When surface temperatures fall below freezing and the dew point is also below freezing, water vapor can deposit directly as ice crystals. The white coating that appears on grass, windows, and car roofs is frost.

Clear, calm nights favor frost because radiational cooling is strongest under those conditions. Objects cool faster than the air, so frost can form even when the official air temperature is slightly above freezing. Hoar frost grows delicate feathery crystals; rime frost builds up when supercooled droplets freeze on contact.

Frost damages tender plants and can make roads slippery. Farmers and orchardists often use wind machines or heaters to protect crops on frost nights. The first and last frost dates of the season help define the local growing season.

Once the sun rises, the ice usually sublimates or melts quickly.