Nattfrost

Nattfrost is a meteorological term used in Norwegian weather reporting to describe frost that forms during the night when surface temperatures fall to freezing or below. It is most likely on clear, calm nights when radiative cooling causes heat to escape from the ground faster than it can be replenished by the air above. As a result, dew or moisture near the surface freezes, producing a frost layer on vegetation, roads, and other exposed surfaces.

There are different frost forms associated with nattfrost. White frost occurs when liquid water freezes on contact with a surface, while hoarfrost forms when moist air deposits ice crystals directly onto surfaces through deposition. Nattfrost tends to occur in autumn and spring in inland valleys and higher elevations, and is less common along the coast where maritime temperatures remain milder. The timing often comprises the late night to pre-dawn hours, with the risk highest on clear nights with little wind.

Impact and management: Nattfrost can pose risks to agriculture, horticulture, and even ornamental landscapes, especially for sensitive crops, blossoms, and young shoots. It can also affect transportation by creating slippery surfaces. Protective measures include covering plants with frost cloths, moving potted plants indoors, delaying sowing or transplanting, using windbreaks, and, in some cases, sprinkling water to form a protective ice layer or using other frost mitigation practices. Forecasters issue nattfrost alerts when freezing temperatures are expected, aiding planning for farming and gardening activities.