Climatic condition at a temperature below 0 °Celsius or 32 °Fahrenheit. In general, the air temperature is measured at the height of 1.25 metres above the ground; at this height, temperatures are always slightly higher than at ground level. In meteorology, frost at the height of the ground is called ground frost. Basically, a distinction is made between radiation fro st and adjective frost. Radiant frost occurs on dry clear nights with no wind, when the heat radiated by 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 low-lying depressions. The adjective (meaning "added") frost is caused by the inflow of already cooled air from elsewhere. This can be from very distant areas such as through a long valley.