Sailing upwind in gusts

Moderator: GreenLake

Re: Sailing upwind in gusts

Postby Interim » Thu Mar 03, 2016 5:30 pm

Is "thermal" the force behind sea breezes and land breezes? If so, then my answer is no. I sail on lakes that range from 3,000 to 10,000 acres. I don't think it is big enough to create that dynamic.

We do have surrounding terrain that can create wind shadows, and altered currents as you referred to in a previous post. But the unstable systems seem to me to be a bigger factor than the geography. We inland sailors are pretty active trimmers :)

--john
1979 DSII
Interim
 
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Re: Sailing upwind in gusts

Postby GreenLake » Thu Mar 03, 2016 11:08 pm

Even smaller lakes show sea/land breeze effects, but they are short lived (almost "pulsing") because the smaller size of the water means that a lake breeze can run out of air (after, say 20 min) so that the system collapses, but then it restarts later. If you have strong gradient winds most of the time you might not see the effect of thermals as separate, because it would mainly moderate a bit the gradient wind.
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Re: Sailing upwind in gusts

Postby Interim » Wed Mar 09, 2016 9:53 am

If that's the case, that may explain part of the puffiness we get, and the direction shifts. We can have big temperature swings (30 degrees or so), which I guess would contribute to that.

--john
1979 DSII
Interim
 
Posts: 141
Joined: Mon Sep 30, 2013 8:40 am
Location: Nebraska

Re: Sailing upwind in gusts

Postby GreenLake » Wed Mar 09, 2016 2:25 pm

There are temperature swings that come from having different air masses (cold vs. warm) move through. That's associated with frontal systems and what they bring is called the "gradient" wind, because it is caused by pressure gradients between high and low pressure systems.

Thermal effects are those caused by different rates of warming between land and water. On land, only the topmost layer gets heated by the sun, so it warms up quickly and gives rise to rising air. For water, the situation is different, because sunlight penetrates, so a much deeper layer gets warmed up, and more slowly, so the water stays cooler than the air. That sets up a circulation where the land sucks in air from over the water, heats it, pumps it high in the atmosphere where it flows out over the water, to cool off and sink.

That's the sea breeze. It blows in a zone parallel to shore, a zone that widens as the day goes on. In some places (with deeper inlets) it can take until late afternoon for the sea breeze to reach. An overall (gradient) wind pattern that moves air offshore can help the sea breeze more than the reverse pattern.

For smaller, land-locked lakes the lake breeze can run out of air, as the hot land all around sucks up the air.

I sail on a bay, sometimes, where during the day there are local breezes (like a lake breeze) but where every afternoon at about five, the larger sea-breeze pattern comes in from the full body of water, after an initial calm period. Very typical that we sail out, run of out wind, take a nap, and then fight a stiffening breeze trying to get home. Always during summer days with lots of sunshine and the water still cold. The sea breeze is very steady (not puffy) because it flows close to the ground with a low ceiling (above which the return flow happens). That supposedly limits the turbulence because really large scale eddies can't form.
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