The Beaufort Wind Force Scale, The Saffir-Simpson Hurricane Scale, & the Fujita Scale of Tornado Intensity


0
calm
1light airjust sufficient to give steerage way.light breezeSufficient wind for working ship.
2light breezeThat in which a well-conditioned man-of-war with all sail set and "clean full" would go in smooth water from1 to 2 knots
3gentle breeze3 to 4 knots
4moderate breeze5 to 6 knotsmoderate breezeForces most advantageous for sailing with leading wind and all sail drawing.
5fresh breezeThat to which she could just carry in close "full and by"Royals, &c.
6strong breezeSingle-reefed topsails or topgallant sails.strong windReduction of sail necessary even with leading wind.
7moderate/
near gale
Double-reefed topsails, jib, &c.
8fresh gale/
gale
Triple-reefed topsails, &c.gale forcesConsiderable reduction of sail necessary even with wind quartering.
9strong galeClose-reefed topsails and courses.
10whole gale/
storm
That which she could scarcely bear with close-reefed main topsail and reefed foresailstorm forcesClose reefed sail running, or hove to under storm sail.
11storm/
violent storm
That which would reduce her to storm stay-sails
12hurricaneThat which no canvas could withstand.hurricaneNo sail can stand even when running
The Beaufort Scale has become the standard method of judging wind force. It is not the oldest, since rating the strength of wind by number and/or assigning names to the ratings had been done for some time. In 1806 Francis Beaufort (later Admiral and Sir) wrote out a numerical scale in his log. This was evidently copied from a British Admiralty pamphlet. Beaufort, however, refined and specified the scale, and in 1838 it was officially adopted by the Admiralty. In 1906 a report further refined the scale with corresponding wind speeds and descriptions of effects on land. Beaufort himself had specified the sailing conditions that went with each rating, as seen in the chart at right.

Separate wind scales for tornadoes and hurricanes did not come until the 1970's. The Fujita (or Fujita-Pearson) Scale for tornadoes was proposed in 1971 by T. Theodore Fujita and Allen Pearson. Soon thereafter, the Saffir-Simpson Scale for hurricanes was formulated by Herbert Saffir and Robert Simpson. Thus, the movie Twister was anachronistic when it had Helen Hunt's father warn, in 1965, that they might have an F5 tornado headed toward them. The scale had not been invented yet.

There has been some recent revision of some of these systems. I have not yet updated the presentation here.

The complete Beaufort Scale has thirteen divisions, as shown above (starting with zero). Beaufort grouped these into 5 categories of winds, or finely divided using five "breezes," four "gales," and four other designations.
light
gentle
moderate
fresh
strong
whole
Breezes and gales both use the same simple scale of adjectives in the table at left. It always seemed a little odd to me that the designations jumped directly from "breezes" to "gales." One might expect an intermediate category, like simple "winds." Indeed, Beaufort himself grouped "strong breeze" and "moderate gale" as "strong winds" [note]. The Fujita and Saffir-Simpson scales are almost entirely subdivisions of hurricane force winds. Only the F0 tornado merely has gale force winds. The complete table below gives wind speeds, the appearance of objects affected by the wind (as described in 1906), the kinds of damage to be expected from tornadoes and hurricanes, and other details, like barometric pressure, for hurricanes. Gale force winds mean that a "tropical depression" becomes a "tropical storm," which is then given a name by the appropriate authorities. Warning flags for water craft are also given. At the left of the table, the Beaufort, Fujita, and Saffir-Simpson Scales are given B, F, and S numbers, respectively. Note that severe hurricanes are often attended with tornadoes as well. Hurricanes in the Western Pacific are called "Typhoons," and those in the Indian Ocean, "Cyclones." Tornadoes used to often be called "Cyclones" also. Modern terminology is that all low pressures systems, which includes hurricanes, tornadoes, and less severe phenomena, are "cyclones."

In 1927, a German captain, Petersen, provided "State-of-Sea" descriptions for each wind force. These are given at right. The term "white horses" is not familiar to me, at least from American usage. This seems to mean simply "whitecaps."

B0Calm<1 mphSea like a mirror.
smoke rises vertically
B1Light Air1-3 mphRipples with the appearance of scales are formed, but without foam crests.
direction of wind shown by smoke but not by wind vanes
B2Light Breeze4-7 mphSmall wavelets, still short but more pronounced. Crests have a glassy appearance and do not break.
wind felt on face; leaves rustle; ordinary vane moved by wind
B3Gentle Breeze8-12 mphLarge wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.
leaves and small twigs in motion; wind extends light flag
B4Moderate Breeze13-18 mphSmall waves, becoming longer; fairly frequent white horses.
wind raises dust and loose paper; small branches move
B5Fresh Breeze19-24 mphModerate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray.
small trees in leaf begin to sway; crested wavelets appear on inland waters
B6Strong Breeze25-31 mphLarge waves begin to form; the white foam crests are more extensive everywhere. Probably some spray.
large branches in motion; telegraph wires whistle; umbrellas used with difficulty
B7Moderate Gale32-38 mphSea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
whole trees in motion; inconvenience in walking against wind
Small Craft Warning  32 mph
B8
  
Fresh Gale / Gale39-46 mphModerately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.
twigs break off trees; generally impedes progress
Tropical Storm39 mph
Gale Warning  
F0Tornado, Fujita Scale 040-72 mph
minor roof, tree, and sign damage
B9Strong Gale47-54 mphHigh waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility
slight structural damage occurs; chimney pots and slates removed.
B10
  
Whole Gale / Storm55-63 mphVery high waves with long overhanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole, the surface of the sea takes a white appearance. The "tumbling" of the sea becomes heavy and shock-like. Visibility affected.
trees uprooted; considerable structural damage occurs
Whole Gale / Storm Warning  55 mph
B11Storm / Violent Storm64-73 mphExceptionally high waves (small and medium-sized ships might be lost to view for a time behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected.
very rarely experienced; accompanied by widespread damage
B12Hurricane >74 mphThe air is filled with foam and spray. Sea completely white with driving spray; visibility seriously affected.
devastation occurs
Hurricane Warning  
F1Tornado, Fujita Scale 173-112 mph
roofs damaged; barns torn apart; weak trailers flipped and torn apart; cars thrown from roads; sheet metal buildings destroyed
S1Category I Hurricane, Minimal74-95 mph
barometer >= 980 mb (hPa), 28.94 inches; storm surge 4-5 ft
damage primarily to shrubbery, trees, foliage, unanchored mobile homes, and small unsecured coverings (i.e. carports). No significant damage to well anchored structures. Some damage to poorly constructed signs. Low lying coastal roads inundated. Minor pier and marina damage. Small craft exposed to open moorings may be torn free
S2Category II Hurricane, Moderate96-110 mph
barometer = 979-965 mb (hPa), 28.91-28.50 inches; storm surge 6-8 ft
considerable damage to foliage and shrubbery, smaller trees uprooted. Major damage to exposed mobile homes. Extensive damage to poorly constructed signs. Possible damage to roofing, windows and doors. No major damage to secure buildings. Coastal roads and low-lying escape routes cut by rising waters 2 to 4 hours prior to storm arrival. Considerable damage to piers. Marinas flooded by storm surge. Small craft in open moorings ripped free from mooring. Evacuation of low-lying areas and shoreline residences required
Alma, 06/08/66, 110 mph, 970 mb
S3Category III Hurricane, Extensive111-130 mph
barometer = 964-945 mb (hPa), 28.47-27.91 inches; storm surge 9-12 ft
foliage torn from trees; large trees blown down. Practically all poorly constructed signs destroyed. Some damage to roofing and windows that are unbraced. Mobile homes unsecured destroyed. Serious flooding of coastal areas and smaller buildings destroyed along shoreline; larger structures near coast damaged by battering waves and debris. Low-lying escape routes cut by rising water inland 3 to 5 hours before hurricane center arrival. Terrain continuously lower than 5 ft above mean sea level may flood as much as 8 miles or more inland. Evacuation of shoreline and low-lying surrounding area where hurricane is estimated to come ashore may be required to be evacuated
Bob, 08/19/91, 115 mph, 953 mb
F2Tornado, Fujita Scale 2113-157 mph
strongly built schools, homes, and businesses unroofed; concrete block buildings, weak homes, and schools destroyed; trailers disintegrated
S4Category IV Hurricane, Extreme131-155 mph
barometer = 944-920 mb (hPa), 27.88-27.17 inches; storm surge 13-18 ft
shrubs and trees uprooted; all signs blown down or destroyed. Extensive damage to roofing, windows, and doors. Complete failure of roofs on smaller structures. Complete destruction of mobile homes whether secured or not. Terrain continuously lower than 10 feet above mean sea level may flood requiring massive evacuation of residences as far as 6 miles or more inland. Major damage to lower floors of large structures near shore line due to flooding and debris. Low-lying escape routes will be cut off 3 to 5 hours prior to hurricane center arrival due to flooding from storm surge. Major erosion of beachheads and coastal formations
David, 08/30/79, 150 mph, 924 mb
Hugo, 09/15/89, 140 mph, 918 mb
Andrew, 08/23/92, 150 mph, 922 mb
S5Category V Hurricane, Catastrophic>155 mph
barometer < 920 mb (hPa), 27.17 inches; storm surge >18 ft
shrubs and trees blown down and uprooted; considerable damage to roofs of all buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on several residences and industrial buildings. Extensive shattering of glass from pressure variation and blown debris. Some complete building failures. Smaller buildings are overturned or destroyed. Complete destruction of mobile homes. Major damage to lower floors of large structures less than 15 ft above sea level within 750 yards of shore. Low-lying escape routes cut off due to flooding 6 to 8 hours prior to hurricane center arrival. Massive evacuation of residential areas on low-lying ground within 5 to 10 miles of shore may be required with possible extension up to 15 miles inland
Camille, 08/18/69, 165 mph, 909 mb
Gilbert, 09/14/88, 160 mph, 888 mb
F3Tornado, Fujita Scale 3158-206 mph
strongly built schools, homes, and businesses have outside walls blown away; weaker homes completely swept away
F4Tornado, Fujita Scale 4207-260 mph
strongly built homes have all interior and exterior walls blown apart; cars thrown 300 yards or more in the air
F5Tornado, Fujita Scale 5 ("the finger of God")261-318 mph
strongly built homes are completely blown away

My introduction to meteorology came with the Life Science Library book Weather, by Philip D. Thompson, Robert O'Brien, and "the Editors of LIFE" [Time Incorported, 1965], which I acquired, about the time it was published, when I was in Junior High School. Some of the descriptions here are still drawn from that volume. I did have an actual meterology class at the University of New Mexico in 1968, but I got less out of it than I might have and failed to hold on to any class materials. Now, however, complete background to all of this can be found at The Weather Channel website. The following descriptions have been taken from that source, from Everything Weather, The Weather Channel's CD-ROM, and from other sources, web and print, that I have lost track of. A detailed history of the Beaufort Scale is now to be found in Defining the Wind, The Beaufort Scale, and How a 19th-Century Admiral Turned Science into Poetry by Scott Huler [Three Rivers Press, New York, 2004]. I was unaware of the sailing condition descriptions, or indeed of the history of the Scale, until finding this book. The Petersen State-of-Sea descriptions can be found at the British Met(erological) Office.

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The Beaufort Wind Force Scale, Note


0
calm
1
light air
2
light breeze
3
gentle breeze
4
moderate breeze/breeze
5
fresh breeze
moderate wind
6
strong breeze
fresh wind/wind
7
moderate/near gale
strong wind
8
fresh gale/gale
9
strong gale
10
whole gale/storm
11
storm/violent storm
12
hurricane
It may be silly, but it bugs me that the Beaufort terminology should jump directly from a "breeze" to a "gale." As I use the words, "wind" can refer to all movement of air, but the very same word also denotes something that is rather more than a "breeze" but still less than a "gale." I don't think of a breeze as much of a wind at all, while a gale is something quite strong and unusual, at least on land, or apart from a winter storm. A breeze is pleasant, and will not cause any inconvenience or rearrange any objects, except the lightest. A real wind, however, many be annoying, render some activities unpleasant, and will move some things around, even if only leaves, that will require attention. But then a gale:  no outside activities will be pleasant, and the wind can cause some damage.

I remedy this with the additions at left. Here we simply would have three "breezes," three "winds," and three "gales." This is how it might be done in Chinese, where many things, like rank, are divided into "high," , "middle" , and "low" , degrees. I expect that the exclusive use of "breeze" and "gale," without using "wind" for specific forces, might be to avoid ambiguity. In the traditional Beaufort Scale "wind" is not used with two meanings, the general and the specific. That is quite reasonable. My proposal then, would certainly not be for any usage where ambiguity might cause some inconvenience or danger. The table simply represents my intuition about ordinary usage and meaning, that a breeze is not quite a wind, while a gale is much more than just a wind.

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Clouds

Clouds are the mountains of the sky. They can, indeed, be taller than any mountains of the earth, reaching up to 40,000 feet or, rarely, even to 60,000 feet, far beyond Everest. On the other hand, they exist on a vastly different time scale. The tallest clouds can develop and disappear in less than a day, while earthly mountains grow and erode over millions of years. In Los Angeles, there is usually not much to be seen in the way of interesting clouds. Much of the weather is simply clear, and in the spring and summer a marine layer of fog and stratus clouds moves in. Occasionally in the summer, thunderheads develop over the San Gabriel mountains and the storms may, though sometimes not for several years, move over the Los Angeles basin. Winter storms off the Pacific, usually lasting no more than a day or two, are responsible for most of the average rainfall of 15 inches. Growing up in Los Angeles, I didn't feel like I saw much in the way of cloud variety until I lived in New Mexico, Lebanon, Hawaii, and Texas. New Mexico was especially noteworthy for the colors that would play on the clouds:  The setting sun could fill the same sky with yellow, pink, orange, and cherry red on different clouds. In Hawaii, where clouds would build up over the windward mountains daily in the rainy season, one striking memory is of the full moon shining on the towering, isolated thunderheads. There was, however, limited thunder from those clouds, which would drop some rain in the valleys and foothills and then disperse, often not even getting Waikiki wet. More violent weather came with the occasional winter storm (a "kona" storm, since the wind may blow from leeward, against the trade winds), or with the rare hurricane.

Clouds are classified by form and by altitude.

The basic forms, with symbols, are cumulus ("heaped up," in Latin), stratus ("spread out," the neuter form of which is stratum, used for extensive layers of similar rock in geology), and cirrus ("lock" or "curl" of hair). Cumulus clouds tend to form from rising air, from 6,000 feet on up, and so are classified as "vertically developing" clouds. Stratus clouds, below 8,000 feet, may be rather like an elevated fog bank; or, altenatively, fog can be thought of as a stratus cloud at ground level. While cumulus clouds mean that air has risen to an elevation where the temperature is at the dew point, so that the water condenses, with stratus clouds the temperature of the air itself may have fallen to the dew point. Cirrus clouds are ice crystals at high altitude, from 18,000 to 40,000 feet; their whispy structure comes from scattering by the wind.

Besides stratus, low level clouds can include nimbostratus (between 3,000 and 10,000 feet), from which rain falls (nimbus simply means "cloud," or "raincloud") and stratocumulus (below 8,000 feet), where cumulus clouds stretch out in a solid layer, showing a lot more structure than stratus clouds, whose outlines can be very indistinct.

At high altitudes (18,000 to 40,000), cirrus clouds can form a fairly solid layer, becoming cirrostratus, or they can take on a lumpy structure, with grains or ripples (a "mackerel sky"), suggestive of cumulus clouds, becoming cirrocumulus.

At middle altitudes, from 6,000 to 18,000 feet, too low to cumulonimbusfreeze into cirrus but higher than ordinary stratus, are altostratus (above 15,000 feet) and altocumulus. The "alto-" element is from Latin "altus," which originally meant "grown" but came also to mean "high," as it is used in these names. Altocumulus can be part of the development of cumulus and cumulonimbus clouds.

The highest -- all the way up to 60,000 feet -- most spectacular, and most violent clouds result when the air, rising to form cumulus clouds, continues to rise, thunderstorm drawing moisture to high altitudes and generating extreme conditions. This makes a cumulonimbus cloud. The swelling top spreads out into a characteristic anvil shape, which then may be blown away by high level winds. Because of freezing air at those altitudes, the streaming clouds from the anvil will often take a cirrus form. Freezing air can also cause hail to form, as ice crystals begin to fall and then are lifted back up again and again to grow larger and larger. When the hail finally falls, descriptions of its size are borrowed from the produce counter and from sports:  Small hail is described as "pea" sized, whence we move up to "grape" sized, "ping-pong ball" sized, "golf ball" sized, "baseball" sized, and, most devastating of all, "grapefruit" sized. Baseball and grapefruit sized hail can break any window, ruin a car, flatten a field of crops, or even kill somebody. The movement of wind, water, and ice up and down the column of clouds also generates differentials in electrical charge, which are then discharged as lightning. Isolated thunderstorms, generated by summer heating, may pass over with a minimum of damage. Spring cold fronts, however, pushing still cold air from the north under warm moist Gulf air, as in Texas, can produce massive squall lines of thunderstorms, stretching for hundreds of miles, filling the sky with vast gray clouds, rain, wind, hail, lightning, and, worst of all, tornadoes. These conditions get called, with charming understatement, "severe weather." Short of hurricane force winds, or actual tornadoes, nature provides no more awesome weather. During one overnight thunderstorm and flash flood that I witnessed in Austin, Texas, on the eve of Memorial Day in 1981, where serious flooding and deaths occurred, the flashes of lightning were so frequent that they were right on top of each other and the night was literally without darkness.

The Beaufort Wind Force Scale, The Saffir/Simpson Hurricane Scale, & the Fujita Scale of Tornado Intensity

Note on Dew Point

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Copyright (c) 1998 Kelley L. Ross, Ph.D. All Rights Reserved

Note on Dew Point

The dew point tells us the absolute humidity. The more commonly used "relative humidity" is the percentage to which the air is saturated with moisture. The dew point is simply the temperature at which the air would be saturated, would have 100% relative humidity. Warmer air can hold more moisture, so that air that would be saturated at 75oF, with 100% relative humidity, would only have about 50% relative humidity if the temperature rises to 95oF without any moisture being added. While people complaining of summer humidity often say that both the temperature and the relative humidity must both be 90 or more, this really does not happen and would be deadly if it did. Typical summer dew points in Texas are around 75oF, as in the example just cited. The highest dew point I've ever heard of was 80o, reported by The Weather Channel at places in eastern Pennsylvania and New Jersey as a line of thunderstorms was arriving in June 1998 (I was nearby in New Jersey -- it was humid). Typical summer dew points in Los Angeles are in the 50's. Extremely humid summer conditions in Los Angeles usually only mean dew points in the 60's. Autumn arrives in Los Angeles in October, when dry air (and sometimes a Santa Ana wind) arrives with the first cold front, dropping the dew point precipitously. October 20, 1996, the dew point in Van Nuys was reported at 37o, the 21st at 14o, and the 22nd at 9o. October 7, 1997, the dew point was reported at 39o and the 15th at 25o. October 5, 1998, the dew point was reported at 13o and the 17th at 19o. Although understanding the dew point in abstraction, its real connection to the feel of the air was not obvious to me until I began seeing dew point isotherm maps on The Weather Channel in the 1980's.

Since air can hold about twice as much moisture for every 20 degrees Fahrenheit, this may be used to write very simple equations for the dew point. In the following equations, D is the dew point in degrees Fahrenheit, T is the air temperature, and H is the relative humidity written as a whole number percentage (i.e. "50" instead of "0.5" for "50%"). The first equation gives the dew point for the temperature and relative humidity, which is usually what one can easily determine, while the second equation gives the relative humidity from the temperature and dew point.

D = T - 20*((2 - log H)/log 2)
log H = 2 - ((log 2*(T-D))/20)

C = 5*(F - 32)/9
C = (5*(F + 40)/9) - 40
F = (9*C/5) + 32
F = (9*(C + 40)/5) - 40

A more precise value for the dew point can be derived from the equations below. For the main equation, the value "X" must be calculated first, based on the relative humidty. The temperature and dew point here will be in degrees Celsius (or centigrade). Celsius and Fahrenheit can be converted back and forth with the equations in the box at right. (Note that the equations that add and subtract 40, in precisely the same way, may be easier to remember than the more traditional conversion equations.)

X = 1 -(.01*H)
D = T - (14.55+.114*T)*X - ((2.5+.007*T)*X)3 - (15.9+.117*T)*X14

If we have a temperature of 73oF (22.8oC) at 24% relative humidity, the Fahrenheit equation gives us a dew point of 3l.8oF. The Celsius equation gives us a dew point of 1.1oC, or 34.0oF. An error of 2.2oF, or 1.2oC, is not bad, especially considering how much easier the Fahrenheit equation is to use.

My favorite personal experience with dew point was when I drove from Austin, Texas, to Tularosa, New Mexico, in June 1982. A cold drink in Texas causes a large amount of moisture to condense on a glass. It "sweats"; and the water collects on the table or in a coaster, sometimes so much that it makes it seem like the glass is leaking. Arriving in New Mexico at my aunt and uncle's place, the next day I had a glass of iced tea sitting on the broad arm of a wooden chair. After half an hour, I checked the glass to make sure that there wasn't any condensing water that might damage the wood. There wasn't. At an elevation of over 4000 feet, far from the Gulf Coast, the relative humidity might only have about 5% or so.

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Copyright (c) 1998 Kelley L. Ross, Ph.D. All Rights Reserved

Snow, Sleet, Ice, and Rain

The temperatures of the air produce different phenomena of precipitation. The most familiar are snow and rain. In the former, ice crystals fall through air that is below freezing from the clouds to the surface. Snow then accumulates in drifts on the ground. The water content of snow can vary, so that some times it is dry, light, and fluffy, other times wet, heavy, and dense. Wet snows can adhere, so that drifts can appear to flow or sag without breaking apart. With rain, water droplets fall through air that is above freezing. Puddles accumulate on the ground, or the water flows away.

Other forms of precipitation occur when the air temperature changes from above freezing to below between the clouds and the surface. If the boundary occurs high enough, there is time for rain droplets to freeze before hitting the ground. This is sleet. As solid little ice pellets, sleet will tend to fall faster than snow and will hit surfaces with some noise. A snow fall is silent. Sleet can sound like small hail. When it accumulates on the ground, sleet can look a bit like snow, but the consistency and the color will look a bit different.

If the boundary between the air above freezing and below freezing is close enough to the surface, we get freezing rain. This is one of the rarer of these phenomena, and its effects are striking and dangerous. Rain drops hit surfaces and freeze. This produces a glaze of ice on amost anything and a surface of "black" ice on streets. An accumulation of ice becomes heavy. Branches, trees, and power lines can easily be brought down in an ice storm. Driving is treacherous. In the daylight after an ice storm, everything looks like it is wrapped in glass. This can be extraordinarily beautiful, but it may be purchased at a terrible price in damage and even lives.

Hail is produced when updrafts carry rain drops high into clouds, and into freezing air at altitude, even during warm times of the year. The rain drop freezes and may fall, or it may be carried up into the clouds again and receive a further coating of ice. This can happen over and over again, producing ever larger hail stones. Hail is thus usually characterized by its size, ranging from pea sized to grapefruit sized. The latter, of course, can cause sever damage or injury when it finally falls to the ground.

The Beaufort Wind Force Scale, The Saffir/Simpson Hurricane Scale, & the Fujita Scale of Tornado Intensity

Clouds

Note on Dew Point

Philosophy of Science

Home Page

Copyright (c) 2007 Kelley L. Ross, Ph.D. All Rights Reserved