How to Predict Frost with Your Own Weather Station

A regional forecast tells you whether frost is possible somewhere in your area. Your own weather station tells you whether it is likely in your garden. Frost depends on very local conditions — dew point, wind, sky cover, terrain — so a station standing a few meters from your plants often beats the official forecast. This guide gives you a practical rule set you can check in under a minute every evening.

How can you tell if there will be frost tonight?

Check four things in the evening: a dew point below about 2–4 °C (36–40 °F), a clear or mostly clear sky, wind lighter than roughly 8 km/h (5 mph), and a temperature that is already falling steadily. When all four line up, frost by morning is very likely. A dew point above about 6 °C (43 °F) makes frost unlikely.

The dew point is the strongest single predictor, and it is a reading most people ignore. On a calm, clear night the air temperature tends to bottom out near the evening dew point, because once the air reaches it, condensation releases heat and slows further cooling. A low dew point means dry air with nothing to brake the temperature drop. If you are hazy on what the reading actually means, see dew point versus humidity explained — it is the difference between guessing and predicting.

Why does frost form on clear, calm nights?

On a clear night the ground radiates its heat straight out to space and cools quickly. Clouds would reflect some of that heat back, and wind would mix warmer air down to the surface — remove both and nothing interrupts the cooling. The surface chills toward the dew point, and if that lies below freezing, ice crystals form.

Meteorologists call this a radiation frost, and it is the type gardeners deal with most. The National Weather Service defines frost as ice crystals deposited directly from the water vapor in the air onto a surface that has cooled below freezing — no rain or drizzle required. The setup is usually a settled area of high pressure: dry air, light winds, and skies that clear out after sunset. That is one of the practical reasons to glance at what your barometric pressure trend means in the afternoon — a high, steady barometer in the cold season is a standing invitation for an overnight frost.

What is the difference between ground frost and air frost?

Air frost means the air temperature at the official measuring height of about 2 m (6 ft) drops below 0 °C (32 °F). Ground frost means ice forms on grass and other surfaces while the 2 m air temperature is still above freezing. Grass can carry frost with the air at +3 °C (37 °F).

The gap exists because the ground radiates heat faster than the air above it. On a strong radiation night the coldest layer is the few centimeters right at the surface, which is exactly where seedlings live. Your station's thermometer sits well above that layer — typically 1.5–2 m up if you followed a sensible weather station siting guide — so treat its reading as optimistic for plants. A practical margin: if your station shows 3 °C or lower on a clear, calm night, assume the grass can frost.

Which parts of a garden frost first?

Low-lying spots frost first. Cold air is denser than warm air, so it drains downhill like water and pools in hollows, valley floors, and against solid walls or fences at the bottom of a slope. These frost hollows can run several degrees colder than a bed only a few meters higher up.

The reverse is also true: some spots are naturally protected. A south-facing wall stores daytime heat and releases it overnight, and anything under a tree canopy or an overhanging eave receives some of its radiated heat back. Walk your garden on the first frosty morning of the season and note where the white patches are — that map is worth more than any forecast for deciding where the tender plants go next year.

How do you check frost risk with your own station?

Do a four-step check an hour or two after sunset: read the temperature trend, the dew point, the wind speed, and the sky. If the temperature is falling steadily, the dew point is below about 4 °C, the wind is close to calm, and the stars are out, prepare for frost.

1. Temperature trend. Open your 24-hour chart. A steady fall of around a degree per hour after sunset, with no sign of leveling off, is the radiation-night signature.
2. Dew point. Below about 2–4 °C (36–40 °F): high risk. Above about 6 °C (43 °F): relax, the moisture in the air will slow the drop before freezing.
3. Wind. Under roughly 8 km/h (5 mph), there is not enough mixing to stir warmer air down to the surface. A steady 15 km/h breeze usually keeps frost away.
4. Sky. Step outside. If you can see stars across most of the sky, radiative cooling is running at full strength. An overcast sky acts as a blanket.

Which plants need protection at what temperature?

Tender annuals — tomatoes, basil, zucchini, dahlias — are damaged at or just below 0 °C (32 °F) and should be covered whenever frost is likely. Half-hardy plants tolerate a light frost. Hardy vegetables such as kale, leeks, and spinach shrug off harder freezes, and some taste sweeter after one.

Exact tolerance varies with variety, plant maturity, and how quickly the temperature falls, so keep a margin rather than cutting it fine. Fleece, cloches, or even an upturned pot over a young plant is usually enough for a light radiation frost. The RHS guide to frost damage covers protection methods and recovery in more depth.

How do you set a frost alert?

Set a temperature alert with a threshold a few degrees above freezing — 3 °C (37 °F) is a sensible tripwire — so the warning arrives while there is still time to go out and cover plants. Pro Weather emails you the moment your own station's temperature crosses the threshold.

The alert is edge-triggered: it fires once when the temperature crosses the line, not repeatedly all night, and it is driven by the thermometer in your garden rather than a forecast for the wider region. That matters in frost-hollow gardens that freeze when the official forecast says +2 °C. Setup takes a minute in the dashboard — see the alerts documentation for the details, or the broader walkthrough on setting up weather alerts for a home station.

Common questions

At what dew point does frost form?

There is no single frost dew point, but the evening reading sets the floor. A dew point below about 2–4 °C (36–40 °F) on a clear, calm night means the air can cool to near or below freezing before condensation slows the drop — high frost risk. Above about 6 °C (43 °F), frost is unlikely.

Can frost form above 0 °C (32 °F)?

Yes — on the ground. Thermometer readings are taken about 2 m above the surface, but on clear, calm nights the ground radiates heat faster than the air and runs several degrees colder. Grass, car roofs, and seedling leaves can frost while your station still shows +2 to +3 °C. This is ground frost, as opposed to air frost.

What conditions cause frost overnight?

Clear skies, light wind, dry air, and long nights. Clear skies let the ground radiate heat to space, calm air prevents warmer air from mixing back down, and a low dew point means little moisture to slow the cooling. This combination — a radiation frost setup — typically arrives with settled high pressure in spring and autumn.

How do I set a frost alert?

Use a temperature threshold alert on your own weather station rather than relying on regional forecasts. Set it at about 3 °C (37 °F) so you are warned before the frost, not during it. On Pro Weather, add a temperature alert in the dashboard and it emails you when your station crosses the threshold.