Talk:Wind gradient

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"climb into increasing wind"[edit]

>>During climb into increasing wind, the airspeed increases, allowing the plane to gain energy and climb faster.<<

That's not strictly true, but I'm not sure I can correct it without it confusing the hell out of everyone. It's one of those cases where if you know what's going on, you don't need the explanation, but if you don't, then a simple explanation will not help much. So I'll offer this explanation for comments. BTW, most people don't realise that you control the speed of a plane with the elevators, not the engine, the engine controls the rate of energy input into the system (otherwise how would a sailplane fly fast?) If you don't get that, then don't bother with the rest of this comment.

The energy of a sailplane is dependant on groundspeed, not airspeed, so a 1000kg plane travelling at 20m/s groundspeed has a kinetic energy of 0.5mv2 due to its motion, and a potential energy of mgh due to its height. An increase in wind speed affects neither of those things.

There is a lower airspeed limit called the stall speed below which the aircraft cannot fly, a pilot who flys below stall speed near the ground is called a dead pilot. This is fairly automatic on take-off, the plane remains a wheeled vehicle when below stall speed.

Once the plane has taken off with groundspeed=airspeed (assuming zero wind near the ground), and climbed through the wind gradient a little, the wind speed adds to the ground speed to increase the effective airspeed, and the pilot suddenly finds himself with an excess of airspeed over stall speed margin. He can then choose to slow the aircraft down by climbing, converting kinetic energy into potential energy in the process.

The energy remains constant, but the height and speed have changed in that manouvre.

To put it another way, a height has been reached at a safe airspeed on a windy day that could not have been reached on a still day with the same engine power, but the aircraft is covering the ground more slowly as a result.

I have not yet found a way to extract energy continuously from a wind gradient, by turning/climbing/diving repetatively. Am I being thick? Sailplanes and birds extract energy from updraughts, especially slope soaring on the windward sides of waves and hills. AFAIKS, dynamic soaring allows a little extra height to be gained for a given energy input, and nothing more.

Do we want to be that pedantic with energy? Speaking as a physicist, I say yes, but we could obscure the reasonable with the accurate NeilUK 07:31, 4 May 2006 (UTC)

Dynamic soaring[edit]

`AFAIKS, dynamic soaring allows a little extra height to be gained for a given energy input, and nothing more.' ... actually, it can do a lot more than that. R/C sailplanes have been able to reach speeds of 300 mph using dynamic soaring, which could be converted into 3000 feet of altitude if you could ignore air resistance. (Of course, you can't, but a 2000 feet gain seems attainable in the sort of plane (sleek, low drag) that could reach 300 mph via dynamic soaring.)

As for the `During climb into increasing wind, the airspeed increases, allowing the plane to gain energy and climb faster' claim, it's accurate enough. Your airplane knows nothing of it's groundspeed -- all that matters is it's airspeed, and anything that increases it's airspeed gives it more energy. It's all a matter of perspective ... dougmc 02:06, 10 May 2006 (UTC)

Merger?[edit]

Should this article on wind gradient be merged with wind shear? It looks like the articles define it identically. Thegreatdr 14:47, 28 November 2006 (UTC)

As said elsewhere, I'm not averse to merging this with wind shear, as long as gradient goes into shear and not vice versa. Evolauxia 23:49, 28 November 2006 (UTC)

I am against merging this with wind shear, since wind shear is a more abrupt phenomena, and therefore deserves its own page. [[ ]] 12:27, 5 February 2007 (UTC)

I am for merging. I understand that in aviation the terms may have separate meanings, but mathematically/meteorologically (and, as far as I can tell, these definition are the most common), the terms are interchangable. I'm not saying to remove the separate definition for wind shear vs. wind gradient aeronautically, but instead put a subsection for the different meaning groupings in the overall article.JeopardyTempest 09:25, 22 March 2007 (UTC)
If it gets merged, then we still need an article on gradient wind, which incorrectly redirects here. Gradient wind is all about the flow around high and low pressure systems. For example the wind speeds around a low pressure system are often less than those around a high pressure system for the same isobar spacing. Do not redirect gradient wind to wind shear which is a more localised phenomenon. JMcC 10:21, 3 April 2007 (UTC)
It is now merged. You are correct, gradient wind should not redirect here. Thegreatdr 13:24, 28 May 2007 (UTC)
I have restored this page. Wind gradient (at least in the aviation sense) has nothing to do with tropical cyclones. Dhaluza 21:12, 28 May 2007 (UTC)
Wind shear has a variety of effects...which can all be added to wind shear. It sounded like we had a consensus. Expect your additions to this article to also appear in that article soon. Thegreatdr 00:38, 29 May 2007 (UTC)
WP:CCC. Wind gradient has a common usage in many fields usually relating to vertical change in wind speed due to surface friction. And windshear (not wind shear) is an aviation term mostly associated with downbursts. Wind shear may be a more encompassing term, but that does not mean that everything needs to be subsumed under that heading. Let's see what else I find on wind gradient, and then see what fits where. This article had been a turned into useless redirect anyway, so what's the hurry? Dhaluza 02:46, 29 May 2007 (UTC)
The merger was proposed months ago, so there obviously is no hurry. You yourself just said that wind shear is more encompassing, and a simple check of a search engine will prove it is the more common term. Wind shear is only abrupt when moving at high rates of speed, since it acts over hundreds to thousands of feet, whether it is horizontal or vertical. Two no's does not change the consensus, since people both within this talk page and wind shear favored merger. Thegreatdr 10:20, 29 May 2007 (UTC)
Wind shear is a much broader term in aviation than only to microbursts, the usage is similar to meteorology, the term is heavily used for changing winds in clear air as well as around convection. Wind gradient is almost never used in meteorology, while they may be interchangeable in some aviation circles, wind shear is still used there too (and predominately in my experience). A discussion of the aviation aspects along with the meteorological in the wind shear article makes sense. There is no entry in the Glossary of Meteorology (or probably any other professional glossary) for wind gradient; a Google Scholar search returns 22,500 returns for "wind shear" vs. 888 for "wind gradient". Wind gradient should be merged into wind shear. Evolauxia 06:22, 3 June 2007 (UTC)
This article now has over 30 references, and most of them use the terminology "wind gradient" although some use "wind speed gradient." The latter term is more specific and descriptive, but the first is more common in this specific case. Since wind shear is a more general term, it naturally has more GShits. Wind gradient is not specifically a meteorological phenomenon, although it is a function of wind. It is used in relation to the wind's effects on things on the ground, and sound propagation, which are engineering applications. It is also used specifically in gliding, in both American and British English, by the authors of the primary flight training references in both countries. Dhaluza 10:38, 8 June 2007 (UTC)
I'm still in favor of merging this article, with effects near the surface going into planetary boundary layer or Ekman layer rather than wind shear since that is how you define wind gradient. Aviation effects would go into wind shear. As bad as this sounds, when you ask the common person about wind shear, it has to do with crashing planes moreso than meteorology. Thegreatdr 14:43, 8 June 2007 (UTC)
strongly oppose merger. These are both vast topics and have distinct places in the encyclopaedia... obviously there is some common ground, but that doesnt justify merger. There is common ground with air and atmosphere, but they shouldnt be merged, for example. Cuvette 14:28, 30 August 2007 (UTC)

Name change[edit]

It is becoming clear that this article is becoming increasingly about the effects to wind shear due to surface friction, so what about changing the name of this article to surface friction? I'm not in favor of two articles about the same topic with nearly identical names. At least a name change would imply some difference between the articles. Thegreatdr 11:28, 31 May 2007 (UTC)

  • If you check the extensive references, you will see that Wind Gradient is the term in common usage in reliable published sources. There is some overlap with wind shear, which is a more general term. Your suggested title of surface friction is not supported by the references. Actually it is related to surface roughness, but this is a much broader term still (i.e it covers everything from the surface of a BB to the Earth). Dhaluza 04:09, 3 June 2007 (UTC)
    • Only two of your references come close to being meteorological texts. While surface friction affects non-meteorological categories, there is no meaningful difference between wind shear and wind gradient. They are the same thing. I'm throwing the merge tag back on this article. Thegreatdr 04:23, 3 June 2007 (UTC)
      • The term has wide usage outside meteorology, including in various branches of engineering and aviation. Wind gradient is a special case of wind shear, and there is more than enough material for a separate article on it. I would oppose the merge until the wind shear article is improved. Once it is, I think it will become obvious that it should be split out. Dhaluza 10:28, 3 June 2007 (UTC)
        • Wind shear now includes the extra information from this article, since there is no difference between the two terms. See your talk page concerning a comment made on the meteorology project page. At least one other person does not see a difference. Thegreatdr 14:07, 3 June 2007 (UTC)
          • I asked that Wind shear be cleaned-up prior to attempting a merge. Frankly adding this content is only making that job harder. Your statement that there is no difference ignores that one is a special case of the other, so that is a distinction with a difference. Please do not remove content from this article in the mean time, as I am not finished developing it. Dhaluza 15:05, 3 June 2007 (UTC)
            • No content has been removed/subtracted from this article, nor will it be until there is a clear consensus and your acknowledgement. Clearly define how they are different. Even wikipedia articles on shear and gradient appear to state gradient is mathmatical (ah, Calculus III), and shear appears to concentrate on physical effects of gradient. Right now, I could live with that differentiation if the content between the articles were divided in this manner, leaving the math to the gradient article and everything else to the shear article. This is my second attempt to reach a compromise and allow two articles on the same topic. A check made by a third party indicates wind shear wins in a landslide over wind gradient in scholarly articles, should no clear differentiation be made between the two articles. This independent check indicates which article would become a redirect for the other, should no clear difference be made between the two articles. Thegreatdr 16:33, 3 June 2007 (UTC)
              • Shear and gradient as abstract concepts are probably related as you describe. But applying your Calc III studies to this would be WP:OR. We are only concerned with how outside experts actually use the term in WP:RS, not how we think or wish they would. Clearly the references show that in common usage in many fields of science and engineering, wind gradient is a term of art used to describe the effect of aerodynamic drag on the lower atmosphere, where the wind profile is assumed to have a logarithmic relationship to height, becoming negligible at a height called "gradient height" where the wind speed is assumed to be the constant "gradient wind speed". As the lede states, it is a vertical shear of horizontal wind near the surface. This is a special case of wind shear, which may or may not involve the surface, and does not have to be limited in these ways. As far as the math, the wind gradient has a special set of simplified equations based on the logarithmic profile, which do not apply generally. In the general case of wind shear, I would expect the math to be much more complex, because of the lack of simplifying assumptions. Dhaluza 16:41, 3 June 2007 (UTC)
                • I don't see how it is WP:OR if I used Wikipedia articles on the terms to come to that understanding. Nevertheless, the effect you are talking about already has at least three existant articles within wikipedia. They are titled planetary boundary layer, ekman spiral, and ekman layer. Gradient wind is a portion of the real wind diverted towards low pressure and away from high pressure along height lines aloft, above the friction/planetary boundary layer. Interestingly enough, I recently added a section (independent of your last comment) concerning the Ekman spiral and planetary boundary layer within wind shear, which would be subarticles. Thegreatdr 17:01, 3 June 2007 (UTC)
                  • Thanks, I added links to them (WP:BTW) Dhaluza 17:34, 3 June 2007 (UTC)
For what it's worth, here's my take on this, some of which may repeat various points made above:
  1. Wind shear is a general term for spatial variation in horizontal wind vectors in any direction (height, latitude or longitude). The wind shear article should be a general article describing primarily what wind shear is and its effects (i.e., generating turbulence in fluid flow and the associated fluxes of fluid properties) and leading on to brief descriptions of notable examples (e.g., planetary boundary layer, upper-troposphere jets, thermal-wind, thunderstorms). These descriptions should be brief because main articles exist for each example. The existing article is more or less pitched this way, although it is currently a bit unfocussed, but that's solvable.
  2. The planetary boundary layer (PBL) article should cover pretty much all aspects of (Earth's) PBL properties: surface friction, wind/momentum, temperature, humidity, stability, chemistry. That is, more and better than it does currently. I suspect that each of these sub-sections has enough information for a separate sub-article, but it's probable best to wait for the amount of information to out-grow the main article before branching off.
  3. It's not clear to me what the boundaries of the wind gradient article are. Maybe it would be easier to decide if a merge is desirable if these boundaries were defined. If I've read Dhaluza correctly above (...wind gradient is a term of art used to describe the effect of aerodynamic drag on the lower atmosphere, where the wind profile is assumed to have a logarithmic relationship to height...), then this is the same as wind in the PBL, is it not? The only things I see in the article that may not be about wind in the PBL are the gliding and avian sections (they're not my expertise). Much of the rest is information I would expect to be in the PBL article, and not at such length in wind gradient. Am I right in suspecting that this article should be something like Effects of boundary layer wind gradient? Okay, maybe not that title exactly, but it would distinguish the meteorological causes and properties of the PBL wind gradient from how different disciplines (aviation, architecture) have to account for its existence. Hats off to the reference count though.
Deditos 16:37, 15 August 2007 (UTC).
Well, yes, the boundaries of the "Wind gradient" article are the same as "Practical application of surface effects on wind in the planetary boundary layer" but it should be obvious which is the title that would be recognized by the broadest group of people. Planetary boundary layer is a term primarily recognized by meteorologists, and I would suggest that article should focus on the aspects of the PBL of interest to meteorologists, i.e. those related to observation and prediction. The aspects of interest to aviators and engineers, i.e. the practical application of this knowledge, should be grouped under the title they would most readily recognize. This seems a reasonable and workable taxonomy. Dhaluza 01:55, 29 August 2007 (UTC)
Further, as you mention in point #2 above, the PBL article should cover all aspects of Earth's PBL, but also as Venus' and Mars' and what has been accepted theoretically about PBLs on other planets as well. The wide net cast by the broad subject of PBL suggests using summary style, with an overview of each topic pointing to a separate main article. For example, in addition to wind gradient, you have a temperature gradient which would be described in brief and pointed to lapse rate as well as sound speed gradient. Merging sound speed gradient into PBL would not be appropriate because it also occurs in the ocean. Lapse rate also stands as a separate article, because it applies to atmospheres on gas planets with no surface. There are many other topics PBL should cover as well, including surface heat, moisture and particle exchange: thermals, evaporation, eolian processes etc. Dhaluza 09:44, 29 August 2007 (UTC)
'Strongly oppose name change. Literature usage supports the present name, whereas the suggested name is circumlocution. Cuvette 14:33, 30 August 2007 (UTC)
You know, looking at this again, I'm largely in agreement with you. I appreciate that the name wind gradient is in common usage, so my query wasn't particularly with that. More that if two articles have 80% overlap of content then they should probably be the same article, even if different communities use different names. However, now I think that the overlap between wind gradient and planetary boundary layer is/would be slight. We just need to make sure that the Background subsection of this article doesn't get edit creep.
I still think that wind shear repeats too much of this article and should summarise more and defer many of the details to this and other articles. Deditos 12:59, 18 September 2007 (UTC).
To Deditos. While I oppose any merge or renaming of any of the articles, I am having trouble following which two articles you think overlap by 80 percent and what your current proposal is. Cuvette 14:26, 18 September 2007 (UTC)

If the use of wind gradient in the literature used to create the article is specifically about Earth's PBL, then the name of this article should be changed to Earth's PBL, with a redirect from this name. That would eliminate any apparent confusion. It is correct to say both this article and wind shear are too detailed and should be worded more broadly. The added detail within wind shear has now been added to the subarticles rather than covered in the broader article. It's odd to have specific info on wind engineering in these articles which was not covered by the appropriate subarticles. I've made the changes in wind shear to make it less specific with more information now in the subarticles, you all can make the appropriate changes within this article. Thegreatdr 21:23, 3 October 2007 (UTC)

Lead problem[edit]

In case this article, by some miracle, remains separate, your lead appears too long and has unique information not found elsewhere down the article. Remember that the lead is meant to be a summation of the article, and should not be the only place where a certain piece of information resides. Thegreatdr 14:52, 8 June 2007 (UTC)


Hellman Exponent[edit]

\ v_w(h) = v_{10} \cdot \left(  \frac {h} {h_{10}} \right)^ a

where:

\ v_w(h) = velocity of the wind at height,  h [m/s]
\ v_{10} = velocity of the wind at height,  h_{10} = 10 meters [m/s]
\ a = Hellman exponent

Could someone who knows explain why :\ a is called the Hellman exponent. I have never seen this exponent referred to as Hellman. Neither "Air Pollution" by J. Colls nor H. Perkins refers to it as Hellman. —Preceding unsigned comment added by 70.53.124.11 (talk) 00:43, 6 January 2008 (UTC)

Have a look at the book here Dhaluza (talk) 00:54, 6 January 2008 (UTC)

Close to GA[edit]

If this article had more images, I'd think it would make GA class. Since its survival as an independent article has now been determined, and it is throughly referenced, I don't see any other obstacles on its path to GA. Thegreatdr (talk) 20:54, 4 July 2008 (UTC)

The lead needs to be expanded as well before GA submission is attempted. I'm tempted to drop the article back to C class due to the lead's shortness/non-comprehensiveness. Thegreatdr (talk) 17:54, 19 February 2009 (UTC)


Gradient in Sailing - > 6 knots[edit]

I've removed comments: "According to one source[38], the wind gradient is not sigificant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the hight of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind."

Since you have the book, you can check that the citation correctly reflects what the book says. Your claim below is that the citation was incomplete, because it didn't deal with taller masts. This is not quite correct, because in the case of winds greater than 6 knots, Bethwaite states that the change in speed is confined entirely to the one to two meters closest to the surface (p. 11). I have no objections to amplyfing the citation to provide additional information. But I cannot understand why you think it should be deleted. So I will restore it, together with additional text and additional citations. See also below.--Gautier lebon (talk) 13:08, 10 March 2010 (UTC)

The article refers to the author's observation of a relatively linear increase in wind-speed relative to height in light airs, and examples and chart given using a Laser class yacht with mast height < 4 meters. The authors chart (pp 18 Section 3.5 fig 3.8) for > 6 knot "Breeze" conditions clearly shows the non-linear exponential increase in wind strength with height, consistent with other data on wind shear and Hellmans exponent estimates. As such, for boats with taller masts, gradient effect would remain significant. Bethwaite, Frank (first published in 1993; new edition in 1996, reprinted in 2007). High Performance Sailing. Waterline (1993), Thomas Reed Publications (1996, 1998, and 2001), and Adlard Coles Nautical (2003 and 2007). 202.180.115.243 (talk) 03:07, 8 March 2010 (UTC)

It appears to me that you misinterpret Fig 3.8. It shows asypmtotic increase for wind speeds greater than 6 knots, which is consistent with what the book says on p. 11 and also with the citation that you yourself provided, see [1]. Please note that the paragraph at the end of that citation assumes a mast height of 1.8 meters, so indeed the wind speed changes significantly from the top of the mast to the water surface. But that says nothing about how the gradient changes higher up.--Gautier lebon (talk) 13:08, 10 March 2010 (UTC)