Reading a wind forecast: knots, gusts and direction

What a knot is

A knot is a speed of one nautical mile per hour. A nautical mile is 1,852 metres, so 1 knot is 1.852 km/h, or about 1.15 mph. Because a knot is already a speed, "knots per hour" is not a meaningful phrase.

Rough conversions are enough on the water. To get km/h, double the knots and take a little off: 10 knots is about 18.5 km/h. To get mph, add about 15%: 10 knots is about 11.5 mph. For metres per second, halve the knots: 10 knots is roughly 5 m/s.

Sustained wind and gusts

Wind is never steady. The sustained or mean wind is the speed averaged over several minutes, by convention at a height of 10 metres (33 feet) above the surface. The averaging period differs between weather services, which is one reason two sources can quote slightly different figures for the same wind. A gust is a brief peak lasting only a few seconds.

The gust figure matters because the force of the wind rises with the square of its speed. A 20 knot wind pushes four times as hard as a 10 knot wind, not twice as hard, and a gust that is 40% faster than the mean pushes about twice as hard. For a kayak, a paddleboard, a dinghy or a small open boat, it is the gusts that knock you off balance, overpower a sail or stop you making headway. Plan around the gust figure rather than the average.

The gap between the two numbers is informative in itself. A forecast of 10 knots gusting 25 describes a much harder day than a steady 15.

Direction means where the wind comes from

Wind direction is named for where the wind blows from. A northerly wind comes from the north and pushes things south. A south-westerly comes from the south-west. Forecasts may also give direction in degrees measured clockwise from north, so 90 is an easterly, 180 a southerly and 270 a westerly.

Many forecast maps draw wind arrows pointing the way the air is moving, so a northerly appears as an arrow pointing down the page, but conventions differ and it is worth checking the legend. Currents and tidal streams, by contrast, are described by the direction they flow towards.

Direction has to be set against the shape of the coast, which is covered in the article on onshore, offshore and cross-shore wind.

The Beaufort scale as a translation

The Beaufort scale runs from Force 0 (calm) to Force 12 (hurricane force) and links wind speed to the appearance of the sea. It turns a forecast number into a picture, and lets you estimate the wind by looking at the water.

Force 2, light breeze, 4 to 6 knots: small wavelets with glassy crests that do not break.

Force 3, gentle breeze, 7 to 10 knots: large wavelets, crests beginning to break, scattered white horses (whitecaps).

Force 4, moderate breeze, 11 to 16 knots: small waves becoming longer, fairly frequent white horses.

Force 5, fresh breeze, 17 to 21 knots: moderate waves, many white horses, a chance of some spray.

Force 6, strong breeze, 22 to 27 knots: large waves begin to form, white foam crests are extensive, spray is likely.

Force 8, gale, 34 to 40 knots: moderately high, long waves, with foam blown in well-marked streaks along the direction of the wind.

These descriptions apply to open water after the wind has been blowing for some time. Sheltered water looks calmer than the wind really is, so white horses appearing in a harbour or on a small lake mean a good deal of wind. For many paddlers and small open boats, conditions start to become hard work around Force 4, well before anything a forecast would call strong, though the limit depends on the craft, the water and the person.

A forecast is an average over an area and a time

A forecast figure from a weather model represents a block of the atmosphere several kilometres across, over a time step that is often an hour or more. It cannot show the lull behind a headland or the acceleration through a gap between islands, and the timing of a change can easily be an hour or two out.

Wind over open water is also usually stronger than over nearby land, because water offers less friction. A forecast made for a town a short distance inland will often understate what is happening out on the water. The reasons different sources give different numbers are explained in the article on why forecasts disagree.

Putting it to use

Read the gust figure first and ask whether you could cope if the wind stayed at that speed.

Check the direction against the coast you will be on, and think about where the wind would push you if something went wrong.

Look at how the wind changes through the day, not only at the hour you plan to launch.

If the water looks windier than the forecast said, believe the water.

CatchWindow location pages show forecast wind speed, gusts and direction with a week-ahead view, which is useful for spotting the calmer days and the general trend. For decisions about going afloat, use the official marine forecast and any warnings for your area as well.