Climatic condition at a temperature below 0 °Celsius or 32 °Fahrenheit. In general, the air temperature is measured at the height of 1.25 meters 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 absence of clouds, fog or haze allows heat radiated from the ground or plant tissue to escape unhindered into the atmosphere. The coldest (and heaviest) air sinks to the ground and collects on the surface and preferentially in low-lying depressions. The adjective (meaning "attached") frost results from the influx of already cooled air from elsewhere. This can be from very distant areas such as through a long valley.