How Frost Forms — Clear Night Science

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 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

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Check overnight low temperatures on Weather Tomorrow.