Wind Gust predictions

[Gcosloy]

Wind represents a significant factor in determining the relative safety and comfort of a kayak trip. I'm relatively knowlegable about how wind is generated but am pretty ignorant about how gusts are. What are gusts? What causes them? Can anyone point me to a decent and simple explanation of this phenomenon? I've already tried researching this topic on the Internet and found the explanations to be too complicated for my feeble mind.

[Phil Allen]

Hey Gene

This post ended up in the boat repair subsection. Probably won't get too many responses here.

best

Phil

[cfolster]

Hey Gene

This post ended up in the boat repair subsection. Probably won't get too many responses here.

best

Phil

Moved!

[leong]

Gene,

I guess you understand that wind is caused by differences in the atmospheric pressure. Over the water wind gusts are caused by wind shear (a change in the wind vector over a distance). The wind gusts speeds are typically 30 to 50 per cent higher than the average wind speed, but stronger gusts are likely in the vicinity of showers, thunderstorms and frontal systems.

Okay, so far, simple enough. However, I don’t think one can really understand wind gusts and how to predict them without the thousands of hours of arduous study necessary for true comprehension.

Perhaps John Huth can add to this.

-Leon

PS
I spent a few hours looking over technical papers discussing wind gust prediction models. Some fancy statistical distribution models are used as well as Bayesian inference approaches. I couldn’t find an easy to understand approach.

[josko]

So hard to figure out what this question's about, but here goes something: gustiness is a representation of three-dimensional flow. Look at a smoke/steam plume from a tall stack to see the eddies and other 3-D flow manifestations. For me, a good way to estimate gustiness is to look upwind. Wind coming from the open ocean is often less gusty that that coming over land or other structures. (A major tactic for sailboat racing on the Charles was to identify vortices coming off the Hancock and the Pru and adjust course accordingly.) Let's think about a single tall structure in the wind flow: it will likely display a vortex (eddy0 trail on the leeward side. Each vortex is 2-3 times bigger than the structure that generated it, and velocity goes from ~0 to twice the mean flow. if you stand on the north shore of the Charles river basin in a southerly flow, you can easily see the gustiness (eddies) coming down from which ever building is directly upwind. It's a good place to look at gustiness and try to relate it to wind flow. For a little more on the topic, see:

http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_streetwhich has some basic equations describing how often gusts will come, how strong they are, and how long they last.

Moving to a more complicated topography, things get, well, more complicated But still, the basic relationship between gust intensity, duration and period holds. Now, if you're at a mouth of a fiord or at the base of a mountain with katabatic winds coming down, there's a whole new level of complexity (For those interested, Google 'jet vorticity' and hang on.)

The short version is, look at the 'Karman vortex street' Wikipedia article, absorb as much as you can, and learn to relate it to upwind topography. It's worked for me. And standing on the bank of the Charles with an open mind's gotta be the single best way to learn about gustiness (i.e. vortex shedding) that I know of.

(And if you've ever raced sailboats in Winthrop bay, you know the only way to win is to get yourself in phase with vortices from jet exhaust. )

[cfolster]

For the simple-minded, like myself, I found the following explanation most helpful (and all I really need): http://host.madison.com/news/local/ask/weather-guys/ask-the-weather-guys-what-causes-wind-gusts/article_3c4ccc46-37db-11e1-ae38-001871e3ce6c.html

I also found the following poem in my search, which I found quite delightful:

Big whorls have little whorls
That feed on their velocity,
And little whorls have lesser whorls
And so on to viscosity.

-- Lewis F Richardson

[Pintail]

Josko, forgive, please, my minor correction; but simply to say that there is "a vortex" behind a building is gross over-simplification. At the downwind side or edge of the building there will be vortices all the way along that edge or side, resulting, perhaps, in one grand turbulent area; but one vortex -- no. Look at the wing of an aircraft (our idealized, "go-to" example, I daresay): there are trailing edge vortices all along the wing, with one greater vortex at the tip.

Leon is correct with his comment about barometric pressure: itself and the winds due entirely to rotation of the earth, along with localized variations. I would not have thought it possible to predict gusts?

Interesting post; but, ultimately, futile, I suspect, Gene.

Edited April 8, 2015 by Pintail

[josko]

True, it IS a gross oversimplification, but it's surprising how often one can estimate nature of gusts by looking at upwind topography.

[jason]

... deleted.....

Perhaps John Huth can add to this.

.. deleted....

My guess is John is back to being busy again http://www.sciencedaily.com/releases/2015/04/150406083258.htm

[JohnHuth]

Yeah, you got me.....I spent the entire past week cranking on physics research-y things.

I haven't given "gustiness" a lot of thought as I write this, but I'll give you my take.

Gusts come from a lot of factors. I absolutely love "von Karman vortex streets" - look this up and you'll see some great images. This is mainly wind forming mini tornadoes - first right handed, and then left-handed, when it flows around an obstacle.

Take a look at the pattern of winds you get in dry leaves as they're carried in the wind - often times you'll see these mini-tornadoes in the pattern of the leaves blowing. Another great observation is to look at how branches of trees or long grass moves in the presence of wind. If wind were uniform, you'd just have a constant displacement of a branch, but they move back and forth under the influence of the wind and their own resistance to motion.

Another factor is the change in speed from the surface into the higher reaches of the sky - there's more friction at the surface, which slows air down, but the flow is not all horizontal - you can get vertical flow that taps into higher speeds as you go higher - you get up drafts and down drafts which are not uniform and create gusts.

Another factor still is associated with convection - hot air rising and cold air sinking - this isn't uniform. One example is the up and down drafts associated with thunderstorms - and can be quite local (and dangerous).

Typically the higher the average wind speed, the larger the 'gustiness' you'll get because there's a bigger difference between the surface flow and the upper level flow. Yesterday (Saturday, April 11th) there was a lot of "swirliness" going on that I noticed,

Typically you'll get more gusts when the conditions for thunderstorms are ripe.

Sorry it's such a complicated answer, but air (a fluid) is like that - complicated.

In the words of Lewis Fry Richardson (a physicist and meteorologist who studied such things):

Big whirls have little whirls that feed on their velocity,

and little whirls have lesser whirls and so on to viscosity