Frozen dew or fog forming at or near ground level. Black frost is a thin sheet of frost without the white colour usually associated with frost, occurring when few or no ice crystals are formed, because air in the lower atmosphere is too dry (H. Mavi and G. Tucker 2004). Air below 0 °C is air frost. Radiation frost (sometimes called ground frost) is a ground-level fog formed by nocturnal radiational cooling of a humid air layer so that its relative humidity approaches 100%, characterized by a low vertical ceiling of the cold air layer. Gołaszewski (2004) Acta Agrophys. 3, 2 concludes that stratification of cold air temperature on spring nights with radiation frost is much stronger than that on nights with advective-radiation frost. Hoar frost, or rime, is a thick coating of white ice crystals on vegetation and other surfaces; Karlsson (2001) Met. Appls. 8 finds a correlation between the amount of hoar frost, average wind speed, and difference between dew point and road surface temperature. See also Whiteman et al. (2007) J. Appl. Met. & Climatol. 46, 1.