Beaufort scale

From Boat Design Wiki

The Beaufort scale is used by mariners and other professionals to give some degree of uniformity to descriptions of weather conditions.

Wind and sea conditions are described by reference to a Beaufort number. For example, a sailor referring to a "Force 7 storm" is talking about conditions that, based on wind speed and wave height, correspond to Beaufort force number 7.

General discussion, about Beaufort force numbers are meaningless (to those not familiar with the scale) without some references and indications of wind strength. Once a sailor (or landlubber) has made a conscious connection between the wind strength, wave height, and the general feeling of the weather at each level, the scale becomes a convenient way to describe a wide range of conditions.

Force Numbers

0

Mean wind speed is less then 1 knot, described as Calm Air. The sea is mirror like in appearance, without ripples and smoke will rise straight up. Powerboats and rowing craft find this an ideal condition.

1

Wind speed is 2 knots or less at sea level, 3 knots at 30 feet above sea level and described as Light Air. Fish scale looking ripples begin to form, but no wave crests, leaves on trees begin to move and sailboats begin to make way. Average wave height is one foot or less.

2

Mean wind speed is 5 knots, 6 at a 30' height and described as a Light Breeze. Small wavelets forming with glossy crests, but not breaking, having an average height of under 2 feet and the water has a noted darkening of color. Flags begin to unfurl, yacht tenders become affected by waves, sailing dinghies move to the windward rail, sailing cruisers carry a full press with a light genoa and small powerboats are still comfortable on plane.

3

Wind speed is 9 knots, with 10 at 30' above sea level and described as a Gentle Breeze. Wavelets begin to break with some foam and an occasional white horse rearing up. Powered yacht tenders begin to reduce speed, sailing dinghy crews have to hike out, sail cruising craft change to a heavier genoa, but in general a sailors delight. Average wave height is just over 2 feet.

4

Mean wind strength is 13 knots with 16 at a 30' height and described as a Moderate Breeze. Small waves form, becoming longer with a healthy mix of white horses, longer tree branches begin to sway and dust is stirred up. Small, powered yacht tenders prefer to stay ashore, small powerboats begin to experience unpleasant motion, sailing dinghies are in full hike or using a trapeze and cruising boats change the genoa to a jib, possibly a mainsail reef or are strongly considering one. Wave height is 3 feet.

5

Wind speed is 19 knots, less then 21 knots at 30' above sea level and described as a Fresh Breeze. Waves lengthen, many white horses with some scattered spray off the tops and power lines begin to whistle. Small craft reduce speed and turn into the sea, if unable to find shelter. Sailing dinghies reef while larger cruisers hand the mizzen and change to a smaller jib. Average wave height is 5 feet.

6

Mean wind speed is 24 knots with 27 at a 30' height and is described as a Strong Breeze. A small craft advisory will have been issued. Large waves begin to form, most with foam crests, spray is present. All trees are moving and louder whistling from power wires. Larger powerboats becoming quite uncomfortable, all others taking to shelter. Ironmen, in sailing dinghies begin to fight the affects of the sea, reefed mainsail and small jib. Smaller sailing cruisers are down to their number 2 jib and full reefed main, larger cruisers have two reefs in the mainsail and smaller jib. Wave height is nearing 9 feet.

7

Wind speed is 30 knots, around 34 knots at a height of 30' and is described as a Moderate Gale (a Near Gale in other places). The sea begins to pile up with breaking waves, wind blown streaks of white foam and trees are violently moving. Large power craft heave to or find shelter. The dinghy ironmen are under full reef (reefed main only or jib only), with larger sailing craft under storm jib and fully reefed mainsail. Motion is becoming violent aboard most moderate size pleasure craft and efforts have been made to dog all hatches, secure all lockers and stow all gear that can become airborne. Average wave height is over 11 feet.

8

Mean wind strength is 37 knots with 40 knots at a 30' height and is described as a Fresh Gale (a Gale elsewhere). Waves are longer and moderately tall, with breaking crests tossed into long easily defined foam streaks and strong, eye stinging spray. Tree branches are torn down, loosely attached items dance and small objects blown about. Only larger sailing craft are remaining underway, are running before the gale or hove off. This is the beginning of "grab your socks" time folks. Wave height is 18 feet.

9

Wind speed is 44 knots, 47 at a height of 30' and is described as a Strong Gale. Waves are high and intimating with thick streaks of wind blown foam and thick spray (enough to affect vision). Wave tops crash and the sea rolls, loose brick work falls out as do weak trees topple, some minor roof damage possible, in short it's now dangerous to be at sea, with large vessels hove to or running downwind, the saltiest aboard are affected by the motion. The waves are over 21 feet.

10

Mean wind speed is 52 knots with 55 at a height of 30' and is described as a Whole Gale (Storm elsewhere). Very tall waves with lingering crests, visibility very reduced with the sea looking near completely white from wind born foam and spray. Violent rolling sea with a hammering aggressiveness. Roofs are damaged, small trees torn whole from the ground, spray is so strong it feels like a thousand dinner forks have attacked all at once on exposed skin. To hell with a St. Christopher's medal, you want the guy himself sitting in you lap, if for nothing else, but to hold your butt down. Wave height is 27 feet.

11 and 12

Wind strength is MEAN and conditions are difficult to describe. Visibility at sea level is next to nil, wave height is exceptional. Nothing but white out conditions, mooring lines snap, breakwater and levy damage, trees are up rooted, roof and building damage, large objects can become airborne and ships can suffer structural damage. Wave height at force 11 is 35 feet and at 12 they are 40 feet plus. Having been through several hurricanes, the motion is indescribable, the noise, wind and violence nearly unbearable.

PAR 4/13/07

Corresponding Wikipedia article: Beaufort scale