The "lee bow effect"

[lw395]

As Flaming himself has said, +/- 3kts of tide can have quite an effect on the effective wind speed

It has quite an effect on the ground wind too, albeit with something of a lag.

 

[DJE]

This is only correct in sailing parlance. In the rest of the world True wind is wind over the ground. There are rather large industries that rely on true wind for planning. It's what meteorological agencies spend billions predicting.

Sadly the small manufacturers of niche sailing instruments chose to make their own stuff up hence the confusion.

I've never heard a forecaster talk about "true wind". The met office and all those large industries are firmly fixed to the ground so the concept of apparent wind is irrelevant. They just use "wind speed".

The big industry where apparent wind does matter of course is aviation. But they use the terms "windspeed" and "airspeed".

"True wind" is a sailing term - always has been.

 

[Buck Turgidson]

I've never heard a forecaster talk about "true wind". The met office and all those large industries are firmly fixed to the ground so the concept of apparent wind is irrelevant. They just use "wind speed".

The big industry where apparent wind does matter of course is aviation. But they use the terms "windspeed" and "airspeed".

"True wind" is a sailing term - always has been.

You're clearly not in the aviation industry.
We use 2 winds. For take-off and landing the wind we receive from the tower is referenced to magnetic north as are the runway headings. En-route we use winds referenced to true north as the deviation can change rather rapidly in some places. The Met Office publishes forecast weather charts in wind referenced to true north.

True wind is the wind over the ground referenced to true north.

see: https://www.metoffice.gov.uk/binari...ts/factsheet_11_weather_charts.compressed.pdf for more details about weather.

 

[lw395]

The only time apparent wind and true wind are different in aviation is landing on an aircraft carrier.
Or maybe in a heli.
Apparent wind in our terms, in a fixed wing plane is generally from dead ahead unless the wheels are on the ground.
They seem to use 'ground wind' for wind speed at 10m above actual ground. But many weather services call it the '10m wind'

Sites like this give wind at other heights or pressure levels:
http://www.wetterzentrale.de/en/top...gem&var=21&time=0&run=0&lid=OP&h=0&mv=0&tr=12

I think it's only marine instrument users who delude themselves true wind is worth measuring to closer than 5 degrees or a compass point?

 

[DJE]

see: https://www.metoffice.gov.uk/binari...ts/factsheet_11_weather_charts.compressed.pdf for more details about weather.

Hmm. 56 pages from the Met Office and no reference to the phrase "True Wind".

 

[BelleSerene]

I think it's only marine instrument users who delude themselves true wind is worth measuring to closer than 5 degrees or a compass point?

Isn't that because it's only boats (and sailing boats at that) that travel sufficiently slowly for a small difference in wind angle to make any difference to their apparent wind and thus to their navigation? If you're flying, a difference of a few degrees in the true (sailing/ water) wind angle makes virtually no difference to the apparent wind you feel, because the vector of your speed-induced wind is relatively so long. So it's not relevant to your navigation. But when you're sailing at just a few knots through the water, a difference of a few degrees in the bearing of the wind over the same water materially changes the angle of the apparent/ relative wind on the boat, and so affects its ability to point where it wants to go, or changes the forward force its sails can generate from that wind.

 

[lw395]

Isn't that because it's only boats (and sailing boats at that) that travel sufficiently slowly for a small difference in wind angle to make any difference to their apparent wind and thus to their navigation? If you're flying, a difference of a few degrees in the true (sailing/ water) wind angle makes virtually no difference to the apparent wind you feel, because the vector of your speed-induced wind is relatively so long. So it's not relevant to your navigation. But when you're sailing at just a few knots through the water, a difference of a few degrees in the bearing of the wind over the same water materially changes the angle of the apparent/ relative wind on the boat, and so affects its ability to point where it wants to go, or changes the forward force its sails can generate from that wind.

A lot of truth in that.
But I don't think air balloonists talk about 5 degree windshifts the way we do?

If you are flying in a fixed wing plane, the apparent wind you feel is your airspeed.
True wind at whatever height you are is drift, headwind or tailwind.
Ground wind is a measure of how scary the landing will be.

Here's a map you can click on to see wind at different heights:
https://aviationweather.gov/windtemp
Is there one of those for Dorset?

 

[Buck Turgidson]

Hmm. 56 pages from the Met Office and no reference to the phrase "True Wind".

"Wind (ddff)
The surface wind direction is indicated on the station circle by an arrow ying with the wind, the point touching the circle. Direction (dd) is measured in degrees from true North. "

 

[johnalison]

The analogy with flying just doesn't work, because there are only three frames of reference, the plane, the air at whatever level one is in, and the ground. When sailing, there are four, the boat, the air, the water, and the ground.

 

[RupertW]

This thread has made me change my mind about 2 things I've been sure about for decades and it's been a long time since that happened on this forum. Good stuff.

 

[Buck Turgidson]

The analogy with flying just doesn't work, because there are only three frames of reference, the plane, the air at whatever level one is in, and the ground. When sailing, there are four, the boat, the air, the water, and the ground.

It describes the correct use of the term true wind. If sailors want to describe the wind/water vector they need to use a different term. Sailing wind seams appropriate to me.

 

[DJE]

"Wind (ddff)
The surface wind direction is indicated on the station circle by an arrow ying with the wind, the point touching the circle. Direction (dd) is measured in degrees from true North. "

This is only correct in sailing parlance. In the rest of the world True wind is wind over the ground. There are rather large industries that rely on true wind for planning. It's what meteorological agencies spend billions predicting.

Sadly the small manufacturers of niche sailing instruments chose to make their own stuff up hence the confusion.

But the Met Office don't use the term "True Wind". So I don't accept your claim for it's meaning in "The rest of the world".
I maintain that it is a widely used sailing term - see any textbook or RYA course syllabus.

 

[lw395]

I can't see any great value in saying what is the true meaning of 'true wind'.
It just pays to be aware that other people might use terms differently.
If you need to avoid any ambiguity, use another term.

 

[Mark-1]

I maintain that it is a widely used sailing term - see any textbook or RYA course syllabus.

I'd be interested in seeing a photo of the relevant page from an RYA text book if anyone has one to hand.

I'm not saying it would be definitive but it would be nice to know the definition of "True Wind" that's actually taught on most UK, sailing courses. (...and if the, universally understood, term 'Ground Wind' is used in RYA text books.)

Ditto other English Speaking countries like Oz, USA.

 

[lpdsn]

I can't see any great value in saying what is the true meaning of 'true wind'.
It just pays to be aware that other people might use terms differently.
If you need to avoid any ambiguity, use another term.

Exactly. The only people that are going to be thinking hard about what the true wind is and how it is being affected by the tide are tacticians on racers and they'll already know what they mean.

 

[Mark-1]

I can't see any great value in saying what is the true meaning of 'true wind'.

A cast iron definition would put me out of my misery - it's driving me crazy. I'm not expecting there to be a cast iron definition, mind you.

It just pays to be aware that other people might use terms differently.
If you need to avoid any ambiguity, use another term.

This. Usage is everything. If Nelson, Noah and Bernard Moitessier turned up on my door step and gave me an unchallengeable definition of true wind, I'd still avoid the term from now on!

 

[Mark-1]

Exactly. The only people that are going to be thinking hard about what the true wind is and how it is being affected by the tide are tacticians on racers and they'll already know what they mean.

The recent Bramblemet thread, where nobody showed any knowledge of the distinction whatsoever, suggests they may not!

 

[grumpy_o_g]

But the Met Office don't use the term "True Wind". So I don't accept your claim for it's meaning in "The rest of the world".
I maintain that it is a widely used sailing term - see any textbook or RYA course syllabus.

The Met Office don't use the term True Wind in that publication, which is not quite the same. I can't actually ever remember hearing a Met Office forecaster using the term True Wind but the tower would use it all the time and they used Met Office forecasts. I never understood why a pilot would be so hung about about a few degrees of accuracy in such a variable thing but they were.

 

[lpdsn]

The recent Bramblemet thread, where nobody showed any knowledge of the distinction whatsoever, suggests they may not!

I didn't follow that thread. Were they just cruisers?

 

[GrahamM376]

Hmm. 56 pages from the Met Office and no reference to the phrase "True Wind".

But it appears in the Glossary of Meteorological terms -

True wind direction
The direction, with respect to true north, from which the wind is blowing. Distinguish from magnetic wind direction. In all standard upper-air and surface weather observations, it is true wind direction that is reported.