Editing Frost (section)

==Types==

===Hoar frost===
{{Wiktionary pipe|hoar frost#English|hoar frost}}
[[File:Frostweb.jpg|thumb|upright|A [[spider web]] covered in air hoar frost]]
[[File:HoarFrost.jpg|thumb|Hoar frost on the snow]]
[[File:LightLTSEM.jpg|thumb|Depth hoar, [[micrograph|imaged]] with [[optical microscope|optical]] (left) and [[scanning electron microscope|scanning electron]] (right) [[microscopy]]]]
{{anchor|hoarfrost_anchor}}
'''Hoar frost''', also '''hoarfrost''', '''radiation frost''', or '''pruina''', refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves.<ref>{{cite web|url=http://www.merriam-webster.com/dictionary/hoar%20frost|title=Hoarfrost – Definition of hoarfrost by Merriam-Webster|work=merriam-webster.com|url-status=live|archive-url=https://web.archive.org/web/20150219065328/http://www.merriam-webster.com/dictionary/hoar%20frost|archive-date=2015-02-19}}</ref> They form on cold, clear nights when conditions are such that [[radiative cooling#Nocturnal surface cooling|heat radiates]] into outer space faster than it can be replaced from nearby warm objects or brought in by the wind. Under suitable circumstances, objects cool to below the [[dew point#Frost point|frost point]]<ref name="RohliRohli2013">{{cite book|author1=Louisiana State University Baton Rouge Robert V. Rohli|author2=Robert V. Rohli|author3=Anthony J. Vega|title=Climatology|url=https://books.google.com/books?id=AxWEAgAAQBAJ&pg=PA381|date=13 December 2013|publisher=[[Jones & Bartlett Publishers]]|isbn=978-1-284-05427-9|pages=381–|url-status=live|archive-url=https://web.archive.org/web/20160519122238/https://books.google.com/books?id=AxWEAgAAQBAJ&pg=PA381|archive-date=19 May 2016}}</ref> of the surrounding air, well below the freezing point of water. Such freezing may be promoted by effects such as '''flood frost''' or '''frost pocket'''.<ref>{{cite web|url=http://www.weatheronline.co.uk/reports/wxfacts/Frost-hollow.htm|title=Weather Facts: Frost hollow – Weather UK – weatheronline.co.uk|work=weatheronline.co.uk|url-status=live|archive-url=https://web.archive.org/web/20130212022607/http://www.weatheronline.co.uk/reports/wxfacts/Frost-hollow.htm|archive-date=2013-02-12}}</ref> These occur when ground-level radiation cools air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.

The word "hoar" comes from an [[Old English]] adjective that means "showing signs of old age". In this context, it refers to the frost that makes trees and bushes look like white hair. <!-- Please leave out the following bit of folk etymology: "may also have association with hawthorn when covered in ... white spring blossom". The speculation is wrong. See OED! -->

Hoar frost may have different names depending on where it forms:
*'''Air hoar''' is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, and wires.
*'''Surface hoar''' refers to fern-like ice crystals directly deposited on snow, ice, or already frozen surfaces.
*'''Crevasse hoar''' consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions.
*'''[[Depth hoar]]''' refers to faceted crystals that have slowly grown large within cavities beneath the surface of banks of dry snow. Depth hoar crystals grow continuously at the expense of neighbouring smaller crystals, so typically are visibly stepped and have faceted hollows.

When surface hoar covers sloping snowbanks, the layer of frost crystals may create an [[avalanche]] risk; when heavy layers of new snow cover the frosty surface, furry crystals standing out from the old snow hold off the falling flakes, forming a layer of voids that prevents the new snow layers from bonding strongly to the old snow beneath. Ideal conditions for hoarfrost to form on snow are cold, clear nights, with very light, cold air currents conveying humidity at the right rate for growth of frost crystals. Wind that is too strong or warm destroys the furry crystals, and thereby may permit a stronger bond between the old and new snow layers. However, if the winds are strong enough and cold enough to lay the crystals flat and dry, carpeting the snow with cold, loose crystals without removing or destroying them or letting them warm up and become sticky, then the frost interface between the snow layers may still present an avalanche danger, because the texture of the frost crystals differs from the snow texture, and the dry crystals will not stick to fresh snow. Such conditions still prevent a strong bond between the snow layers.<ref name="McClungSchaerer2006">{{cite book|author1=David McClung|author2=Peter A. Schaerer|title=The Avalanche Handbook|url=https://books.google.com/books?id=0Bpscs7Gqb8C&pg=PA72|year=2006|publisher=[[The Mountaineers Books]]|isbn=978-0-89886-809-8|pages=72–|url-status=live|archive-url=https://web.archive.org/web/20160506012123/https://books.google.com/books?id=0Bpscs7Gqb8C&pg=PA72|archive-date=2016-05-06}}</ref>

In very low temperatures where fluffy surface hoar crystals form without subsequently being covered with snow, strong winds may break them off, forming a dust of ice particles and blowing them over the surface. The ice dust then may form ''[[yukimarimo]]'', as has been observed in parts of Antarctica, in a process similar to the formation of [[dust bunny|dust bunnies]] and similar structures.
[[File:Frosted Flower.jpg|alt=A photo of a flower with advection frost on the tips of its petals.|thumb|A flower with advection frost on the edges of its petals]]
Hoar frost and [[#White frost|white frost]] also occur in man-made environments such as in freezers or industrial [[refrigeration|cold-storage]] facilities. If such cold spaces or the pipes serving them are not well insulated and are exposed to ambient humidity, the moisture will freeze instantly depending on the freezer temperature. The frost may coat pipes thickly, partly insulating them, but such inefficient insulation still is a source of heat loss.

===Advection frost===
'''Advection frost''' (also called '''wind frost''') refers to tiny ice spikes that form when very cold [[wind]] is blowing over tree branches, poles, and other surfaces. It looks like rimming on the edges of flowers and leaves, and usually forms against the [[windward and leeward|direction of the wind]]. It can occur at any hour, day or night.

===Window frost===
'''Window frost''' (also called '''fern frost''' or '''ice flowers''') forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is a bad [[thermal insulation|insulator]] (for example, if it is a single-pane window), water vapour condenses on the glass, forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double-pane energy-efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles, the frost usually forms on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice [[nucleation|nucleates]]. The patterns in window frost form a [[fractal]] with a [[fractal dimension]] greater than one, but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake, which is shaped by a similar process, but forms in three dimensions and has a fractal dimension greater than two.<ref>{{cite book|last=West|first=Bruce|author2=Mauro Bologna|others=Paolo Grigolini|title=Physics of Fractal Operators|publisher=Springer|date=2003|page=46|isbn=978-0-387-95554-4|url=https://books.google.com/books?id=EgyTpQZOga0C&pg=PA46}}</ref>

If the indoor air is very humid, rather than moderately so, water first [[condensation|condenses]] in small droplets, and then freezes into [[clear ice]].

Similar patterns of freezing may occur on other smooth vertical surfaces, but they seldom are as obvious or spectacular as on clear glass.

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===White frost===
'''White frost''' is a solid [[deposition (phase transition)|deposition]] of ice that forms directly from water vapour contained in air.

White frost forms when relative humidity is above 90% and the temperature below −8&nbsp;°C (18&nbsp;°F), and it grows against the wind direction, since air arriving from [[windward and leeward|windward]] has a higher humidity than leeward air, but the wind must not be strong, else it damages the delicate icy structures as they begin to form. White frost resembles a heavy coating of hoar frost with big, interlocking crystals, usually needle-shaped.

===Rime===
{{Main|Rime ice}}

'''Rime''' is a type of ice deposition that occurs quickly, often under heavily humid and windy conditions.<ref>{{cite web|url=http://www.merriam-webster.com/dictionary/rime|title=Rime – Definition of rime by Merriam-Webster|work=merriam-webster.com|url-status=live|archive-url=https://web.archive.org/web/20150501193835/http://www.merriam-webster.com/dictionary/rime|archive-date=2015-05-01}}</ref> Technically speaking, it is not a type of frost, since usually [[supercooling|supercooled]] water drops are involved, in contrast to the formation of hoar frost, in which water vapour desublimates slowly and directly. Ships travelling through Arctic seas may accumulate large quantities of rime on the rigging. Unlike hoar frost, which has a feathery appearance, rime generally has an icy, solid appearance.

===Black frost<span class="anchor" id="Black frost"></span><span class="anchor" id="Killing frost"></span>===
[[File:Plants affected by Below Freezing Temps.jpg|thumb|Dead plant leaves during [[Winter Storm Uri]] in a backyard in [[Northern Mexico]], with below freezing temperatures.]]
'''Black frost''' (or "killing frost") is not strictly speaking frost at all, because it is the condition seen in crops when the humidity is too low for frost to form, but the temperature falls so low that plant tissues freeze and die, becoming blackened, hence the term "black frost". Black frost often is called "killing frost" because white frost tends to be less cold, partly because the [[latent heat]] of freezing of the water reduces the temperature drop.