There are many circumstances where it is impossible to measure the wind speed
precisely, and yet studying its effects on the surroundings enables a useful
estimate to be made. Nowhere is this more evident than at sea, and in 1805
Captain Beaufort devised a simple numerical scale of wind forces in terms of the
resulting state of the sea.
(if you want the wind speed in other units,
common conversions are here)
| Force |
Speed (kn) |
Speed (mph) |
Description |
On Land |
At Sea |
| 0 |
0 |
0 |
Calm |
Smoke rises vertically |
Sea like a mirror |
| 1 |
1 - 3 |
1 - 3 |
Light air |
Smoke drifts. Wind vane still. |
Small ripples. |
| 2 |
4 - 6 |
4 - 7 |
Light breeze |
Wind felt on face. Leaves rustle, vane moves. |
Small wavelets, glassy crests |
| 3 |
7 - 10 |
8 - 12 |
Gentle breeze |
Leaves and small twigs in constant motion. |
Large wavelets. Crests break. Glassy foam. |
| 4 |
11 - 16 |
13 - 18 |
Moderate breeze |
Raises dust; moves small branches. |
Small waves but fairly frequent "white
horses". |
| 5 |
17 - 21 |
19 - 24 |
Fresh breeze |
Small trees in leaf begin to sway. |
Moderate waves, more pronounced long form.
Many "white horses" |
| 6 |
22 - 27 |
25 - 31 |
Strong breeze |
Large branches in motion. |
Large Waves. Extensive foam crests. Probably
some spray. |
| 7 |
28 - 33 |
32 - 38 |
Near Gale |
Whole trees in motion. |
Sea heaps up. White foam streaks along wind. |
| 8 |
34 - 40 |
39 - 46 |
Gale |
Breaks twigs off trees. Impedes progress. |
Moderately high waves of greater length.
Edges of crests begin to break into spindrift. Foam blown in well marked
streaks in the wind. |
| 9 |
41 - 47 |
47 - 54 |
Severe Gale |
Slight structural damage possible. Tree
branches may break. |
High waves. Dense foam streaks along wind.
Wave crests begin to topple and roll over. Spray may effect visibility. |
| 10 |
48 - 55 |
55 - 63 |
Storm |
Seldom experienced inland. Trees uprooted.
Considerable structural damage. |
Very high waves with long overhanging crests.
Dense large patches of foam blown along wind. Sea surface white. Visibility
reduced. |
| 11 |
56 - 63 |
64 - 72 |
Violent Storm |
Rarely experienced. Widespread damage. |
Exceptionally high waves. Sea covered with
long white patches of foam along wind. Everywhere edges of wave crests blown
to froth. Visibility much reduced. |
| 12 |
64+ |
73+ |
Hurricane |
Widespread severe damage. |
Air filled with foam and spray. Sea
completely white with driving spray. Visibility seriously affected. |
| One Knot |
= 1.15078 Miles per hour |
| |
= 1.85200 Kilometres per hour |
| |
= 0.51444 metres per second |
| |
= 1.68781 feet per second |
| |
|
| One Mile per hour |
= 0.86898 Knots |
| |
= 1.60934 Kilometres per hour |
| |
= 0.44704 metres per second |
| |
= 1.46667 feet per second |
| |
|
| One Kilometre per hour |
= 0.53996 Knots |
| |
= 0.62137 Miles per hour |
| |
= 0.27778 metres per second |
| |
= 0.91134 feet per second |
| |
|
| One metre per second |
= 1.94384 Knots |
| |
= 2.23694 Miles per hour |
| |
= 3.60000 Kilometres per hour |
| |
= 3.28084 feet per second |
| |
|
| One foot per second |
= 0.59248 Knots |
| |
= 0.68182 Miles per hour |
| |
= 1.09728 Kilometres per hour |
| |
= 0.30480 metres per second |
