Why does frost only form on clear, still nights?
The three conditions that decide whether a given temperature produces frost — the sky, the wind and the moisture — and why the same forecast number means frost one night and nothing the next.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 3 min
Short answer
The ground loses heat by radiating it to the sky. Cloud sends most of that heat straight back, so a cloudy night barely cools. Wind mixes the cold layer at ground level into the warmer air above, so a breezy night never builds one. Only when the sky is clear and the air is still does the surface cool far enough and long enough to frost — which is why a forecast a couple of degrees above freezing frosts on a clear calm night and does not on a cloudy windy one.
This is the single most useful piece of weather physics a gardener can hold, because it turns a forecast temperature from a number into a judgement. Once you know it, you stop being surprised by frost.
Step by step
- Look at the sky, not just the number.Radiation to a clear sky is what cools the ground. Cloud acts as a blanket, absorbing that radiation and returning much of it. A solid overcast night at three degrees is safe; a clear night at three degrees frequently is not.
- Look at the wind.Even a light breeze stirs the cold layer sitting on the ground into the warmer air a metre or two up, so the surface never gets as cold as it would in stillness. A frost night is nearly always a calm night.
- Consider the dew point.If the air is moist, condensation releases a little heat as it forms and slows the cooling, and you may get dew rather than frost. Very dry air cools further and faster, which is why a cold dry airmass frosts harder than a damp one at the same starting temperature.
- Notice how long the night is.Cooling is cumulative. A long midwinter night has many more hours to lose heat than a short one in late spring, which is why late spring frosts tend to be brief and shallow even when they happen.
- Use the same physics as protection.Anything overhead that intercepts radiation acts as local cloud: fleece, a cloche, a sheet of newspaper, even the canopy of a tree. This is why a plant under an open-sided shelter escapes frost while an identical one a metre away in the open does not.
Tips
- The classic frost setup in Britain is a ridge of high pressure with clear skies and light winds after a cold front has passed. If that is the forecast, believe in frost regardless of the daytime temperature.
- Cloud arriving part-way through the night stops the cooling at whatever temperature it had reached. This is why forecasts hedge — a difference of an hour in cloud arrival changes the outcome.
- Frost that forms on some surfaces and not others is telling you about those surfaces' view of the sky. Under a tree, under a bench and along a wall are all sheltered from radiation loss.
- The coldest moment is usually just after sunrise, not at midnight — cooling continues until the sun actually starts putting energy back in.
Common mistakes
- Treating the forecast minimum as the temperature your plants will experience — It is a screen temperature under standard conditions. What a leaf reaches on a clear still night depends on how much open sky it can see, and can be several degrees lower.
- Taking fleece off in the evening because the forecast is above freezing — The forecast is a screen figure. If the sky is clearing and the wind is dropping, leave it on — those are precisely the conditions that turn a mild forecast into a frost.
Questions people ask
Why is there frost in the garden but not on the road?
Tarmac stores a lot of daytime heat and releases it slowly all night, and roads are often flanked by buildings that block their view of the sky. Grass has almost no heat store and a clear view upward, so it frosts first.
Can I stop frost by watering the garden in the evening?
Moist soil holds and releases more heat than dry soil, so on a marginal night a well-watered bed is slightly warmer than a bone-dry one. It is a small effect and it is not a substitute for a cover, but it is real and free.
Does spraying plants with water protect them, like commercial growers do?
Commercial frost irrigation works by continuously freezing water on the plant so the latent heat released keeps the tissue at freezing rather than below. It only works if the spraying is continuous throughout the whole frost. Started and stopped, it makes the damage worse, and it is not practical in a garden.