Isregn

Isregn is a meteorological term used in Norwegian to describe freezing rain, a form of precipitation consisting of liquid water that freezes upon contact with surfaces at or below 0°C, producing a glaze of ice. In English, the phenomenon is usually called freezing rain or glaze ice.

Formation of isregn occurs when a sufficiently deep layer of warm air lies above a shallow layer of subfreezing air near the surface. Snow or rain falls from above into the warm layer, melting into liquid rain. As the drops descend through the cold layer, they become supercooled but do not freeze until they hit a surface that is at or below freezing, where they instantly form an ice coating.

Isregn must be distinguished from sleet, where raindrops freeze into ice pellets before reaching the ground, and from hail, which consists of layered ice formed within strong updrafts in thunderstorms. The resulting ice glaze tends to be smooth and transparent, making it difficult to see on roads and vegetation.

Impacts of isregn include hazardous travel conditions, ice accumulation on roadways, power lines, and trees, and potential damage to infrastructure. The glaze can persist for hours after precipitation ends, depending on surface temperatures and sunlight.

Forecasting isregn involves assessing the presence of a warm atmospheric layer aloft over a shallow subfreezing layer near the surface. Meteorologists use surface observations, vertical sounding, radar, and weather models to evaluate the likelihood and potential severity of freezing rain events.

See also: Freezing rain, glaze ice, sleet, rime.