Weather guide

Ice and frost on event sites

Winter markets and events put people on paths, ramps and grass in the coldest months. This guide sets out what the Met Office means by each kind of frost and ice and what HSE says about walkways.

Air frost and ground frost

The Met Office says an air frost occurs when the air temperature falls to or below the freezing point of water. It is usually measured at a height of at least one metre above the ground.

A ground frost is ice forming on the ground, objects or trees whose surface is below freezing. The Met Office says a ground frost can occur without an air frost when the ground cools quicker than the air. Its fact sheet on water in the atmosphere puts it plainly: a ground frost may occur when the air temperature does not get down to freezing point.

Surfaces differ too. The Met Office notes that concrete and road surfaces hold on to warmth better than grass, so grass can frost when they do not.

Sources: Met Office: What is frost and ice?; Met Office: National Meteorological Library fact sheet 3: Water in the atmosphere (archived copy)

Glaze and black ice

Glaze is a smooth, transparent ice that forms when drizzle or rain hits a cold surface. The Met Office says it can be mistaken for a wet surface and can be highly dangerous. Glaze on roads and pathways is often called black ice.

Source: Met Office: What is frost and ice?

What HSE says about walkways

HSE says cold weather can cause frost and ice to form, which may create slippery surfaces. Its advice on icy conditions is practical.

  • Grit when frost, ice or snow is forecast. Grit too when walkways are likely to be damp or wet and the ground is at or below freezing.
  • The best times to grit are early in the evening before the frost settles and early in the morning before people arrive.
  • Start with the places most likely to ice: entrances, car parks, walkways, shortcuts, slopes and areas that stay in shade or wet.
  • Divert people to less slippery routes and barrier off the others. Cover smaller areas overnight with an insulating material.
  • Take warning cones away once the ice or snow has gone. Otherwise people will ignore the signs.

Sources: Health and Safety Executive: Slips and trips: frequently asked questions; Health and Safety Executive: Slips and trips: environment

Snow on marquees

Snow on a fabric roof is a different risk from ice underfoot. MUTA's advice on heating marquees when snow is possible is in the snow guide.

Where a forecast fits

A forecast gives the air temperature for your site. It cannot tell you the temperature of your paths. Because the ground can freeze before the air does, treat a forecast near freezing as the prompt to plan gritting and check surfaces on the day.

How Gustline helps

Gustline's ice and frost alert watches the forecast air temperature for your site and warns when it is forecast to fall to your threshold, 1°C unless you change it. It is worded as freezing conditions and ice risk because the forecast has no ground temperature. 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 event

Sources

Last updated 4 October 2026.

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