Wind veer aloft

[srah1953]

I read that wind direction veers with altitude and that is one of the reasons why you have (more) sail twist aloft. If I visualise this on starboard tack, it makes sense but on port tack you would need less twist on high than low down because the wind would be further forward higher up.
I'm sure I'm missing something so I'd welcome enlightenment.
Thanks

 

[andygc]

Try this. It's not just Coriolis effect. http://www.oceansail.co.uk/Articles/WindTwistArticle.html

 

[srah1953]

Excellent. Thank you very much. That article is very helpful - now if I can just remember it out on the water.

 

[dom]

I took a quick look at the above link and (unless I'm mistaken), the Coriolis stuff is a complete red herring in the sub-25m world us sailors habitate! Think of Coriolis as an immensely very weak force that takes quite some time and space to gain any traction. So all this port/starboard effect depending on what hemisphere one is in can be ignored.

However, it is true that wind speed can be somewhat higher at 25m altitude than at sea level; in particular at low wind speeds (sub-10kts). This means that as you sail forwards the apparent wind swings a little (i.e. teensy!) less aft at 25m than it does at sea level, for which you may want to set a little more twist in your sail.

But this is a simple apparent wind effect which is symmetric on either port or starboard tack. The only exception would be if you are using NASA instrumentation, and by that I mean the rockety Yank one!

 

[oldbilbo]

So who else reckons Vertical Wind Shear is only marginally relevant? Go tell it to competition glider pilots, balloonists, and these guys.....

 

[Javelin]

Dom nailed it.

A better page to look at is at http://syr.stanford.edu/SAILFLOW.HTM

 

[srah1953]

I don't see what the Sandford document says that changes the proposition of the ocean sailing article. It seems to accept that wind speed increases as you go up the mast and therefore gives rise to twist.
It doesn't seem to me that this difference in wind speed with height is materially affected by apparent wind, which is a function of the induced wind created by the boat speed. But the induced wind is the same at every level on the mast – they don't travel at different speeds – so the twist must be created by something else, which the ocean sailing site says is the Coriolis effect.
I do question the conclusion of ocean sailing which says that there is increased twist on starboard tack because of what it calls the addition of apparent wind veer and true wind veer. But I can't really see where this apparent wind veer comes from. As the boat moves faster, the apparent wind increases, but it increases at all heights by the EXACT SAME AMOUNT. So, it would seem that the degree of twist, which is a relative function, would be more or less the same. Using some purely hypothetical numbers, when the boat is stationary, let's say the wind at ground is 10 knots and at the mast top it's 15 knots. When the boat moves at 5 knots, the wind at ground will be (just under) 15 knots and at the mast top just under 20 knots. Relative twist shouldn't have changed significantly.

 

[charles_reed]

I don't see what the Sandford document says that changes the proposition of the ocean sailing article. It seems to accept that wind speed increases as you go up the mast and therefore gives rise to twist.
It doesn't seem to me that this difference in wind speed with height is materially affected by apparent wind, which is a function of the induced wind created by the boat speed. But the induced wind is the same at every level on the mast – they don't travel at different speeds – so the twist must be created by something else, which the ocean sailing site says is the Coriolis effect.
I do question the conclusion of ocean sailing which says that there is increased twist on starboard tack because of what it calls the addition of apparent wind veer and true wind veer. But I can't really see where this apparent wind veer comes from. As the boat moves faster, the apparent wind increases, but it increases at all heights by the EXACT SAME AMOUNT. So, it would seem that the degree of twist, which is a relative function, would be more or less the same. Using some purely hypothetical numbers, when the boat is stationary, let's say the wind at ground is 10 knots and at the mast top it's 15 knots. When the boat moves at 5 knots, the wind at ground will be (just under) 15 knots and at the mast top just under 20 knots. Relative twist shouldn't have changed significantly.

Perhaps this will help comprehension.

As wind speed goes up one lets out the sheet to avoid being overpressed. Wind speed @ 2m is frequently 50% windspeed @ 15m. therefore the top of your mainsail will need to be @ a higher angle of incidence than the bottom. That's why you always try and get those telltales on the training edge of your main lifting.

That's apart from wind-shear which can give you brown trousers when you're coming in to land.

 

[Roberto]

The boundary layer (the section of the atmosphere from sea level up to where the wind shows purely geostrophic behaviour) can be divided in a few sub-layers, depending on the conditions of the air.
The lower level, the surface layer, the one in contact with the surface of the earth, can be anything from about 30 to 100-200m thick, depending on the conditions. In this level the friction forces are widely predominant against any other, so basically the wind speed vector only changes in magnitude, while its direction remains constant. There is no "wind shear", if one considers shear as change in direction.
The layer above this one, aka the Ekman layer, is where COriolis comes into play and the wind direction begins to veer.<:
If one keeps on going higher, wind direction will be mainly sensitive to other factors like cold/warm air advection, etc etc backing or veering , etc.


A "normal" mast height is inside the surface layer, so it is only affected by changes in wind speed, not direction. For a given boat speed, an increase in true wind speed with height means the apparent wind shifts towards the stern with height, hence the need for twist, etc etc.


What can happen in some particular situations, the upper wind can begin to occasionnally interfere with the surface layer, that is where one mast may also see changes in wind direction with height (not only speed); the typical case is the onset of sea breeze in a particularly stable surface layer, the "new" wind -sea breeze- slowly drops in height, and for some minutes one can see the top of the sail reacting to a different wind from the one at deck level, it becomes almost untrimmable; once the "new" wind is widespread, the mechanics come into play again and the surface layer shows again only changes in wind speed with height.

 

[Javelin]

Where you do get a difference in wind dir is with gusts.

A rule of thumb on the sea is when sailing on Port tack and you see a gust coming then the chances are it will be a lift.
If on Starboard it's more likely to be a header.
This is I believe due to the Coriolis effect.~
As the opposite is true in the southern hemisphere.

This does not happen every time but enough for me to have won a few races being aware of it and either standing on a bit longer or throwing a quick tack in.

There maybe veer aloft but its usually due to other factors like the presence of land and resulting wind bend which will differ from the gradient wind direction.
However I don't believe we would see any significant veer between sea level and 40 or 50 ft though we would see an increase in speed.

 

[Alan ashore]

My experience is that wind shear varies a lot. I have only sailed in the northern hemisphere, and I am quite convinced that the wind is indeed much more often veered aloft than backed, but occasionally it is significantly backed aloft.
It is a rare day when there is no sheer, and optimum twist thus nearly always differs between tacks. Of course the only way to establish the optimum twist is to experiment, watching all the tell-tails AND monitoring performance.

 

[dom]

Where you do get a difference in wind dir is with gusts.

A rule of thumb on the sea is when sailing on Port tack and you see a gust coming then the chances are it will be a lift.
If on Starboard it's more likely to be a header.

Tks for the excellent link to stanford sailing - v. well explained. BTW I have always believed (in fact almost swore by) this "gust" rule of thumb, but recall a US academic paper a couple of years ago which poured cold water on it. Interestingly it concluded that in different weather conditions, the wind could either veer or back in gusts, and that the pattern would remain more or less constant as long as the weather dynamic remained unchanged. Irritatingly, I can't find the paper - I'll post if I do!

But here is Mr Singleton on the same topic: http://weather.mailasail.com/Franks-Weather/Gusts-And-Other-Wind-Changes

 

[William_H]

Interesting this corieolis effect. relatively recently it was explained to me and suddently made a lot of sense with our local wind conditions. Sometimes known as the Fremantle Doctor for most of summer and autumn we get a sea breeze every afternoon around lunch time. A soth west wind. Cooling after the warm morning.
The coast is essentially straight running north south. So I was aware that the land heating up causes air to warm and rise sucking in cooler air off the sea. The thing is it is not a westerly wind but rather a south westerly wind. The wind in flowing to the low pressure areas turns left (southern hemi) and so is amazingly predictable in direction and usuallys strength around 15 to 18 knots. Also surprisingly in summer nights the opposite happens and an easterly wind springs up when the land cools but ocean remains warm so the easterly wind is predictably north east. Usually not so strong unless the basic weather system aids it. So Mr Coreiolis certainly works around here to provide a very repeatable almost boring summer sailing conditions on a macro scale.
Regarding wind at different heights I certainly notice in very light wind conditions that the bigger boats with taller masts get more wind than us little tackers. olewill

 

[srah1953]

Quote “A "normal" mast height is inside the surface layer, so it is only affected by changes in wind speed, not direction. For a given boat speed, an increase in true wind speed with height means the apparent wind shifts towards the stern with height, hence the need for twist, etc etc.”

To Roberto - Can you explain the above please- if there is no change in true wind direction with height, why would the apparent wind shift aft with height, in that the “additional” wind created by the boat's forward movement is the same at every height – no one section of the mast travels faster than any other?

 

[Roberto]

To Roberto - Can you explain the above please- if there is no change in true wind direction with height, why would the apparent wind shift aft with height, in that the “additional” wind created by the boat's forward movement is the same at every height – no one section of the mast travels faster than any other?

Indeed the "additional" wind created by the boat is the same at every height, the change in speed is in the true wind, the higher you go, the faster the wind (up to a limit). Suppose the true wind speed is 10kt at 5m height and 13kt at 10m height: the apparent wind at 5m height will be given by the vector sum of the boat speed and the 10kt true wind.
The apparent wind at 10m height will be given by the vector sum of the boatspeed and the 13kt true wind.

In the picture, the black vector is constant and is boat speed, giving a wind component right against the direction of boat movement; the red vector is true wind, which has the same direction but increases in magnitude left to right. The green vectors are the apparent winds at the two heights, the "longer" the red vector (hence the true wind speed) the more open and longer will the green vector be.

If one wants to have a sail profile with the same angle of incidence to the apparent wind (which is the wind the sail is working in), then the upper sections of the sail should be "opened up", twisted, to account for the variation in apparend wind direction.



An interesting aside: boundary layer theory explains how the surface layer wind speed gradient with height (how much the wind speed shanges with height, always talking about a few tens of meters) is very sensitive to air stability. In very stable air, the friction will be felt mostly by the layers in proximity to the surface of the earth, the upper air will be quicker --> the wind gradient will be very pronounced, a lot of difference in wind speed with height, hence the need for a lot of twist.
Conversely, in unstable conditions, the mixing of air will distribute wind speed more regularly, and the lower layers will experience a wind speed very similar to the upper layers --> less twist.

A very rough evaluation of air stability-instability can be had onboard by comparing water temperature to air temperature. Cold air over warm water tends to be unstable, warm air over cold water shows more stability.

That explains why for a given wind speed at masthead, when the wind is relatively "cold" (unstable) the boat tends to heel more, as overall there will be more wind, for a given anemometer speed the lower sections of the sail will get more wind than in warm air, people onboard have the the impression that "air is heavier". The opposite in warm air.

(Re cold/warm air, some difference in wind dynamic pressure comes also from the variation in air density, though this component has a lot lower magnitude than the difference in wind gradient)

 

[srah1953]

Hi Roberto
Thank you for this, which makes sense (even to my addled brain).
You seem to know what you are talking about. Do you have a view on whether the wind changes direction in gusts? I would have said that there seems to be a lift in gusts (and I think it is a lift (rather than a veer or backing) ) and applies on either tack. And, if it is a lift on either tack, then it's not the result of Coriolis. I'll have to pay more attention to this out on the water to see if there is a consistent effect.
Srah

 

[Roberto]

whether the wind changes direction in gusts? I would have said that there seems to be a lift in gusts (and I think it is a lift (rather than a veer or backing) ) and applies on either tack.

There are several different factors to consider.
From the mechanics/vector point of view, if one defines "gust" as a temporary increase in wind speed only (without a change in direction) then yes we are in the situation you describe: an increase in true wind speed will make the apparent wind lift on both tacks. An example I am sure anyone knows: if the boat is going at slow speed, the increase in wind will first lift the apparent wind, we get a better heading, the boat will then begin to accelerate -which causes the apparent wind angle to close, but as the gust ends one finds himself with a fast boat in a slow true wind, which causes a massive turn of the apparent wind towards the bow --> sails may be backed, or anyway a large change in heading bearing away will be needed.

At a large scale, if we are in gusty territory, the gust is often an incursion of wind from the upper layer (think about cloud height) into the surface layer. That wind is veered to surface wind. When that wind arrives on the surface, it interacts with the surface layer characteristics: if the lower layer is, say, "normal", then the upper wind will temporarily replace the surface wind and we will experience an increase in wind speed *and* a veer in direction, which is the average case and understandably has different implications on the two tacks.
If the surface layer is particularly stable, dense, it will oppose a lot more resistance to the upper wind invasion (especially in light ocnditions), and the veer may not happen.

Similarly, at the same scale, if we are not yet in gusty territory but are sailing towards an isolated cumulus, we will experience very different gusty wind directions depending on which side of the cloud we are approaching. A cloud with precipitation will have another different pattern of wind distribution. And so on.

Then, there is the gust microstructure: the distribution of wind speed vectors *inside* a gust.
As an example, one common case of gust has an "open hand" speed distribution, the air splashes on the surface of the sea and then spreads away radially, like the fingers of an open hand.
Depending on the boat position with regards to the whole gust, wind direction felt onboard will be completely different. If the gust comes a few lengths in front of the boat, we will first be headed (say by the little finger of the hand), then the wind will veer towards its average direction (medium finger), then we will get a lift with the last side of the gust as the true wind veers (index finger).
If we catch the gust halfway, we will just see the increase in speed (constant direction, medium finger), then the final lift of the index finger.

In all, it is a complex matter as it involves relationships between phenomena at very different scale: from the structure of a single gust, to the conditions of the surface layer (stability/instability, state of the sea surface, etc), from cloud types (isolated, bands, precipitating or not, etc) to longer term evolutions during the day, etc. In terms of sailing it involves appreciating what is happening on the water with a time frame of seconds/minutes say as the gust hits, what may happen in 10s of minutes (how are gusts distributed over water for example), what may happen in a few hours as for example the atmosphere changes due to heating, precipitation, major changes of winds, etc.

Put it all together and from a sea anemometer one may record whatever type of gust behaviour, increase in speed and veering, backing or unchanged direction.


Just to give you an idea, this is the representation of a large scale gust front from a thunderstorm (sorry can't remember the Reference), can you imagine the confusion at sea level..



Addition
If you are interested, here is an example of wind speed changes with heights in three different types of surface layer, stable, neutral, unstable. As you can see, the total amount of wind against a sail can change a lot.

 

[srah1953]

Thank you again. No more questions. My curiosity is sated.