Light breeze	1.6–3.3	4–6	4–7	0.3–0.5	Small wavelets; crests have glassy appearance but do not break	Wind felt on face; leaves rustle; vanes begin to move
3	Gentle breeze	3.4–5.4	7–10	8–12	0.6–1.0	Large wavelets; crests begin to break; scattered whitecaps	Leaves and twigs in constant motion; light flags extended
4	Moderate breeze	5.5–7.9	11–16	13–18	1.5	Small waves becoming longer; numerous whitecaps	Dust, leaves, and loose paper raised; small branches move; flags flap
5	Fresh breeze	8.0–10.7	17–21	19–24	2.0	Moderate waves taking longer form; many whitecaps and chance of some spray	Small trees in leaf begin to sway; whitecaps on inland waters
6	Strong breeze	10.8–13.8	22–27	25–31	3.5	Large waves forming; white foam crests extensive, and spray probable	Larger branches of trees in motion; flags pop; whistling in wires; umbrellas unstable
7	Moderate gale	13.9–17.1	28–33	32–38	5.0	Sea heaps up, and white foam from breaking waves begins to be blown in streaks; spindrift appears	Whole trees in motion; resistance felt in walking against the wind
8	Fresh gale	17.2–20.7	34–40	39–46	7.5	Moderately high waves of greater length; edges of crests break into spindrift; foam is blown in well-marked streaks	Twigs and small branches broken; progress generally impeded
9	Strong gale	20.8–24.4	41–47	47–54	9.5	High waves; dense streaks of foam; sea begins to roll; spray may reduce visibility	Slight structural damage occurs; slate blown from roofs
10	Whole gale	24.5–28.4	48–55	55–63	12	Very high waves with overhanging crests; sea surface takes on white appearance as foam in great patches is blown in very dense streaks; rolling sea is heavy; visibility reduced	Seldom experienced on land; trees broken or uprooted; considerable structural damage occurs
11	Storm	28.5–32.6	56–63	64–72	15	Exceptionally high waves; sea covered with long white patches of foam; small and medium-sized ships might be lost to view behind waves; visibility further reduced	Very rarely experienced on land; usually accompanied by widespread damage
12	Hurricane	>32.7	>64	>73	>15	Air filled with foam and spray; sea completely white with driving spray; visibility greatly reduced

Table 2 Saffir-Simpson Scale for hurricane intensity. The maximum sustained winds are given in meters per second, knots, and miles per hour.

Category	Sustained winds			Central pressure (mb)	Storm surge (m)	Effects
	(m s-1)	(knots)	(mph)
Tropical storm	17–32	35–63	39–73			Beaufort force 8–11
1	33–42	64–82	74–95	>980	1.0–1.7	No real damage to buildings. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some flooding of coastal roads and minor damage to piers.
2	43–49	83–95	96–110	979–965	1.8–2.6	Some damage to doors, windows, and roofing material. Considerable damage to vegetation, mobile homes, and piers. Coastal low-lying escape routes flood 2–4 hours before the storm center arrives. Small craft in unprotected anchorages break moorings.
3	50–58	96–113	111–130	964–945	2.7–3.8	Some structural damage to small residences and utility buildings, with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures, with larger structures damaged by floating debris. Terrain continuously lower than 5 feet above sea level may be flooded 8 miles or more inland.
4	59–69	114–135	131–155	944–920	3.9–5.6	More extensive curtainwall failures, with some roofs on small residences failing completely. Major beach erosion. Major damage to the lower floors of structures near the shore. Terrain continuously lower than 10 feet above sea level may be flooded inland as far as 6 miles; massive evacuation of residential areas could, therefore, be required.
5	>69	>135	>155	<920	>5.7	Complete roof failure on many residences and industrial buildings. Some complete building failures, with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet above sea level and within 500 yards of the shore. Massive evacuation may be required for residential areas on low ground within 5 to 10 miles of the shore.

Fujita Scale: Similarly, the Fujita Scale categorizes tornadoes in terms of their maximum wind speed and the damage they cause (Fujita 1981; Glickman 2000, p. 322f.). Table 8 shows the Fujita Scale. Fujita (1981) associated the lower surface-level wind speed limit for a tornado category with the Fujita number F through the expression:

U = 6.30(F + 2)^3.2

where U is the wind speed in m s^-1.

Table 3. Fujita Scale to describe tornado intensity. The range for maximum wind speeds is given in meters per second and miles per hour.

Fujita scale	Wind speed range		Damage specifications
	(m s-1)	(mph)
F0	18–32	40–72	Beaufort force 8–11. Light damage. Some damage to chimneys; branches break off trees; some shallow-rooted trees pushed over; damage to sign boards.
F1	33–49	73–112	Moderate damage. The lower wind speed is the beginning of the hurricane range. Surfaces of roofs peeled off; mobile homes pushed off foundations or overturned; moving autos pushed off the road.
F2	50–69	113–157	Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped off or uprooted; light-object missiles generated.
F3	70–92	158–206	Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in a forest uprooted; heavy cars lifted off the ground and thrown.
F4	93–116	207–260	Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown; large missiles generated.
F5	117–142	261–318	Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 m; bark ripped from trees; incredible phenomena occur.
F6+	>142	>318	Tornadoes are not expected to reach F6 wind speeds.