Talk:Virga

Please help Chech Wikipedia!

Good evening (or whatever phase of day may be in your place), I'd like to beg of you for further explanation of following sentence. Please use easier words, if possible. My knowledges of English aren't so good.
Thank you very much
Milhaus, czech wikipedia User:147.251.242.166 ---

The red light can be caught by the streamers of falling precipitation, while aloft winds push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.

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Hi Milhaus,

It was not my line, but:

The streamers of rain below the cloud at sunset are visible because of the setting sun. Air movements (wind currents) below the clouds push the streamers to one side in an arcuate pattern resembling a comma below the cloud. The streamers of precipitation evaporate some distance below the clouds, typically creating an irregular whispy lower edge to the streamers of precipitation, or virga.

Hope that helps - Vsmith 02:07, 14 October 2005 (UTC)

Above copied from my talk page for reference here. Vsmith 02:33, 14 October 2005 (UTC)

Acronym

I removed "It also is an acronym for "Variable Intensity Rain Gradient Aloft", meaning the rain gradient varies in intensity dependent upon altitude. As the precipitation evaporates as it falls, its intensity lessens, hence; virga." as I've never heard of that in meteorology and seriously doubt it's veracity. The name comes the derivation from Latin. Dan100 (Talk) 13:20, 25 January 2007 (UTC)

Since the last posting somebody undid that change again, putting the "acronym" explanation back in. I put it back to the Latin yesterday. Sorry for having made a change without checking here first, as I see the "Latin vs. acronym" thing has gone back & forth like a ping-pong ball. I do hope it will be agreeable to give the legitimate Latin etymology, and then as additional information refer to it as a backronym since it is, apparently, pretty popular and many people think it is really an acronym. Better to address that than leave it out, since it keeps coming back like a bad penny. brucemcdon – 02:49, 4 February 2008 (UTC)

Funny. The statement that the "acronym" is not real but rather a made-to-fit "backronym" has been marked "citation needed." I'll leave that one alone for now. (How would you do that, cite a place where it gives the "acronym" explanation, and say that they're wrong? Or maybe by citing an authoritative weather source such as NOAA and note that they don't say it's an acronym.) In principle it would seem more pertinent to include a citation for the real (Latin) etymology (example: virga. Dictionary.com. The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. http://dictionary.reference.com/browse/virga (accessed: June 22, 2009)), rather than citing a source for the fact that the made-up acronym (backronym) is not the origin (though it clearly has been force-fitted to the meaning) of the word. Somehow, though, it doesn't seem appropriate in an encyclopedia to cite a dictionary for an etymological question.

As a side note, since I included the assertion that the word is not a legitimate acronym, by calling it a backronym, that explanation has spread out to hundreds of online resources, all drawing from here or subsequently from each other, usually without attribution. Researching the question has become increasingly recursive. brucemcdon – 16:19, 22 June 2009 (UTC)

Compressional heating

"... this is usually due to compressional heating, because the air pressure increases closer to the ground."

The use of the term "compressional heating" is somewhere between wrong and confusing. For starters, it's not clear what is being compressed. I think the sentence is trying to say that the atmospheric pressure, experienced by the droplet of water, increases as the droplet falls, and that that implies that heating must be occurring. It certainly is true that atmospheric pressure is a function of altitude. But where is the heating? It could be argued that friction between the droplet and the atmosphere, as the droplet falls, is creating some heating. It is also true that atmosphereic temperature is a function of altitude. But that does not mean in itself that there is an active heating process going on. If there were, then the source of the heat has to be identified. I think the term "compressional heating" comes from a misunderstanding of adiabatic heating, which requires, in this case, the pressure to change in time (work done to the atmosphere), which it is not (unless you're considering a secondary effect- where the evaporating droplet cools the warm air through which it is passing, lowering its volume- now as a function of time- and a corresponding replacement of an air mass above the virga area, with its corresponding decrease in pressure). It takes a bit to think all this through, and I rather suspect that if someone actually reads through this and then comments on it, there will be an argument. But I think I've got it right. blackcloak (talk) 05:41, 4 January 2009 (UTC)

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"Conversely, precipitation evaporating at high altitude can compressionally heat as it falls, and result in a gusty downburst which may substantially and rapidly warm the surface temperature. This fairly rare phenomenon, a heat burst, also tends to be of exceedingly dry air."

This is patently false as stated, although the author(s) is/are admittedly trying to grapple with some complicated phenomena that can be difficult to explain. Adiabatic heating OF THE AIR (not the precipitation) can and does take place in descending air in the lee of mountains and as a result of downdrafts caused by thunderstorms (although rarely noticed in thunderstorms, because of other phenomena operating in the other--cooling--direction). What is really false here, however, is the idea that precipitation heats as it falls as a result of adiabatic (or compression) effects. (Air, not precipitation, heats as it falls due to adiabatic effects.) What happens to falling precipitation is not simple. First of all, precipitation evaporates as it passes through dry air, taking heat out of the air as the evaporation occurs (in much the same way that evaporating water on the skin takes heat out of the skin). That is, evaporation as such is always a cooling process. This can result in further cooling of a cold air layer, as is seen when falling rain cools the air that it falls through, at times causing the air to drop sufficiently in temperature that it then freezes the precipitation, as in the case of sleet or freezing rain. In NO case does the evaporation cause the air to heat up, nor directly cause the falling precipitation to heat up. Second, falling precipitation through dry air always COOLS the air through evaporation, but the drop in altitude of the air itself causes heating OF THE AIR to occur due to adiabatic effects. Third, warming of the precipitation can occur due to conduction effects as the precipitation falls through hot air, typically associated with surface heating. The range of possibilities is indeed enormous as a result of these often countervailing effects: (1) adiabatic (pressure) effects, (2) evaporation qua cooling effects, and (3) conduction effects (which typically work to cool precipitation, but can in certain circumstances actually warm it). A warming downburst actually can occur due to a combination of forces--with adiabatic effects being typically the most important. The author(s) is/are on to something quite important here, but what is needed is a clearer differentiation between adiabatic effects and other effects. To complicate matters even more, two things are being affected: air and precipitation. Landrumkelly (talk) 04:49, 29 May 2009 (UTC)

'…commas attached…'

I'd love to be convinced this is true… Zetetic Apparatchik (talk) 17:48, 4 June 2009 (UTC)

Cloud seeding

Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighboring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming.[citation needed]

Really? There is unlikely to be much horizontal movement of hydrometeors and, by definition, they have evaporated. The only references I can find to this phenomena are copies of this page. With no citation, I would be inclined to remove this paragraph. Jgb2 (talk) 13:26, 23 December 2012 (UTC)

Reference to geographical extent of phenomena

The text implies that virga is limited to the western US and Canadian prairies, as well as a couple of other places in the world. I can tell you that I have regularly seen virga in every part of the US. It may be most common in the west, but it is very common everywhere. — Preceding unsigned comment added by 64.134.217.160 (talk) 14:51, 19 July 2013 (UTC)

Virga == Phantom Rain?

Is Virga same as Phantom Rain? Virga is an observable streak from a raincloud, is phantom rain like full clouds raining?--RicHard-59 (talk) 09:46, 6 March 2014 (UTC)

External links modified (January 2018)

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Sublimation?

In the first sentence of this article a 'Virga' is described as "an observable streak or shaft of precipitation falling from a cloud that evaporates or **sublimates** before reaching the ground". The wikipedia page for 'sublimation' states that "sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state". Now, as far as I am aware, clouds are a gaseous form of water, and liquid water evaporates into its gaseous (cloud) form - i.e. in the water cycle, water transforms between liquid and gas, and vice versa, with no intervening solid state (and thus no "transition of a substance directly from the solid to the gas state").

I have very little familiarity with meteorology or chemistry, so I would appreciate it if someone would tell me that the use of sublimation here is correct or not (and, if not, please amend the article appropriately by removing the word "sublimates".

Cheers