Surface wind backing

[gerardflyagain]

I've read different reports on here and elsewhere on how much the surface wind backs and decreases from various heights. What is the rule that the PPL exams use?

(This link says that the process is reversed at night, is that right?)

[welkyboy]

Surface wind is on the surface so how can it change with height! You're reading far to much to pass PPL exams.

[kghjfg]

Originally Posted by welkyboy

You're reading far to much to pass PPL exams.

Might be the other way round!

Gerard, buy the text books, they're really not that expensive. I'd recommend the AFE ones, though some like the Pooleys ones.

[engineno9]

I seem to remember it backs by 30 degrees and speed decreases by 50% at the surface (10 degrees and 30% over water due to less friction) but as usual happy to be corrected.

It's to do with the surface friction reducing the speed and therefore reducing the Coriolis effect I think, so doubtful that it could be any different at night.

EDIT: Just found this which explains it: http://ww2010.atmos.uiuc.edu/(Gh)/gu...r/fw/fric.rxml

[gerardflyagain]

welkyboy the questions that are coming up talk about the veering/backing as height from surface wind increases.

kghjfg I have bought and read the Pooley's Air Law and Meteorology book. When I ask questions on here it is because the book has conflicted with something else (e.g. PPL Cruiser, or a recent law change) or I'd like to understand in more detail.

Thanks engineno9. This is the link I posted's explanation for a difference at night: "At night, surface friction decreases as surface cooling reduces the eddy motion of the air. Surface winds will back and decrease. During the day, as surface friction intensifies, the surface winds will veer and increase."

[ChickenHouse]

PPL theory may best be learned in an ancient procedure called talking in a group. A forum or some virtual know-all-see-all-Marjory-the-Trash-Heap never replaces brain activity.

First frame of reference is printed books, the internet is full of garbage and no quality assurance is visible and trust in posted threads, as more then just a hint where to find the real wisdom, is not advisable.

The wind variations, daily changes, geostrophical wind systems, Coriolis effects and height dependency there is a vast of very good met'books available. Weather is easy, once you get the basic physics and if you did not reach that point, you may get some lessons from a skilled, or discuss in a group (loop closed)

[oggers]

engine is right. Your Pooleys will most likely be correct whereas the link you read is merely someone's badly written attempt at an explanation.

When I was at groundschool we were taught by bona fide meteorologists so I have no reason to doubt the explanation I was given. What follows is a summary of what I understood from those lessons and the notes provided:

Geostrophic wind at 2000' agl is parallel to isobars. Surface wind speed is less than geostrophic wind speed due surface friction. Less wind speed also means less geostrophic effect therefore the surface wind is approx 30º off the geostrophic. In the northern hemisphere the result is that the surface wind 'backs and slacks' from the geostrophic wind. Southern hemisphere it would veer and slack but that doesn't roll off the tongue so well.

So - in the northern hemisphere - the surface winds 'back and slack' whilst the winds aloft 'veer and increase'.

That is the basic theory. At night, due to the cool dense air, surface friction increases so the surface wind backs and slacks even more.

[gerardflyagain]

ChickenHouse, I agree that books and groups is the best method. Group learning is not always available to me though. Despite the limitations of variable quality of information, I feel I still make progress this way. Because I often end up comparing what the book says, what PPL Cruiser says, what an 'expert' says, and through racking my brains reach a logical conclusion.

Thanks for the explanation oggers