How to Read Gustiness in a Forecast: Gust Values, Spread and Terrain

Two sessions in the same fifteen knots can feel like different sports. In one you ride smoothly; in the other the kite drops and then yanks. The difference is gustiness, and it is visible in the forecast — if you know where to look.

What the gust row actually shows

In Windguru and Windy a second figure sits beside the average wind speed: the gust. It is not the maximum for the day but the computed short-term peak within the same interval.

Reading it on its own is useless. Only the spread works.

• Spread up to 20 % (15 and 18 knots) — smooth wind. Comfortable and predictable.
• 20 to 40 % (15 and 20) — working gustiness. Fine, but you will use your depower.
• Over 40 % (15 and 25) — broken wind. The kite lives its own life, and teaching is pointless.

The practical rule: size your kite by the average and your readiness by the gust. If the spread is over forty percent, go a size down and work the bar.

Where gustiness comes from

Three main causes, and they show up in the forecast differently.

Thermals. Heated ground releases warmth in vertical currents that stir the air. Hence the daytime peak of gustiness in hot weather and its decline towards evening. Thermal gustiness is predictable by time of day.

Terrain. Wind flowing over a hill, a treeline or a building breaks into a turbulent wake, and that wake extends roughly ten to twenty times the height of the obstacle. A twenty-metre stand of trees spoils the wind for two to four hundred metres behind it. None of this is in the forecast: the model does not know about your trees.

Atmospheric instability. Cold air over a warm surface mixes vigorously. This is the case where, after a front passes, the wind strengthens and turns ragged at the same time.

What a forecast will never show

Models compute the atmosphere on a grid of several kilometres. Anything smaller than a grid cell is smoothed away.

So forecasts miss: turbulence behind a specific obstacle, local sea-breeze circulation, wind acceleration through a gap between hills, and shoreline effects.

Which leads to a conclusion that saves wasted trips: a forecast describes the flow, not your spot. The correction for the spot is yours to make, and you refine it with every session.

Direction matters more than speed

The most underrated row in any forecast.

The same spot gives smooth flow on an onshore wind and broken flow on an offshore one, because the air has passed over land, buildings and trees. The difference in wind quality between those two cases is larger than the difference between 15 and 20 knots.

That is why experienced riders check direction first and speed second. Eighteen knots from the wrong side is worse than twelve from the right one.

What to do about it

Plan the time of day. Thermal gustiness peaks in the middle of the day. Mornings and evenings are smoother.

Get further from shore. A turbulent wake decays with distance: a hundred metres further out is already different wind.

Go smaller. In broken wind a small kite with power in reserve is more comfortable than a big one at its limit: you keep your depower range.

Check stations, not only models. Live readings from the nearest weather station show gustiness more honestly than any forecast.

Common questions

What spread between average and gust is normal?

Up to 20 % is smooth, 20 to 40 % workable, over 40 % broken. Only the first range is suitable for lessons.

Why is the wind at my spot stronger than forecast?

Usually local acceleration: a gap between hills, a headland or a shoreline effect. The model cannot see it.

When is the wind smoother, morning or evening?

Usually both, because thermal activity is weaker. Mixing is strongest in the middle of the day.

Is gustiness more dangerous than strong wind?

For a beginner, yes. A steady twenty-five knots is predictable; a ragged fifteen is not.

Which forecast is best for gusts?

Local models on a fine grid usually sit closer to reality than global ones. But none of them will see turbulence behind a specific obstacle.

What to read next

How to read a forecast at all — Windguru and Windy. On sea breeze and thermal wind — a separate article. Kite sizing by wind — the size table.