Fog at outdoor events
Autumn and winter events open in the dark and close in the dark. This guide sets out how the Met Office measures fog and where a fog forecast helps an event team.
How the Met Office defines fog
The Met Office says fog is the name given when visibility is less than 1,000 metres. When visibility is over 1,000 metres it calls it mist.
In its forecasts for the public, fog generally means visibility under 180 metres. Dense fog is visibility below 100 metres, which the Met Office says can cause severe travel disruption.
It also says the sudden onset of coastal fog can be dangerous, because it causes disorientation as it cuts visibility.
Source: Met Office: What is fog?
Where a forecast fits
Fog matters most where people and vehicles share a route: arrivals, car parks and site traffic. A forecast of fog gives time to plan lighting, signs and stewarding before the day. Visibility can change quickly, so a forecast does not replace a look at the site.
How Gustline helps
Gustline's fog alert warns when forecast visibility at your site falls to your threshold, 1,000m unless you change it. That is the Met Office definition of fog. With that threshold it turns Red at 100m, the Met Office line for dense fog. 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
Last updated 4 October 2026.
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