Külgaääri

Külgaääri is a Finnish term that translates to "cold edges" or "frosty fringes." It is used in a meteorological context to describe the specific atmospheric conditions that lead to the formation of frost on surfaces, particularly during the transition periods of autumn and spring. This phenomenon typically occurs when the ground temperature drops below freezing, but the air temperature remains slightly above freezing, or vice versa.

The formation of külgaääri relies on several factors. A clear night sky allows for radiative cooling, where heat escapes from the ground into space. If the air near the surface contains sufficient moisture, and the temperature of exposed surfaces like grass or car windshields drops to or below the dew point, then condensation can occur, followed by freezing. This results in the delicate ice crystals that characterize frost.

Külgaääri can be a sign of the changing seasons. In autumn, its appearance signals the impending winter and the end of the growing season. Conversely, in spring, it indicates that while the air may be warming, ground temperatures are still susceptible to freezing overnight. This can pose a risk to early-blooming plants and agricultural crops, necessitating protective measures such as covering them or using frost-fighting equipment. The term emphasizes the delicate balance of temperature and moisture that creates these visible signs of cold.