Wednesday, November 21, 2007

Lake Winds

I accepted the challenge from TK. I promised I would explain in more simple language what Stuart Walker was trying to communicate in his Sailing World article Unlocking the Mysterious Lake Winds.

Since accepting the challenge last Wednesday I've studied the article. I've puzzled over the meaning of many of the statements in it. I've tried to make sense of them by referring to Stuart Walker's book Sailor's Wind. But that left me even more confused.

And then, out of the blue, came an email from Tom Donlan, a J-22 sailor from Annapolis, who is also a professional writer and a great fan of Dr. Walker's thinking and writing. To my huge relief Tom had taken it upon himself to "translate" the first few paragraphs of Walker's article. Magic!

So here we go. Walker as translated by Donlan. With a few random jottings from Tillerman along the way...

Walker wrote...
Lake sailing is more complex than sailing on the open sea, but more predictable. A lake, like any body of water enclosed by land, is subject to gradient winds, always offshore, and by thermal winds -- the lake breeze itself as well as winds generated by the surrounding terrain.

Believe it or not, even this introductory paragraph left my feeble mind confused. I got stuck on that phrase, "gradient winds, always offshore". Whaaaat? Last Wednesday I took my almost-two-years-old granddaughter down to the local lake to indulge in one of her favorite games, throwing rocks into what she calls the "ocean". There was a big low pressure area over Hudson Bay creating a strong southerly "gradient wind" over southern New England. (Don't worry if you don't know that term "gradient wind". Tom will explain it soon.)

Emily and I stood on the northern shore of Lake Whippersnapper with that wind blasting into our faces. And I kept thinking about Walker's words. Gradient wind always offshore? Sure feels like it's blowing onshore to me.

So here is Tom Donlan's opening sentence...
Lake sailing is more complex than sailing on the open sea because the wind is always coming from the land and is therefore more shifty and puffy.

Oh, now I get it. Of course the wind over a lake has been blowing over land before it reaches the lake, even though in different parts of the lake it will be blowing offshore, onshore and even parallel. to the shore. That's all the good doctor was trying to say. My bad.

But wait. Walker used two technical terms in that first paragraph. "Gradient" and "thermal" winds. I think I know what they mean, but other readers might not. If you don't, then the rest of the article might not make much sense to you. So Donlan explains these terms ...
No matter where you sail, there are two kinds of winds: "Gradient" winds are the winds from a weather system; "thermal" winds are those generated by local temperature differences between land and water.

Fair enough.

In the first sentence of his second paragraph Walker says...
A lake is an oasis of calm in the surrounding land; a pool of cool marine air trapped above its surface protects it from the gradient winds and facilitates the development of thermal winds confined to the lake surface and its surroundings.

Donlan's version is...

A lake, surrounded by higher land, is less likely to be affected by gradient winds, which blow across the lake higher up than our masts. The smaller the lake, the more true this is. On the lake surface, the dominant winds are likely to be local thermal winds. There may be several different thermal winds in the course of a day.

(Tom confesses that he added the sentence "The smaller the lake, the more true this is". And the last sentence about several different thermal winds is really a lead-in to the next thought where Walker gets into discussing those different winds.)

But wait. I've sailed on some pretty small lakes, probably the smallest lakes that anyone in their right mind would even consider organizing sailboat racing. And I agree that such lakes are shielded to some extent from gradient winds by the higher land around them. And though it's less intuitively obvious I could even accept Walker's point that the lake is protected from gradient winds by a pool of cool marine air trapped above its surface. But I'm not sure I accept the implication that small lake sailing is dominated by thermal winds rather than gradient winds.

I remember many summer Sunday mornings on a small lake in New Jersey waiting for the wind to come in. And if there was a gradient wind of any strength at all it almost always reached the lake surface at some point during the day and was the dominant wind for sailing that day.

What's your experience on this question? Perhaps my New Jersey lake had some geographical factors that made it unusual?

Anyway, thanks to Ton Donlan for helping us out in understanding the insights of Dr Walker. There is lots more to come from both of them next Wednesday in our series on Walker's Words of Wisdom.


Derek said...

I am not sure why this is, but that little lake in NJ that you have so many fond memories of does seem to have some definite wind patterns to it. Besides the various geographic shifts, there seems to be a some what consistent pattern of the wind moving to the South / Southwest in the early afternoon when the gradient wind is light from another direction... particularly the Northwest.

Tim said...

I have found that some of the smaller ponds in the uk have some interesting effects near the banks.

The topographical features and the local effects of trees and buildings seem to have the most significant affects. Local knowlegde and good observation are vital if you hope to do well.

Tillerman said...

derek - It's certainly true in my experience on that lake when the gradient winds are light or non-existent that thermal effects start to come into play. And with the lake having a long axis running roughly NE-SW, and a big hill to the NW, the effect you describe could well be a thermal effect. Maybe that will be explained somewhere further down in Walker's article.

tim - you're absolutely right. Wind shadows, wind curves, and channeling are very common. I also know of one lake where in very light conditions there is a very local thermal wind generated by a large parking lot!

Pat said...

Thinking about these things, I sail on one lake where the long axis of the lake is aligned with the prevailing wind direction, and on another lake where its long axis is nearly perpendicular to the prevailing winds. That orientation may have some effect, but it's hard to untangle it from all the little local shoreline effects.

At least I did recently read a Dr. Stuart Walker article that made perfect sense on the first reading. But, it wasn't about the wind; it was about sportsmanship and nasty competitors, "Using the Power of Intimidation" in September's Sailing World.

As something to do on a non-sailing day, it might be interesting to write up a zoo or bestiary of skipper types. Type A screamers, marshmallows, clueless souls, the tactically rigid, protest-room lizards, frostbiters, and other curious sorts would be interesting to understand.

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