Wind and tide.

Allan

Allan

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I have heard, in the past. of a relationship between wind and tide but always dismissed it as being, at best, slight. Last weekend I was sailing in the Bristol channel with spring tides (8-13m depending on which end I was sailing) . I had 0 to 24 knots of apparent wind and all of the best winds were at high tide. Is there a relationship? If so, why?
Allan
 
Allan

there are a number of factors but the first you have already alluded to.

The apparent wind is what your sails actually feel. In a no breeze but tide situation, if you are moving across the surface at say 6 knots due to a strong tide you have now got 6 knots apparent wind against you, as you would now be sailing upwind your own boat speed will further increase the apparent wind, which in turn further powers you.

Another key factor is the effect that the movement of water has on the air just above the surface. Liken it if you will to a travelator. If the tide/travelator is moving at X knots it drags the person/air above it along as well.
 
Doug_Stormforce said:
Allan

there are a number of factors but the first you have already alluded to.

The apparent wind is what your sails actually feel. In a no breeze but tide situation, if you are moving across the surface at say 6 knots due to a strong tide you have now got 6 knots apparent wind against you, as you would now be sailing upwind your own boat speed will further increase the apparent wind, which in turn further powers you.

Another key factor is the effect that the movement of water has on the air just above the surface. Liken it if you will to a travelator. If the tide/travelator is moving at X knots it drags the person/air above it along as well.
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Yes I fully agree with the point about the apparent wind and worked with some of that at the weekend. With a tide of a bit less than 4kts I had a SOG of a little under 10kts. With the lack of wind at low tide that would have been nothing without the engine.
I can see your point about the "travelator effect" but the true wind direction was relatively constant even when the tide turned. It may have been coincidence but the strongest winds seemed to be either side of high tide and I wondered if there was some sort of explainable relationship.
I should have said before that most of the time the true wind was just ahead of the beam (across the tide) and only moved forward due to the boat speed.
Allan
 
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Allan said:
I have heard, in the past. of a relationship between wind and tide but always dismissed it as being, at best, slight. Last weekend I was sailing in the Bristol channel with spring tides (8-13m depending on which end I was sailing) . I had 0 to 24 knots of apparent wind and all of the best winds were at high tide. Is there a relationship? If so, why?
Allan
Click to expand...

When I was dinghy racing as a kid we were always firmly of the view that (nearish the coast) the wind blew harder at high tide because we'd been lifted up higher into the air where there was generally more wind and blew less at low tide a) because were were lower and out of the wind and b) there was generally more shelter from any nearby land. I think we just based that on observation but as long as you're near land it doesn't seem totally implausable to me today.

Maybe Mr Keeling will explain?
 
oldsaltoz said:
Have you considered the effect that for several square miles the air has been pushed up several metres by an incoming tide.:confused:
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No, I had not really thought about that. I will have to give that some thought. One of the reasons for asking about this was to try and help me plan some of the trips I have planned this year. Most of them will be out of the Bristol channel, so any link between tides and wind will be less.
Allan
 
toad_oftoadhall said:
When I was dinghy racing as a kid we were always firmly of the view that (nearish the coast) the wind blew harder at high tide because we'd been lifted up higher into the air where there was generally more wind and blew less at low tide a) because were were lower and out of the wind and b) there was generally more shelter from any nearby land. I think we just based that on observation but as long as you're near land it doesn't seem totally implausable to me today.

Maybe Mr Keeling will explain?
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Mmmmmm, yes I can understand how that would work inshore but at the time I was from 2 to more than10miles offshore.
I have always thought that any layering of the wind started at sealevel and the layers moved up and down with the tide. You now have me wondering if the tide was lifting me up into a faster moving layer of air?
Allan
 
Turbulence

Allan said:
..... You now have me wondering if the tide was lifting me up into a faster moving layer of air?
Allan
Click to expand...

You can think of it like that but what happens is that the boundary layers are changing shape based on a lot of variables. They are settling down into a more laminar flow regime.

See http://en.wikipedia.org/wiki/Boundary_layer

toad_oftoadhall is quite correct in his observations about wind strength with regards to height. The effect though is more to do with the turbulent area generated by a bluff of land.

The cross sectional area, profile of this bluff, wind speed and direction will all impact the strength and length of a turbulent area. Dont think that the turbulent area is indicated by confusing wind speeds and direction, it may be in close, but the majority of the turbulent area will be just a pressure area that extends upwind. The most obvious feature is simply a reduction in the steady speed of wind for some miles out to sea i.e. it reduces the pressure differential at a very local area which results in less air movement. In close to the land it may even be a reversal of wind direction.

This implies that as the tide rises, the cross sectional area and profile of the bluff that generates the pressure area changes. The negative effect of the pressure area gets less at the local area and the wind speed increases.

Basically you are not getting lifted into a clear stream by the tide, rather, the wind has less resistance to over come and turbulent effects are reduced.

However, if the land is gently sloping up and away from the sea, then drops away again, you may also have an effect which actually causes the air to accelerate to get over the profile, the shape of land promotes faster air flow. The air compresses, the pressure drops and the velocity increases (wing profile). Even on this optimum shape there is a turbulent area falling back from the edge.

The height from sea level to the bit of atmosphere where the wind direction is affected by the jet stream, as opposed to weather system, is probably so high that any rising of sea level by tide has a negligible effect on sea level wind speeds.

When racing this type of local knowledge can be exploited to great advantage but its not straight forward to predict.
 
.... or of course it could be that it just happened to get windier around the time of high tide....

Bizarrley i have seen it very windy at LOW tide as well!
 
I think it's more likely that, over a short period, high tides occur at around the same time of day and it is the time of day that affects the wind rather than the tide.
 
There is a definite correlation between tide and wind, in that the wind conditions, if they are set to change, will almost always do so at the turn of, or half tide.
The 'travelator effect' is something I have argued for years. When the ebb sets off at the Lizard, in SW wind, the wind will freshen, likewise for E wind when the flood sets in. Seems like sod's law, just to make things as bad as possible. But if the air is being carried away by the 'drag' of the water, then it will try to get back to where it was, against the tide, and as the waves kick up the drag effect becomes greater. I asked about this in the met section of the YM stuff, but no real answer.
 
Old wives tail or truth....?

I often work in the towns along the shore of the Cromarty Firth.

One fine still summers day I was working outside, when suddenly from nowhere a strong wind blew up. I commented to the owner "where's that wind come from all of a sudden"

His reply was "high tide"

Since then I've paid more attention to the weather, and a strog wind an hour either side of high tide, on an otherwise still day, is not uncommon.

When we get the boat back in the water this year, I'll see if this is true also of the Moray firth, and whether it only happens on shore, or out in the water as well.
 
Hi all,

Do I 'wade in' from a weathmans viewpoint? There was a readers question in YM a couple of months ago to which I wrote a reply.

The tide does undoubtedly have an effect on wind speed and hence direction. Now, this is something that you would not be able to model and so don't expect it to be included in any GRIB or internet forecasts; many human forecasters would struggle with it too. Much is down to a yachtsmans 'sixth sense'.

In my opinion the main effect is down to temperature. I've not come across any sceintific papers on this, and one day when I have more time on my hands (yeah!) I'd like to look into it further.

You see, there are a couple of scenarios. A change in tide can bring a change in water temperature as eddies within the water bring warmer/colder water to the surface. This must have an impact on the air above the surface, changing the temperature and increasing the temperature gradient, hence leading to an increase in wind.

The height or the tide has to have an impact too, as as the tide rises the column of atmosphere above the surface effectively becomes less (even a 1 or 2m change could make a difference) and so this compression effect must have an impact too.

Justing thinking further into a sea breeze scenario. This relies on a temperature contrats between the land and sea. If cold water were to upwell, this decreases the sea surface temperature, increases the gradient beweetn the land and sea temperature and hence increases the sea breeze speed. The opposite would be true of warm water.

Similarly with fog. If cold water upwells and there is warm air on top this must increase the chance of mist and fog?

I could go on but you are no doubt asleep by now!

Simon
 
Oh my!

Thank-you Simon and everyone else for your replies. When I asked the question I thought someone may reply with one reason which I could consider and either try to use or dismiss as too slight to work with. I now have all of the above to think about!
I think I will print out this thread and, over the next few months, try to relate the theories to each situation. As I sail mostly in the Bristol channel at least I have some of the largest tides to learn from.
Allan
 
A surfer's perspective....

I surf along the Bristol Channel coast and have noticed several affects the tide has on wind speed and direction. I've searched for some scientific reason for this also and can't really find anything. Like several posts have said it's probably a factor of many things not just one. Anyway, my main observation is that the wind speed seems to increase on the flood and decrease on the ebb in the Channel. The increase and decrease is quite marked, from 5mph to 15mph especially on warmer days. I can always rely on the wind being significantly lighter than forecast on the ebb (lighter wind generally better for surfing).

As for direction, the wind always seems to come more onshore on the flood. ie it could be NW on the ebb but back WNW or even W as it turns ... and it stays that way for the whole of the flood. The opposite can be said for the ebb, the wind seems to veer more offshore as it turns to go out ... and again it remains that direction for the whole of the ebb.

So in summary. As the tide goes out the wind drops and turns offshore. Unfortunately (for surfers) due to the huge tidal range in the Bristol Channel swell heights decrease on the ebb (wave / current interaction).
 
jeffbeanpole1 said:
I surf along the Bristol Channel coast and have noticed several affects the tide has on wind speed and direction. I've searched for some scientific reason for this also and can't really find anything. Like several posts have said it's probably a factor of many things not just one. Anyway, my main observation is that the wind speed seems to increase on the flood and decrease on the ebb in the Channel. The increase and decrease is quite marked, from 5mph to 15mph especially on warmer days. I can always rely on the wind being significantly lighter than forecast on the ebb (lighter wind generally better for surfing).

As for direction, the wind always seems to come more onshore on the flood. ie it could be NW on the ebb but back WNW or even W as it turns ... and it stays that way for the whole of the flood. The opposite can be said for the ebb, the wind seems to veer more offshore as it turns to go out ... and again it remains that direction for the whole of the ebb.

So in summary. As the tide goes out the wind drops and turns offshore. Unfortunately (for surfers) due to the huge tidal range in the Bristol Channel swell heights decrease on the ebb (wave / current interaction).
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Some 'scientific reasons' perhaps!

http://www.seabreeze.com.au/forums/Kitesurfing/General/How-can-tide-affect-the-wind/ ;)
 
I suspect that there are several factors that may or may not be present or important on any particular occasion. Many of the suggestions made in this discussion could apply at some time or other. However, it is a feature of weather that we all tend to formulate rules on a few occasions and then generalise. Being cynical by nature, I have learned to think of 1 as few, 2 as several and 3 as many. The hoary old one that gusts always veer is a case in point. The statement that an offshore wind will veer as it crosses the coast is another.

Further, always be careful when a local says that something or other "always" happens. When David Houghton was forecasting for Olympic or other major events, he always used to canvass local opinions. But, he once estimated that, in this context, “always” meant between 30% and 40% of the time.

Simon is correct in saying that the temperature of the sea is important. That must have an effect on the stability of the air and, therefore, the depth of air through which mixing will take place. Effects quoted about changes of wind at change of tide might just be the result of the tide pushing you towards the wind or away from it with a resultant few knots difference in the apparent wind. Low tide with an offshore wind might mean air travelling over a rougher surface and be more affected by friction. I can imagine height of tide being a factor in the Bristol Channel or the Channel isles but less so down the west of France.
 
SWMBO noted a similar effect many years ago. As we sail in estuaries & vast areas of shallow water, I wondered if the covering up of a few score square miles of hot sand as the tide came in might affect the Sea Breeze phenomenon.

But what would I know?
 
Anything that can create temperature differences in the atmosphere is likely to affect the wind. Any roughness of the surface over which air is flowing will affect the wind. Any change in the stability will affect the wind.

It is virtually impossible to give a definitive answer to the questions posed in the original post or in the many experiences quoted since. As Simon rightly said, it is a matter of using your own experience. Forecasts, however good the numerical weather prediction models are, or however good are the forecasters that interpret them, can only achieve so much.

Forecasts do really well over periods of up to 5 days on the large scale. When it comes to local, small scale effects, there is always going to be a need for the sailor to interpret the forecast using intelligence and native wit. Even were NWP models able to describe detail on a one mile scale, the problem of communicating that information to us in any broadcast or written verbal form is simply impossible. Further, even if we could receive all that amount of information in some computerized form or other, there remains the problem of predictability. Small weather details have short lifetimes. Roughly speaking, something of scale 1 mile can last for 1/2 hour. A thunderstorm, say 10 miles across, has a 6 hour lifespan.
 
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