Wind is moving air. It brings clouds, rain, warmth or cold, and it is the most intuitive of weather quantities: everyone knows what a light breeze or a gale is. To speak precisely you need a direction, a mean speed, gusts and a shared scale, the Beaufort scale.
Direction, mean speed and gusts
A wind's direction is the one it comes from: a westerly blows from the west towards the east. Speed is given in kilometres per hour, in metres per second (1 m/s equals 3.6 km/h) or in knots, the sailors' unit (1 knot equals 1.852 km/h).
Wind is never steady. Stations measure, at a height of 10 metres, a mean speed over 10 minutes, which is the "wind" of the bulletins, and the gust, that is the maximum of the speed averaged over 3 seconds. Gusts exceed the mean wind, the more so over rough terrain or in unstable air, and it is gusts that cause the damage.
Why the wind blows
Wind arises from pressure differences: air moves from high towards low pressure, and the bigger the difference over a short distance, the stronger the wind. The Earth's rotation deflects it, to the right in the northern hemisphere and to the left in the southern, so the wind circles around pressure centres rather than heading straight for them.
A practical rule follows, Buys Ballot's law: in the northern hemisphere, if you stand with your back to the wind, low pressure is on your left; in the southern hemisphere it is on your right. Near the ground, friction slows the wind and turns it towards low pressure; that is why wind is stronger out at sea, on a hilltop or at height than in a valley bottom or a town.
The Beaufort scale
Devised in 1805 by the British admiral Francis Beaufort to estimate wind from its effect on a ship's sails, the scale has been adapted to land and to measured speeds. It has thirteen forces, from 0 to 12. The table below gives for each the mean speed in km/h and the effects you can see on land.
A few landmarks help. Up to force 3 the wind is a pleasant breeze. From force 4 to 6 it becomes a nuisance: paper flying about, branches in motion, umbrellas turned inside out. From force 7 walking becomes hard. Force 8, a gale, snaps branches; force 10 uproots trees; force 12 is a hurricane.
| Force | Speed (km/h) | Name | Effects observed on land |
|---|---|---|---|
| 0 | <1 | Calm | Smoke rises vertically. |
| 1 | 1–5 | Light air | Smoke shows the wind direction, but vanes do not move. |
| 2 | 6–11 | Light breeze | Wind felt on the face; leaves rustle. |
| 3 | 12–19 | Gentle breeze | Leaves and small twigs in constant motion. |
| 4 | 20–28 | Moderate breeze | Dust and loose paper raised; small branches move. |
| 5 | 29–38 | Fresh breeze | Small leafy trees sway. |
| 6 | 39–49 | Strong breeze | Large branches move; wires whistle. |
| 7 | 50–61 | Near gale | Whole trees in motion; walking against the wind is hard. |
| 8 | 62–74 | Gale | Twigs break off; walking against the wind is very difficult. |
| 9 | 75–88 | Strong gale | Slight structural damage: roof tiles are blown off. |
| 10 | 89–102 | Storm | Trees uprooted; considerable damage. |
| 11 | 103–117 | Violent storm | Widespread, severe damage. |
| 12 | 118 and over | Hurricane | Devastation. |
A worked example: Toronto
For Toronto, using the forecast of 2026-11-02 08:13: over the next 48 hours the strongest mean wind reaches 15 km/h, which is force 3 on the Beaufort scale (Gentle breeze). The strongest forecast gusts reach 44 km/h.
Frequently asked questions
What wind force is dangerous?
From force 8, or 62 km/h of mean wind, branches break and walking becomes very difficult; force 10 (89 km/h) uproots trees. Gusts, stronger than the mean wind, are what make it dangerous.
What is the difference between mean wind and a gust?
The mean wind is the speed averaged over 10 minutes. The gust is the maximum of the speed averaged over 3 seconds: a brief burst stronger than the mean wind.
How do I convert knots to km/h?
Multiply by 1.852: 10 knots is about 18.5 km/h. For metres per second, multiply by 3.6.
Sources
- World Meteorological Organization (WMO), Manual on Codes, WMO-No. 306: the Beaufort scale and its speed limits in kilometres per hour; Guide to Instruments and Methods of Observation, WMO-No. 8: 10-minute mean wind and 3-second gust.
- Beaufort, F., wind force scale of 1805, based on the effect on a ship's sails.
- Today's example: hourly mean wind and gusts from the Open-Meteo forecast at 10 m, over the 48-hour window.