Climatic condition at a temperature below 0 °Celsius or 32 °Fahrenheit. In general, the air temperature is measured at a height of 1.25 metres above the ground; at this height the temperatures are always slightly higher than on the ground. In meteorology, frost at ground level is called ground frost. Basically a distinction is made between radiation frost and adjective frost. Radiation frost occurs on dry clear nights with no wind, when the heat radiated from the ground or plant tissue escapes unhindered into the atmosphere due to the absence of clouds, fog or haze. The coldest (and heaviest) air sinks to the ground and collects on the surface and preferably in deep depressions. The adjective (meaning "added") frost is caused by the influx of already cooled air from other places. This can come from very distant areas such as a long valley.