What Is Frost?
Frost forms when surfaces cool below 0°C and water vapour in the air deposits directly as ice crystals on those surfaces. It is not frozen dew — frost forms through deposition (vapour to ice, skipping liquid).
Conditions for Frost
- Clear skies: Clouds act like a blanket, trapping heat. Without them, heat radiates into space and surfaces cool rapidly.
- Calm wind: Wind mixes air and prevents surfaces from cooling below the air temperature. Calm nights allow ground-level cooling.
- Dew point near or below freezing: If the air is dry, there is not enough moisture to form frost even if it is cold enough.
Types of Frost
- Hoar frost: The classic white, feathery crystals on grass, cars, and fences. Beautiful and photogenic.
- Rime frost: Forms when supercooled water droplets in fog freeze on contact with surfaces. Creates thick, white coatings on windward sides.
- Black frost: Not actually visible ice — it is when temperatures drop below freezing without frost forming (too dry). Dangerous for plants because it kills tissue without warning.
Frost and Gardening
| Frost Level | Temperature | Plant Impact |
|---|---|---|
| Light frost | 0 to -2°C | Tender plants damaged |
| Moderate frost | -2 to -5°C | Most annuals killed |
| Hard frost | Below -5°C | Hardy plants at risk |
Frost Protection Tips
- Cover tender plants with fleece or cloches before nightfall
- Water soil in the afternoon — moist soil releases more heat overnight
- Plant frost-sensitive species near walls or under trees for radiant heat protection
- Park your car in a garage or under a tree to reduce windshield frost
Check overnight low temperatures on Weather Tomorrow.
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