Frost

Frost or freezing occurs when the temperature of air falls below the freezing point of water (0 °C, 32 °F, 273.15 K). This is usually measured at the height of 1.2 m above the ground surface. There is a rather subjective scale to show several degrees of frost severity: (1) slight frost: 0 to −3.5 °C (32.0 to 25.7 °F), (2) moderate frost: −3.6 to −6.5 °C (25.6 to 20.3 °F), (3) severe frost: −6.6 to −11.5 °C (20.2 to 11.3 °F), (4) very severe frost: below −11.5 °C (11.3 °F). Frost is not necessary to get ground frost or hoar frost; they can form even if air temperature is above freezing point if the surfaces have been chilled by heat emission during a cold night. [Frost (temperature). Wikipedia]