What is the difference between an air frost and a ground frost?
The two frosts a forecast reports, why ground frost happens far more often and matters more to seedlings, and how to read a forecast that mentions one but not the other.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 3 min
Short answer
Air frost is measured at standard screen height, roughly a metre and a bit above the ground, and means the air itself has reached freezing. Ground frost is measured at ground level, where surfaces radiate heat away fastest and get colder than the air above them. Ground frost happens far more often, arrives earlier in autumn and lasts later into spring — so a forecast that says no air frost can still take out your seedlings, your strawberries and your bedding.
This distinction sounds like meteorological pedantry until the first time a forecast promises no frost and you go out to blackened courgettes. The difference between the two measurements is routinely several degrees, and everything low-growing lives in the colder one.
Step by step
- Know where each is measured.Air temperature in a forecast comes from a thermometer in a ventilated screen about 1.25 metres above short grass. Grass minimum, which is what produces a ground frost report, comes from a thermometer lying at the top of the grass itself.
- Understand why they differ.On a clear still night the ground radiates its heat straight out to the sky and cools fast. The thin layer of air resting on it cools with it. Air a metre higher up is not in contact with that cold surface and stays warmer, which is why the screen reading is higher.
- Translate a forecast.If the forecast minimum is a couple of degrees above freezing and the night is clear and still, expect ground frost. If the forecast minimum is at or below freezing, expect both, and expect the ground frost to be harder than the number suggests.
- Work out which one affects each plant.Anything at or near ground level — seedlings, salad, strawberries, newly planted bedding, low herbaceous growth — experiences the ground frost. Anything up on a stem, on a wall or in a raised container is closer to air temperature.
- Use height as protection where you can.Standing a tray of seedlings on a bench rather than on the ground genuinely raises its minimum temperature. So does bringing pots up onto a wall or a table on a marginal night.
Tips
- Published frost dates are usually air frost dates. Your effective ground-frost season is longer at both ends, sometimes by weeks.
- A concrete or paved surface holds heat better than bare soil, and bare soil better than grass or mulch. That is why the lawn frosts white while the path beside it does not.
- Frost on car roofs and shed roofs but not on the ground means the air is close to freezing and radiation losses are doing the work — the classic marginal night.
- A cloche or a sheet of fleece raises the ground-level minimum by intercepting the radiation loss. That is exactly why it works and why an open-sided shelter still helps.
Common mistakes
- Reading a forecast minimum of two or three degrees as safe for tender seedlings — That is the screen figure. At ground level, on a clear still night, the surface can be at or below freezing while the screen reads several degrees above it.
- Assuming a frost that did no visible damage to shrubs did none to what was underneath them — The shrubs were in warmer air. The seedlings at their feet were in the cold layer, and cold damage often does not show for a day or two.
Questions people ask
Which one is a 'frost warning' about?
Public frost warnings are generally about air frost and about roads and people rather than plants. For gardening you want the grass minimum if the forecast provides it, and the sky-and-wind conditions if it does not.
Is a hard frost different again?
It is an informal term for a frost cold enough and long enough to freeze the ground rather than just to whiten surfaces. It is a matter of degree and duration rather than a separate measurement.
Why does frost form when the air is above freezing?
Because the surface it forms on is not. A leaf, a car roof or a blade of grass radiating heat to a clear sky can drop below freezing while the air around it stays above, and the moisture in contact with that surface freezes.