Heat at outdoor events
Summer shows, festivals and fairs put crowds in the sun for hours. This guide sets out how the Met Office defines a heatwave and what UKHSA asks event organisers to plan for.
What the Met Office counts as a heatwave
The Met Office sets a heatwave temperature threshold for each county. A heatwave is at least three days in a row with a daily maximum temperature at or above that threshold.
The Royal Meteorological Society notes that the county thresholds run from 25°C to 28°C. They were updated in 2022.
Sources: Met Office: How do we define a heatwave, and will we have one next week?; Royal Meteorological Society: MetMatters: Heatwaves
What UKHSA advises for events
UKHSA publishes hot weather advice for mass gatherings and events. It asks organisers to plan ahead.
- Identify hot weather as a potential risk in the event risk assessment, with plans in place to reduce its impact.
- Provide enough drinking water throughout the event.
- Have a plan to increase shaded areas.
- Plan activities for cooler times of day, such as the early morning or the evening.
- Watch the Heat-Health Alert system, which gives yellow, amber and red alerts.
Source: UK Health Security Agency: Hot weather advice: mass gatherings and planning events
Where a forecast fits
A heatwave is defined over three days. One hot afternoon can still matter to a crowd standing in the sun. A forecast days ahead gives time to order water and shade before the day.
How Gustline helps
Gustline's heat alert warns when the forecast maximum temperature for your site reaches your threshold, 25°C unless you change it. That is the lowest of the county heatwave thresholds. It looks at each forecast hour, not a run of three days. It comes in the same email and text as your wind alerts.
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 eventSources
- Met Office: How do we define a heatwave, and will we have one next week?
- Royal Meteorological Society: MetMatters: Heatwaves
- UK Health Security Agency: Hot weather advice: mass gatherings and planning events
Last updated 4 October 2026.
Read next
- Thunderstorm risk at outdoor events
Writing a lightning policy from PLASA guidance: plan for forecast storms, set action distances and know where people can shelter. A forecast is not detection.
- Met Office weather warnings for event organisers
What yellow, amber and red Met Office warnings mean for an outdoor event. Each colour combines impact and likelihood, so no colour means a set wind speed.
- Wind limits for marquees and tents
A marquee's wind limit comes from its design certificate, not a general figure. What MUTA guidance says about design wind load and wind management plans.
- Heavy rain and waterlogging on event sites
The Met Office counts rain as heavy above 4mm an hour. What MUTA says about saturated ground and anchors and what HSE says about wet surfaces.