Wednesday, November 28, 2007

Lake Winds - Part 2

The second in a series of articles discussing Stuart Walker's article in Sailing World on Unlocking the Mysterious Lake Winds with help from a "translation" of part of the article provided by Tom Donlan.

In Part 1 of this series, we set the scene by talking about "gradient" and "thermal winds" and why local thermal winds are often the dominant factor in lake sailing.

In the second part of the second paragraph of his article, Dr Walker discusses the kinds of wind to expect on a lake on a clear day when the sun is shining. This is classic Walker.... one sentence running on for nearly 70 words... technical words explained in phrases in parentheses... text galloping madly on and and on.

So let's take it slowly and find out about each kind of wind one by one. I've split Walker's words of wisdom into numbered bullet points, one for each type of wind, to make it somewhat easier to follow.

Cold lakes can only expect a sailing wind on clear days in the presence of insolation (surface heating by the sun), and the winds that are then most likely are
  1. a morning offshore downslope wind,

  2. an afternoon onshore lake breeze,

  3. or an afternoon offshore expansion differential wind (created when the pressure levels above an elevated lake fail to rise as much as those above the surrounding heated lowlands).

  4. Then thermal turbulence (heated air rising and expanding and cooler air sinking above the near-shore heated land) brings the cool air on the upper mountain slopes down to the lake

  5. or brings the cool marine air onshore in a lake breeze

  6. and brings upper, higher velocity airflow in an overflowing gradient or expansion differential wind offshore onto the lake surface.
The first two winds seem fairly straightforward. Expect air flowing over the land on to the lake in the morning. And then as the land warms up in the sun, expect a thermal breeze blowing towards the land in the afternoon.

Or as Donlan puts it (with my bullet points added again)...

On a clear, sunny day, a lake sailor may experience
  1. a morning offshore wind coming down the slope from surrounding hills and then

  2. an afternoon thermal lake breeze blowing toward the warm shore.
So far so good. If only life were so simple.

Another option according to Walker is an offshore breeze in the afternoon, or as he puts it, "an afternoon offshore expansion differential wind (created when the pressure levels above an elevated lake fail to rise as much as those above the surrounding heated lowlands.)"

You see, this is why I find Walker hard to read. I trust his information is correct because of his reputation but I don't find that last idea very helpful. I sort of understand the physics of why the situation he describes would cause an offshore wind. But I want to know when that condition would occur. And why. And how I can predict when it will happen. It's not much use to be told in effect, "in the afternoon the wind may blow offshore; then again it may not."

Let's see if Donlan conveys that thought any more clearly...

A lake high in the mountains, however, may generate an afternoon wind blowing off the shore if there is a great deal of temperature difference between the water and the surrounding land.

Aha. I think I get it now. If the water is warm and the land is cold, then the land may not warm enough to generate a thermal onshore breeze, so you get an offshore breeze in the afternoon instead. At least I think that's what he means. And at least there's some information in the Donlan version that I can use to predict what will happen in the afternoon.

And if the thermal onshore breeze doesn't kick in, Walker says you can get any or all of winds 4, 5 and 6 which basically means
  • it could be offshore
  • it could be onshore
  • it could be the gradient wind.
Thanks Doctor. I feel like one of your patients and you have just described that I have some awful medical condition and when I ask you what it means you tell me, "Well you could go blind, or impotent, or you might die tomorrow." Thanks Doctor.

Let's see how Donlan delivers the prognosis...

Heated air rising and expanding above the warm land near shore and cooler air sinking above the water near shore can create turbulence. This disruption can do almost anything.

It can bring the cool air on upper mountain slopes down to the lake, like the morning breeze. It can bring the cool moist air over the lake onshore in a lake breeze. It can even bring strong breezes from the overhead gradient wind onto the lake surface, in whatever direction the gradient wind happens to be blowing. Sometimes it can do all of these things simultaneously within short distances.
lol as they say on the forums. lmao even. I love that phrase, "This disruption can do almost anything." And then having described the three kinds of winds you might see, he delivers the coup de grace, "Sometimes it can do all of these things simultaneously within short distances."

Ah. That's the lake sailing I know. Anything can happen anywhere. Some wind. No wind. Wind from almost any direction. I'm sailing a beat and that boat on the same leg 50 yards away is on a run. Yes, that's lake sailing.

So, to sum up, what have we learned today? On a sunny day on a lake, expect an offshore breeze in the morning that might be replaced by a thermal onshore breeze in the afternoon if the land warms up enough in the sun. If not, anything can happen and probably will.

Comments welcomed. Especially if I have misinterpreted what the experts say. Does this explanation so far align with your experience of lake sailing?


Carol Anne said...

There was one race up at Heron (elevation 7200 feet) this summer that really showed the "anything can happen" circumstance.

It was a distance race, with multiple roundings of various navigational buoys in the lake. Black Magic was uniformly ahead of the rest of the fleet by about one leg.

Every time we rounded a mark, the wind shifted, and so every single leg of the race for us was upwind. Conversely, for all of the rest of the fleet, everything but the first half of the first leg was downwind. So they had their spinnakers up nearly the whole time.

Up at Dillon in Colorado, one can often see two boats, heading right at each other, both with spinnakers up. You would think that before they meet, one of them would have to take the spinnaker down. Not necessarily. In the middle of the lake is often a puddle of totally dead air, so both boats end up without the spinnaker.

Pat said...

Except for the J24 whose crew turned on her motor, shifted it into reverse, and went backwards around other boats on the lake, thereby keeping her spinnaker filled and drying in the sun.

A contribution from the navigator:
Wherever you go, there you are.

mj said...

I think by "afternoon offshore expansion differential wind", Walker may be referring to an upslope, anabatic wind. This would be a case where the wind you experience on a mountain lake is air that is on its way from a high pressure source in a valley lower in elevation (than the lake) to lower pressure over or near the lake. Lake Garda in Italy would be the classic example of this. I believe Walker writes about this lake extensively in the Sailor's Wind. The Columbia River Gorge would be an example of this phenomenon in the US, though other effects come into play there (I think).

What repeatedly confuses me in this article, is the use of the terms offshore and onshore (which you pointed out earlier in this thread).

bwanapete said...

All this depends on the size of the lake, and the hills around it. Lake Champlain - wind blows north or south. Ontario, the rules may apply. NY's Finger Lakes - depends on the hills. Smaller lakes - mostly ignore the rules.

What do others think?

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