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Old 22-12-2011, 15:53   #1
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Differences Between Ground, Apparent, and True Wind Direction

I'm somewhat confused over the differences between Ground, Apparent, and True wind direction. I understand Ground and Apparent (I think), but am confused over True. Here's my understanding.

First, all are represented by vectors (speed and direction).

Ground: This one is simple, I hope. It's a wind vector relative to the ground. But is it relative to true, or magnetic north?

Apparent: This is the difference between the Ground wind vector and the boat's movement vector. This difference is the wind vector against the boat - very useful for sailing. This seems straight forward too. I assuming the vector direction for both is relative to the same thing (true or magnetic). And the boat's speed is speed over ground (SOG)?

And what the heck is True wind? Is it a synonym for Apparent wind? Or is Ground wind relative to magnetic north, and True winds is relative to True north?

I'm confused.
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Old 22-12-2011, 16:18   #2
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re: Differences Between Ground, Apparent, and True Wind Direction

I've never really heard "ground" used when talking about wind. True indicates its direction of origin, so a NW winds blows SE out of the NW, so for sake of argument so that's 290 degrees. Nothing special there, and easy to observe just standing on the beach.

If you start moving north along the beach, the wind will seem to come more from the NNW than "true" NW, so your "apparent" wind is NNW because essentially you are creating your own wind as you move through the air.

The differences in true and apparent become smaller as the wind speed increases, and when vessel speed stops there is no difference.

An obvious example is to drive down the freeway and roll down your window and stick your hand out. Unless you are in a hurricane, the apparent wind is always dead ahead because you are going so much faster than the environmental wind.

"Ground" I know when talking about speed or course (speed over ground SOG, or course over ground COG). Speed over ground normally comes from GPS (or someone who knows how to do set and drift calculations manually). Moving for 10 hours at 5 knots should move you 50 nautical miles, but if you were directly moving against a 1 knot current you'd travel 40 knots. The SOG indicator would show 4 knots and your knotmeter (which is only measuring the water rushing past the hull) would show 5.

Not sure if that helps.
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Old 22-12-2011, 16:25   #3
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re: Differences Between Ground, Apparent, and True Wind Direction

True wind ( I think ) is the direction a baloon will travel if you let it fly, the apparent wind is what the boat captain feels blowing on his face while standing on the deck of his boat (its what the sails respond to) . Ground wind...I don't know.
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Old 22-12-2011, 16:31   #4
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re: Differences Between Ground, Apparent, and True Wind Direction

There is a long discussion about this in the archives.

Apparent Wind = the wind speed and direction in relation to your boat. So in a dead calm motoring ahead at 10 knots, you would have a 10 knot apparent wind at 0 degrees.

True Wind = the wind speed and direction in relation to the WATER. In the example above, you would have Apparent Wind of 10 knots and True Wind of 0. Your instruments calculate True Wind by taking Apparent Wind (the only thing your wind instruments measure directly) and taking out the vector for speed through the water. Which is why you get no true wind readout if you have not got STW data -- speed through the water data -- in your network.

Ground Wind = the wind in relation to land, not water. In the example above, if there is no current and no leeway, Ground Wind = True Wind. If you are motoring directly against a 5 knot current, however, then you have Apparent Wind of 10, True Wind of 0, and Ground Wind of 5. To calculate Ground Wind, your instruments take out the vector for SOG, rather than STW. Ground Wind is of no relevance to sailors unless we are sailing in a strong tidal current and are trying to anticipate what the wind will be like when the tide changes.


Before GPS, it was impossible to calculate Ground Wind. That is because the only speed data we had was STW. Possibly the idea of True Wind versus Ground Wind arose because instrument makers who suddenly had access to SOG data on the network needed some way to differentiate true wind calculated that way versus True Wind calculated using STW.

True Wind, as calculated with STW data, is actually what we really need, since our boats sail in the interface between water and air. If we were sailing in a 1000 knot current, we wouldn't give a damn that we have a 1020 knot true ground wind in the same direction. We would only feel or care about the 20 knots differential between the water and wind -- the true wind -- that's what we sail in. That is why no one bothered, I guess, to change the definition once instrument makers had access to SOG data.

This will be rather academic for anyone who sails in waters without strong currents. For them, SOG and STW, and therefore True Wind and Ground Wind, will be more or less the same most of the time, but for perhaps some effect of leeway. For others, like me, who sail in the English Channel where the tide rips at up to 10 knots (Alderney Race), the disinction is not academic.


Don't be confused by the fact that meteorologists use True Wind to describe the difference between wind and land. Of course they do -- they are sitting on land, not on water. That in no way contradicts our usage of True Wind as being the difference between wind and water.


You asked which kind of wind is expressed in relation to North, what kind of North, or your bow. Don't get confused by this. "True" in "True Wind" has nothing to do with True North. It means true as in -- without the effect of the boat's motion. Any wind can be expressed any way you want -- in relation to your bow, in relation to either kind of North. But your instruments will generally tell you wind in relation to your bow, which is what you need for sailing. Ground Wind, which you don't need for sailing -- which is irrelevant for sailing here and now (but may be relevant for planning), would be useful to know in relation to North, to the extent you care at all. Which North -- why that depends on how you think, and how your instruments are set up. I always use Magnetic for everything, so that is what I would want to know, personally.
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Old 22-12-2011, 16:36   #5
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re: Differences Between Ground, Apparent, and True Wind Direction

If the wind is blowing due south at 10 knots, thats the true wind. If you are sailing due south in that wind moving at 6 knots, the apparent wind is 4 knots. Where it gets fun is when you veer off of true south and start accelerating.....
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Old 22-12-2011, 17:15   #6
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re: Differences Between Ground, Apparent, and True Wind Direction

Thanks, Dockhead, that explains it. Ground is relative to ground, and True is relative to the water surface. With SOG you can calculate Ground wind, and with STW you can calculate True wind. Thanks for the clear explanation.
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Old 22-12-2011, 18:21   #7
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Quote:
Originally Posted by Matt sachs
If the wind is blowing due south at 10 knots, thats the true wind. If you are sailing due south in that wind moving at 6 knots, the apparent wind is 4 knots. Where it gets fun is when you veer off of true south and start accelerating.....
No, that is ground wind. If the water has a south setting current of 5 knots, then true wind is 5 knots instead of the 10 knots ground wind.

Dockhead is 100% correct.

cheers,
Nick.
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Old 22-12-2011, 23:33   #8
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re: Differences Between Ground, Apparent, and True Wind Direction

Ground (speed) is all about boat speed relative to ground (sea floor) nothing to do with the wind..
True wind is wind relative to ground.
And apparent wind is relative to boat.
All headings true north.
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Old 23-12-2011, 00:40   #9
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re: Differences Between Ground, Apparent, and True Wind Direction

Dockhead has given a very good description.

Part of the problem is the terminology. Ground wind is really what the actual wind is doing ignoring all movement of the boat. Before sophisticated electronic marvels like GPS became available this could not be calculated. So we are stuck with terminology where true wind became defined as including the boat movement due to current.

You can set up some of the better instruments to read true, or ground wind (as well as apparent). I find ground wind more useful, but others disagree
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Old 23-12-2011, 01:22   #10
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re: Differences Between Ground, Apparent, and True Wind Direction

Quote:
Originally Posted by s/v Jedi View Post
No, that is ground wind. If the water has a south setting current of 5 knots, then true wind is 5 knots instead of the 10 knots ground wind.

Dockhead is 100% correct.

cheers,
Nick.
I don't think that what Matt said is wrong and I don't think it is at odds with what Dockhead posted. He didn't say anything about current. Sailing DDW at 6 knots in 10 knots of true wind gives 4 knots apparent.

Quote:
Originally Posted by TeddyDiver View Post
Ground (speed) is all about boat speed relative to ground (sea floor) nothing to do with the wind..
True wind is wind relative to ground.
And apparent wind is relative to boat.
All headings true north.
I think ground wind is going to be a problem as I don't believe it has a definition beyond what manufacturers of GPSs have created. I have looked in Duttons, Chapmans, and Annapolis, none mention ground wind. Bowditch does not mention ground wind in its glossary but does have apparent and true. Bowditch defines true wind as wind relative to a fixed point on earth.

Others like Annapolis ignore the possibility of current and say true wind is what you see when the boat is not moving through the water, or also what you see when you are on land.

Before GPS, wind and STW instruments computed a number they called true wind, which if you go by Bowditch'es definition would be incorrect if you were in current, but as you didn't have anything more than a guess as to current I don't think anyone made an issue of it.

When I first heard the term ground wind I defined the terms as Dockhead did as that made the most sense to me.

What really throws a wrench in the works is a definition that Maretron has come up with. (This looks like it caused a 20 page argument on another forum.)

From:
Maretron : Marine Electronic Instruments

Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat. The apparent and true wind are interesting to see how the wind is effecting or pushing the boat (i.e., when sailing or docking), the ground wind is important for understanding the weather (i.e., where is the low pressure).

Extra credit: Did this not become an issue during the days of Loran because there was little or no networking and/or computing power?

John
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Old 23-12-2011, 02:19   #11
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re: Differences Between Ground, Apparent, and True Wind Direction

The terminology is certainly a mess.
As long as everyone understands what is meant its OK, but that is often the case.

Many sailors would expect there “true wind” instruments to display 0 if there was no wind and they were at anchor, or tied to the dock. This is not the case, if there is any current present, at least for the way 99% of boats have their instruments set up.
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Old 23-12-2011, 04:06   #12
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re: Differences Between Ground, Apparent, and True Wind Direction

Well, I guess my confusion was justified after all. This is quite the mess.

Interestingly, my confusion came to a head during a conversation with Maretron at the Ft Lauderdale boat show. I simply could not understand the difference between True and Ground wind as he was explaining it. Even after convincing him that I understood vectors and vector math (which is where most people start the explanation), I still could not make sense of it. I later read the section of their manual quoted earlier, and that confirmed my confused.

I suppose each of us just needs to figure out what our own instruments are telling us, even though they may all be different.
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Old 23-12-2011, 08:01   #13
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re: Differences Between Ground, Apparent, and True Wind Direction

phew...I thought for a minute I was wrong, but I was confused...
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Old 23-12-2011, 08:26   #14
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re: Differences Between Ground, Apparent, and True Wind Direction

If everybody starts to understand that the True from True and Apparent is different from the True from True and Magnetic, it'll help

As long as we do not start compensating for continental drift (!!), ground wind is the wind referenced to ground, as in solid rock above or below water. Ground wind is what is measured by a weather station that is mounted ashore. It is what is broadcast in weather forecasts.

Wind direction is always True, never Magnetic (at least it is True when organizations like NOAA talk about it).

cheers,
Nick.
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Old 23-12-2011, 09:18   #15
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re: Differences Between Ground, Apparent, and True Wind Direction

Quote:
Originally Posted by s/v Jedi View Post
If everybody starts to understand that the True from True and Apparent is different from the True from True and Magnetic, it'll help

As long as we do not start compensating for continental drift (!!), ground wind is the wind referenced to ground, as in solid rock above or below water. Ground wind is what is measured by a weather station that is mounted ashore. It is what is broadcast in weather forecasts.

Wind direction is always True, never Magnetic (at least it is True when organizations like NOAA talk about it).

cheers,
Nick.
Well I had to read your first sentence about ten times before it started to parse out and make sense to me. Yeah I think arguing about whether you reference direction from true north, magnetic north, or relative to your lubberline is a completely different topic.

Ok, I reread the Maretron definition a few more times and I think they're saying the same thing we have been, but have made a very poor choice of words IMHO.

Replace the words "compass rose" with fixed point on earth, and remove "bow" and you get:

Ground wind is ... referenced to a fixed point on earth, ... app and true are referenced to the boat.

as opposed to this:

Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat.

John
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