öökülmad

Öökülmad, also known as frost or ground frost, refers to a phenomenon where the ground surface temperature drops below freezing, even if the air temperature remains above freezing. This typically occurs on clear, calm nights when there is minimal cloud cover and wind. Heat radiates from the ground into the atmosphere, causing the surface temperature to fall.

The presence of dew or moisture on surfaces is a prerequisite for öökülmad. When temperatures drop below freezing, this moisture freezes, forming ice crystals. These crystals can appear as a delicate white coating on grass, leaves, and other exposed objects. Öökülmad are more common in low-lying areas or valleys where cold air can settle and accumulate.

While air temperature might be a few degrees above zero Celsius, the ground temperature can fall significantly lower due to efficient radiative cooling. This can have important implications for agriculture, as even light öökülmad can damage sensitive plants, especially during the spring when plants are just beginning to grow and are more vulnerable. Farmers often use various methods to mitigate the effects of öökülmad, such as irrigation or wind machines. The term is widely used in Estonia and other Baltic countries.