On a still, cloudless night the grass, the car roof and the twigs lose heat to the sky and cool the air above them. The Met Office explains why that can leave a white frost even when the forecast says it stayed above zero.
Pick the kind of night the forecast describes. The bar shows how likely the Met Office's rule of thumb makes a white frost on the grass and the car.
Based on the Met Office guide to frost and ice. Local spots such as hollows and open grass can be colder than the forecast suggests.
TWO NUMBERS WORTH KNOWING
Some winters bite harder.
Days of air frost across the UK as a whole, from the Met Office series that starts in 1931.
93.4 days
UK days of air frost in 2010
The Met Office's UK series counts days with a minimum air temperature below zero. 2010 has the highest yearly figure from 1931 to 2025, with 24.0 days in December alone.
MOST FROSTY · 2010
31.7 days
UK days of air frost in 2014
The lowest yearly figure from 1931 to 2025 came only four years later. For comparison, the series shows 36.0 days in 2024 and 44.6 days in 2025.
LEAST FROSTY · 2014
Source: Met Office National Climate Information Centre, UK areal series of days of air frost from HadUK-Grid 1 km data (updated 1 October 2026). The figures are averages across the UK, so a cold valley sees far more.
FOUR THINGS TO NOTICE
Read the frost on your street.
Frost forms unevenly, and the pattern tells you how the night went. These come from the Met Office, the RHS and the American Meteorological Society.
Look at the grass first
The Met Office says grass loses heat faster than tarmac, so a grass frost can form when roads and paths stay clear.
Check the car, not the thermometer
A car's thermometer reads the air. The roof and windscreen can drop a few degrees lower on a clear night and frost over above 0°C.
Notice the low spots
The RHS says cold air sinks and flows downhill on frosty nights, collecting in dips and hollows known as frost pockets.
Watch for the wet look
Glaze is clear ice that can be mistaken for a wet surface. On roads and paths it is called black ice, and the Met Office calls it highly dangerous.
AI-generated image, not a photograph of a real person or place.
RIME AND GLAZE
Ice that is not frost.
AI-generated image, not a photograph of a real person or place.
Rime grows when supercooled droplets in fog freeze on whatever they touch, so it builds into the wind on fences, twigs and wires. The US National Weather Service describes it as an opaque coating of tiny white granular ice.
Glaze forms when supercooled rain or drizzle meets the ground, or when ordinary rain falls on a surface well below 0°C. The Met Office says its weight can bring down branches and, more rarely, telephone lines.
Questions
Questions people ask
What is the difference between a ground frost and an air frost?
A ground frost means the ground or objects on it have fallen below the freezing point of water. An air frost means the air, usually measured at least one metre above the ground, is below freezing. The Met Office says ground frosts are common even when the air stays above 0°C.
Why is my car frosty when the forecast said 2°C?
Forecast overnight temperatures are air temperatures. The Met Office explains that surfaces such as a metal car roof cool first on a clear night and can end up a few degrees colder than the air, so frost can form above 0°C.
What makes hoar frost feathery?
The Met Office says feathery hoar frost forms when a surface reaches freezing point before any dew forms, so vapour turns straight to ice crystals. If dew forms first and then freezes, the result is a more globular white frost.
Is rime the same as frost?
No. Frost grows from water vapour, while rime forms when supercooled fog droplets freeze on contact with a surface they drift past. That is why rime builds up on the windward side of fences, twigs and wires.
Why are some gardens frostier than others?
Cold air sinks and flows downhill on frosty nights. The RHS says it collects in dips and hollows called frost pockets, and can pool behind hedges or fences across a slope. Gaps in the barrier can let it drain away.
