Frosts

Frosts are thin layers of ice crystals that form on exposed surfaces when near-surface air temperatures fall below the freezing point and the air is sufficiently humid. They most often occur on calm, clear nights when radiative cooling of the ground and vegetation lowers surface temperatures faster than the surrounding air.

Frost forms by deposition, meaning water vapor in the air changes directly into ice on surfaces rather than freezing as liquid water. The resulting appearance depends on conditions such as humidity and air movement. In many places, frost is most noticeable as a white, feathery coating on plants, machinery, and vegetation.

Two common types of frost are hoarfrost and rime frost. Hoarfrost forms when humid air is cooled and water vapor deposits directly as ice crystals, often creating delicate, needle-like or feathered shapes that rise from the surface. Rime frost occurs when supercooled droplets in fog or mist freeze on contact with surfaces, producing a rough, opaque crust that can appear milky or cloudy.

Frosts are a familiar feature in temperate regions during autumn and spring, especially in low-lying areas or at higher elevations where radiative cooling is intense. They can occur even when air temperatures are not uniformly below freezing, because the surfaces themselves may become much colder than the surrounding air.

Impacts of frost are significant in agriculture and transportation. Frost can damage or kill frost-sensitive crops, delay growth, and cause economic losses. Frost on roads and sidewalks increases slip hazards. Forecasting frost and implementing protective measures—such as coverings, irrigation, or delaying planting—are common agricultural practices.