Definition of "hoarfrost"

[…] It is moſt remarkable on the ſurface of ſnovv, vvhen covered vvith hoarfroſt, and on other bodies vvhere hoarfroſt moſt abounds. In ſuch caſes, I have often found the ſurface to be ſeven degrees colder than the air at the diſtance of tvvo feet; […]

1785, Patrick Wilson, “[Account of Some Experiments on Cold]”, in Andrew Duncan, compiler, Medical Commentaries, for the Years 1783–84. […], volume IX, London: […] J[ohn] Murray, […], and C[harles] Dilly, […]; Edinburgh: W. Gordon and C. Elliott, section III (Medical News), page 426

This striking fact, of the lower temperature of the surface on which dew and hoarfrost are forming, it was always found difficult to explain; for the transition of a body from the state of vapour to the fluid or solid form is always accompanied with an evolution of heat, […] The surface is not warmer in consequence of the formation of the dew or hoarfrost, but the dew or hoarfrost is deposited because the surface is previously cold; and its low temperature, compared with that of the air above, is owing to the discharge of heat by radiation.

1817, John Murray, “Of Caloric”, in Elements of Chemistry. ; London: Longman, Hurst, Rees, Orme, & Brown, book II (Of Repulsion, and the Powers by which It is Produced), section III (Of the Communication and Diffusion of Caloric), page 146

However, high mountains with a long winter and long dry spells, hoarfrosts and infertile soils are not an ideal environment for some annual crops but are well suited for perennial industrial crops and forest species.

2005, Manh-Cuong Pham, “Conclusions and Policy Implications”, in Land-use Change in the Northwestern Uplands of Vietnam: Empirical Evidence from Spatial Econometric Models and Geo-referenced Analyses and Policy Implications for Sustainable Rural Development, Göttingen: Cuvillier Verlag, page 331