Frost
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Frost is a solid deposition of water vapor from saturated air.
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[edit] Frost
If solid surfaces in contact with the air are chilled below the deposition point (see frost point), then spicules of ice grow out from the solid surface. The size of the crystals depends on time and the amount of water vapor available.
Frost is often observed around cracks in wooden sidewalks due to the moist air escaping from the ground below. Other objects on which frost develops are those with low specific heat and high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails. The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights. It is also affected by differences in absorptivity and specific heat of the ground which in the absence of wind greatly influences the temperature attained by the superincumbent air. The formation of white frost on the indoor surface of window panes indicates low relative humidity of the indoor air, otherwise water would first condense in small droplets and then freeze into clear ice.
Because cold air is more dense than warm air, and forms close to the ground, in calm weather cold air pools at ground level. This is known as surface temperature inversion. It explains why frost is more common and extensive in low-lying areas such as valleys and hollows. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as "frost pockets". Frost can form in these areas even when the reported temperature is above the freezing point of water.
[edit] Hoar frost
Hoar frost (sometimes hoarfrost) refers to the white ice crystals, deposited on the ground or exposed objects, that form when the air is moist and surface is cold. Hoar frost is often seen on cold, clear autumn nights.
Hoar frost does not occur exclusively in nature. Hoar frost is also found in and around freezers particularly in industrial cold storage facilities. It occurs in adjacent rooms that are not well insulated against the cold and around entry locations where humidity and moisture will enter and freeze instantly depending on the freezer temperature.
Hoar frost is similar in appearance to rime ice, but the two are distinct. In formation of hoar frost, the water vapour condenses through deposition directly to solid ice whereas rime ice is formed following initial condensation into liquid droplets. Thus, hoar frost is formed when the dew point is warmer than surface but colder than the air temperature, and colder than freezing point. The formation of frost is an example of meteorological deposition.
Hoar Frost is a common cause of avalanches when it forms at the snow's surface and subsequent layers of snow fall on top of it. The layer of hoar frost consists of angular crystals that do not bond well to each other or other layers of snow, causing upper layers to slide off under the right conditions, especially when upper layers are well bonded within themselves, as is the case in a slab avalanche.
[edit] Effect on plants
Many plants can be damaged or killed by freezing temperatures, and by frost. This will vary with the type of plant and tissue exposed to low temperatures.
Plants considered to be tender, such as tomatoes, will die immediately if they are exposed to frost. Hardy plants such as radish will tolerate lower temperatures. Perennials, such as the hosta plant, will die back after first frosts and regrow when spring arrives. The entire visible plant may completely turn brown until the spring warmth, or will drop all of its leaves and flowers, leaving the stem and stalk only. Evergreen plants, such as pine trees, will withstand frost although normally all or most growth stops.
Vegetation will not necessarily be damaged when leaf temperatures drop below the freezing point of their cell contents. In the absence of a site nucleating the formation of ice crystals, the leaves remain in a supercooled liquid state, safely reaching temperatures of -4 °C to -12 °C. However, once frost forms, the leaf cells may be damaged by sharp ice crystals. Certain bacteria are particularly effective at triggering frost formation, raising the nucleation temperature to about -2 °C. In the absence of these "ice-nucleating" bacteria, frost damage can be greatly reduced [1].
The Selective Inverted Sink [2] prevents frost by drawing cold air from the ground and blowing it up through a chimney. It was originally developed to prevent frost damage to citrus fruits in Uruguay.
[edit] See also
[edit] Notes and references
- ^ Lindow, Stephen E.; Deane C. Arny, Christen D. Upper (October, 1982). "Bacterial Ice Nucleation: A Factor in Frost Injury to Plants". Plant Physiology 70 (4): 1084-1089. PMID: 16662618. Retrieved on 2006-08-08.
- ^ Selective Inverted Sink Rolex Awards site (won award in Technology and Innovation category) 1998.
