kuurasta

Kuurasta is a Finnish term that translates to "frost" in English. It specifically refers to a thin, crystalline layer of ice that forms on surfaces when moist air comes into contact with a surface that is below the freezing point of water. This process is known as deposition or desublimation, where water vapor in the air directly turns into ice without first becoming liquid water.

This type of frost is typically delicate and feathery in appearance, often forming intricate patterns on objects like leaves, windows, and grass. Kuurasta can form under clear skies, especially on calm nights when radiative cooling causes surfaces to drop below the dew point and then the frost point. Unlike rime ice, which forms from supercooled water droplets, kuurasta is purely a result of water vapor deposition.

The presence of kuurasta is a common sign of cold temperatures and can contribute to the aesthetic beauty of winter landscapes. While it can sometimes indicate a risk of slippery conditions, it is generally a benign meteorological phenomenon. The specific conditions required for its formation, namely sufficiently low temperatures and high humidity, make it a predictable occurrence in many temperate and polar regions during colder months.