Explainer

Gusts or mean wind: which number to plan around

A wind forecast gives two numbers. They are not the same. Structure limits are written as gust speeds, so the gust is the number to compare.

Two numbers in every forecast

The Met Office explains both in its guide to its forecast tables. The speed shows the average wind speed expected at that point in time. It says the wind will strengthen and lull above and below this value.

The gust represents the highest wind speed between the time shown and the start of the next period. Strong winds are shown in bold at 25 knots (29mph) or above.

The Beaufort scale lists mean wind speeds for each force. Force 5 is a fresh breeze. The gusts inside it run higher.

Sources: Met Office: What does this forecast mean?; Met Office: Beaufort scale

Limits are written as gust speeds

MUTA's guide for fabric structures refers to the maximum service gust speed and says plans should evacuate when speeds approaching it are forecast.

The PIPA inflatable manual says winds and/or gusts greater than 24mph. Bute Park asks organisers to check both the forecast average wind speed and the forecast gust speed for the event day.

HSE's guidance for inflatables says winds above 24mph. If a document does not say whether it means the mean or the gust, compare the gust.

Sources: MUTA: Best Practice Guide 2026; PIPA: Operations Manual IS0010-200, February 2026; Bute Park, Cardiff: Event guide: structures; Health and Safety Executive: Inflatable play equipment guidance

Load rises with the square of speed

A wind management plan published by Merton Council says wind pressure is proportional to the square of wind speed. Between 12 and 24 metres per second the pressure on a structure approximately quadruples.

In plain terms, doubling the speed roughly quadruples the load. A 40mph gust puts about 1.8 times the load of a 30mph gust on the same structure.

Source: Merton Council: Appendix N: Wind Management Plan, Diynamic Festival London, 2018

Which number to plan around

Plan around the gust. Compare the forecast gust with each structure's rated gust limit. Use the mean wind to understand the weather, not to decide whether a structure is safe.

Forecasts can be wrong. Local gusts can be stronger than forecast. On the day, an anemometer at the structure is the reading that counts.

How Gustline helps

Gustline compares the forecast gust, not the mean wind, with the limit you enter for each structure. An alert means a gust is forecast within 5mph of that limit.

Gustline checks the Met Office site-specific forecast for the point you pin on the map, every six hours from a week out and every hour in the final three days. It compares the forecast gust with the rated limit you enter for each structure and checks the weather alerts you leave on for the event. When a level changes it emails and texts the people you choose. Every alert is logged with a timestamp.

Gustline is an early warning for planning. It does not replace checks on site on the day, the official warnings or your event's own plans.

Plans start at £19 for one event. A season pass is £99 a season and a year pass is £179 a year. It covers the UK only.

Set up alerts for your next event

Sources

Last updated 4 October 2026.

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