A forecast may show a wind speed of 25 km/h and gusts of 55 km/h. That is not a contradiction. The figures describe different parts of a flow that is constantly changing. Mean wind summarises the general conditions over a period, while a gust is a short, stronger burst. Both values matter when planning an activity.
What mean wind tells you
Mean wind is calculated by averaging many observations over a set interval. That interval is not identical in every meteorological service. The UK Met Office commonly uses a ten-minute average for observations, while the US National Weather Service defines sustained wind in some products using a two-minute average. Slight differences between services do not necessarily mean that one is wrong.
In an hourly forecast, wind speed normally represents the general flow expected at that time or during that period. It helps describe whether conditions will be calm, breezy or windy, but it does not by itself show the brief peaks that can move an object or destabilise a vehicle.
What a gust is
A gust is a short-lived rise in wind speed. In Met Office observations, gust speed is the highest three-second average measured within the reporting period. In forecasts, the gust figure normally represents the strongest brief speed expected during the displayed hour or interval.
Turbulence, vertical mixing and obstacles make wind gusty. Buildings, trees, slopes and cliffs distort the flow. Showers and thunderstorms can also bring faster-moving air down towards the surface. The wind may therefore seem manageable for several minutes before producing a sudden strong push.
Why the gust may matter more than the mean
- Driving: a crosswind gust can move motorcycles, bicycles, caravans, vans and trailers, especially when leaving a tunnel or passing a lorry.
- Coasts and boating: the peak can raise more spray, complicate a manoeuvre or reach a boat before the mean wind clearly changes.
- Terraces and worksites: awnings, parasols, panels, fences and light materials respond to the short maximum rather than the average.
- Trees and power lines: weakened branches may fail in a gust even when the general wind seems tolerable.
How to read a forecast properly
- Check the unit: km/h, m/s or knots. One knot equals 1.852 km/h.
- Read mean wind, gust and direction together.
- Check the timing of the maximum against your journey or activity.
- Consider exposure: a seafront, bridge or ridge is windier than a sheltered street.
- Check official warnings, whose thresholds reflect local impacts.
A forecast gust does not mean that speed will last throughout the hour or occur in every street. It is a likely maximum for the period and represented area. Terrain and buildings can strengthen or weaken the wind locally.
A practical decision rule
Use mean wind to picture the continuous conditions and the gust to assess the strongest short push you may face. If an activity is sensitive to a sudden loss of balance, side loading or loose objects, give extra weight to gusts. If an official warning is in force, follow the competent authority even when an app shows a lower number.
Technical definitions are available in the Met Office guide to how wind is measured and the National Weather Service wind-gust glossary.