25th December 2005

Frost

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, though only at high altitude in tropical areas. Height: Mainly on the ground, but also on plants, trees, buildings and other low structures. Cause: Water vapor freezing without first forming a liquid. Associated Weather: None. Hazard Warning: Slippery roads. Damage to plants particularly fruit. Frost, like fog, tends to occur on clear nights when the absence of cloud allows heat to rapidly radiate from the ground, resulting in a significant drop in temperature. For frost to form, the temperature must fall to below freezing (i.e. below 32° F or 0° C). True frost, known as hoar frost, occurs when a thin layer of moist air near the ground cools to below freezing and immediately forms ice crystals, without first condensing as liquid (dew). These crystals will coat any cold surface including stone, grass, leaves, berries, and even spiders' webs. Sometimes, hoar frost is so thick and white that it is mistaken for snow. The ice crystals that result from hoar frost have exquisite, jewel-like patterns that branch outward from the edges of leaves and grass stems. These intricate structures are easy to see when hoar frost forms on window panes. This normally happens on the windows of an unheated house, when the exterior temperature falls to below freezing. Because moisture levels inside the house are higher than those outside, hoar frost crystals readily form on the inside of the cold window pane, coating the glass with delightful columns, plates, and feathers of frost. If condensation takes place and dew forms before the air temperature falls below 32° F (or 0° C), the water or dew simply freezes, forming solid droplets rather than delicate ice crystals. These droplets are a form of ice rather than frost, and they occur in the same way as the ice on puddles, ponds, and lakes. When temperatures fall below freezing, the water within the leaves and stems of plants will freeze. This can cause cell damage in the plants and produce a blackening of the leaves. Although this phenomenon is known in some parts of the world as black frost, it is not always accompanied by a frost. Air with a low dew point may cool to below 32° F (0° C) without reaching saturation point, which means that no water vapor is released by the air and no real frost formation can occur.

---

Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.