Hygrometer
The forecast low says 36°F. You have trays of seedlings outside and a decision to make about whether to haul them in, and the number itself is not much help.
Because 36°F on one night is completely safe and 36°F on another is a lost tray. The difference is a figure sitting directly underneath it in the same forecast.
Two things the forecast low does not tell you
First: surfaces get colder than the air. The reported temperature is measured about a metre and a half up, in a ventilated shelter. Leaves, trays and open soil radiate heat to the sky and end up 4 to 8°F below it on a clear, still night.
So a forecast low of 37°F routinely means frost on the ground. This is why "ground frost" and "air frost" are different terms.
Second: how far the temperature will actually fall. And this is where the dew point comes in.
The dew point is a floor
Air cannot cool much below its dew point without giving up water, and giving up water releases latent heat — nearly as fast as the ground is losing it. So cooling stalls when the temperature reaches the dew point.
Which means the same forecast low behaves completely differently depending on what is underneath it:
| Damp night | Dry night | |
|---|---|---|
| Forecast low | 36°F | 36°F |
| Dew point | 34°F | 21°F |
| What happens | Heavy dew forms at 34°F, releasing heat and halting the fall | Nothing to stop the fall |
| Surface reaches | About 34°F | Well below freezing |
| Result | Wet trays, no damage | Frost. Damage. |
The damp night saves the seedlings. The dry night does not, and it looked identical on the forecast.
The rule of thumb
Look at the forecast low and the forecast dew point:
- Dew point well below freezing (under about 27°F) — nothing will stop the temperature falling. Assume frost on exposed surfaces even if the air low is above freezing.
- Dew point near freezing (30–34°F) — dew or light frost forms at that point and cooling largely stops. Usually survivable.
- Dew point above 36°F — the temperature will not get near freezing.
Then apply the surface correction: subtract 4–8°F from the air temperature for anything low and exposed on a clear, still night.
The other two conditions
Frost, like dew, needs a clear sky and light wind.
Cloud radiates heat back down and largely prevents the surface cooling. An overcast night at 34°F is often safer than a clear one at 38°F.
Wind mixes warmer air down across the surface. A breezy night rarely produces ground frost.
So the dangerous night is the one that looks most pleasant: clear, calm and dry.
What to do about it
- Cover — fleece, cloches, even newspaper. The mechanism is blocking the view of the sky rather than insulating, which is why a single layer of fleece works far better than its thickness suggests.
- Move under something. A tree, an overhang, a wall. Anything overhead radiates warmth back down.
- Water the soil in the afternoon. Damp soil stores more heat and releases it overnight, and the extra evaporation raises the local dew point slightly.
- Get things off the ground. The coldest air pools at the lowest point, and cold air drains downhill after sunset.
The hourly forecast shows the temperature and dew point converging, so you can see whether tonight's low is a genuine floor or just a waypoint — which is the whole question when the trays are already outside.