BLUEMOMENT

The 11:00 wind.

How does that work then?

Sailing the west coast of Scotland I am usually in relatively sheltered waters and land is often less than 5 miles away on 2 or 3 points of the compass albeit there might be a 3000 mile fetch to one of the other quarters. When there is a strong breeze (or more) the wind typically blows day and night but on lighter days it often starts in the late morning and stops in the late afternoon/early evening.

Sea and land breezes are the obvious probable cause but the effect can be quite marked on very overcast days and completely absent (i.e. no wind at all) on bright sunny days. Of course the effect of such breezes is unpredictable in relatively narrow firths, lochs and sounds. The diurnal pattern seems quite consistent in comparison.

I don't remember this being covered in my coastal skipper meteorology.

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A very well recognised phenomenon among microlight pilots.In the evening the surface wind frequently drops right away, allowing sunset flights after being grounded all day by strong winds. Usually the pilot will find that above a certain height, perhaps 50 - 100ft, the wind is just as strong as it as on the surface earlier - and this means the landing approach has to be made with plenty of extra speed to allow for the increased wind gradient.

So what causes it? On this I am less clear, but I believe it is to do with the friction between the wind and the ground increasing or decreasing - due to temperature changes I think.

A bit vague but there you go - a start. Perhaps someone can enlighten us further.

Silkie, interesting one....are you talking about a wind that occurs at any particular time of year?

There could be a couple of explanations deopending upon your geographical location, and the time of year.

1. If there has been a sea breeze during the day, the whole circulation mvoes inland. At night, as things cool down, the circulation moves back out to sea. However, you could remain in the calm zone with the wind ceasing and the sea breeze ciculation collapsing before it reaches you.

2. At night the lower parts of the atmosphere become more stable because of the decrease in temperature. As this happens turbulence in the low layers is significantly reduced. You can see this effect taking place sometime when in the evening you feel no wind on-deck, but the anemometer registers 5 or 6kts.

2b. (This isn't directly related to your question but is worth mentioning). Think about what happens to pressure at night. Taking away other factors, air flows from high to low pressure. By day, as temperatures rise inland, air rises and pressure falls, so the pressure is higher over the sea than overland, therefore air flows from the sea to land. By night, temperatures inland fall, pressure increases and may become higher than over the sea at lower levels so the reverse happens, air flows from land to the sea.

These effects are very localised and are dependent upon geography. The best aide to forecasting these situations is you as there is now way that automated forecasts such as GRIB can pick these effects up.

Does that help or is it far too long?

Simon

weatherman wrote:....are you talking about a wind that occurs at any particular time of year?

Well I only notice it during the sailing season

Does that help or is it far too long?

Excellent answer with lots to think about.

There are plenty of geographical effects here where the wind bends round headlands and funnels through narrow sounds and many of these can at least be anticipated if not exactly predicted. There's the famous Sound of Mull trick where the wind can blow from both ends towards the middle and the weather crossroads at the top end of the Firth of Lorne which often produces large and seemingly unpredictable wind shifts.

I tend to think of the sea breeze as the classsic circulation which forms under an overlying offshore wind but obviously there must be lots of other influences caused by heating on even the most overcast days. I didn't know about your point 2 and this is clearly what Nick is referring to from his flying days.

TFT Simon.

Simon - this is Silkie you were talking too - he has a cunning habit of asking very good questions and giving the impression of advanced levels of intellect.
Sadly this is not the case so if you could make what seemed like a very comprehensive answer simpler then I'll do my best to explain it to Silkie.

I wuv you too Claysie-waysie.

(So pleased to have found a use for that emoticon)

Oh buggeroonies
Silkie's turning into Russell Brand