weather term
Frost
A thin layer of ice crystals that forms on surfaces when the air temperature at ground level drops to 0°C or below.
Frost forms in broadly the same conditions as radiation fog, clear, calm nights that let heat radiate away from the ground quickly, which is why the two often occur together. Ground-level temperatures can be several degrees colder than the official air temperature measured at the standard height of around 1.5 to 2 metres, so frost is possible even when the forecast temperature stays just above freezing.