Monday, January 17, 2011

Gusts


Last night I was sitting at my desk enjoying the sound of the winds..and it was quite an audio treat. I would hear the rumble in the distance. It would get closer and closer. Then the house and trees would shake.

As all of you know, the wind is almost never constant, with ebbs and flows. As a result, meteorologists often talk about sustained winds and wind gusts.

Sustained winds are the winds averaged over a period of time, typically two minutes. Gusts are the maximum winds (or actually maximum three-second winds for NWS sensors) during some period (often the same as the sustained winds).

Wind gusts can be a LOT stronger than the sustained winds...a typical value is 30-40% stronger, but on occasions it can be more. Here is an example at the UW for the last 24-h--look at the top row which shows both sustained winds and gusts. Not unusual for the gusts to be 10 knots greater than the sustained winds. Or here are the winds in a hurricane--gusts were nearly twice the sustained winds at times.
A lot of time you can hear the roar of gusts before the strong winds hit, as I noted above. The anticipation is half the fun.

So why are there gusts in the first place?

The key reason is that gusts represent the mixing down of stronger winds from aloft. As many of you know, winds generally increase with height. The surface is relatively rough, with all the houses, trees, hills, etc. that slows the winds down at low levels. Aloft, away from the surface friction and drag. winds are stronger. The atmosphere is also a turbulence place...particularly when winds are strong or the surface is heated by the sun. The turbulence motions (called turbulent eddies in the business) mix the air in the vertical. You have all seen these eddies during the fall when leaves are flying up and down near the surface during windy periods.

Where an eddy brings air down from aloft the winds increase--you have a gust. When the air is moving back up, the winds slow. Last night we had some very strong winds aloft (see the Seattle profiler winds below)...at some points 50 knot sustained winds were only a few hundred meters above the surface. When some of that fast moving air moved down to the surface in an eddy, you had one hell of a gust.

Current numerical weather prediction models only provide sustained winds...they don't do gusts, which are too small a scale to directly simulate. So meteorologists have to add them to the predicted winds.

By the way, you can also see gusts on the water as darkened patches...known as "cats paws" by marine types (see pic).

PS: The weather is going to benign this week. No major events of any kind apparent in the models. Rivers will retreat. Snow level will push downward. And we may see sun on Wednesday.

PSS: Answer to the question below...you can hear the gust before it hits because the speed of sound (around 760 mph) is much faster than the speed of the gust (30-60 mph).

11 comments:

ninaf said...

Hey Cliff,

I just saw this post: http://community.livejournal.com/weathernerds/700710.html (Western Pacific Cyclone)

I was wondering if you could do a post on this. Also, any chance we might feel a little of this breaking apart?

brian said...

Here's the thing I found remarkable about last night's winds (at least in Ballard): even though they were strong gusts, they were confined to low altitudes. The trees were flailing and the garbage can lids were flying at ground level, but the overhanging cloud cover seemed perfectly stationary with respect to the moon.

brian said...

I realized after submitting my comment that I wasn't clear. Your article explains clearly that surface-level gusts are downward-pointing flows from higher-altitude winds. What seemed remarkable last night was that those higher-altitude winds had to be confined to some layer between the ground and the apparently stationary overcast. The appearance was quite startling.

Nathan said...

Cliff,

I'd like to point out a very large, deep low in the north Pacific. 45 N 175 E. This is an unusually massive storm with a pressure of only 933 mb, and probably hurricane force winds. I spotted this on NOAA's 'ocean prediction center' page. If this doesn't weaken, somebody will get hammered in a few days. I think it's likely to follow the usual track to Alaska or the Queen Charlottes. Any comments?

Christopher said...

Fascinating on gusts -- I've often wondered. But you didn't explain why we can hear them before they arrive. Is that because the sound of them hitting the trees or waves travels faster than the gust itself?

As for benign weather forthcoming, I'm sorry for the meteorologists who crave wild weather, but I couldn't be happier personally. Benign is just ducky as far as I'm concerned!

bonobo said...

Thanks Cliff,
I was enjoying those gust down here in Olympia last night, and by watching the clouds race by the moon I figured that they were just mixing down. Thanks for the confirmation and love the blog

davew said...

I was complaining to myself when I kept hearing the media "chief" meteorologists saying rising or lower "snow" level, when I thought they should be saying the "freezing level. Then in your latest post you use "snow" level. What gives?

Warmbowski said...

I lied awake and listened to last nights winds, while sneaking out of bed to peek out the window a couple of times. We are near the bluffs in Broadview, set back about a block. Out of our window we see a beautiful line of tall 70' poplars that stick up into the winds near the bluff. The tops fail in some crazy dance and they create a constant roar. Down by our house the short cherry trees and birches would sit relatively still and only flail occasionally with the downward gusts. It's a gorgeous sight and sound.

Mark said...

I live on Cougar Mountain in Bellevue at about 800 feet elevation. When we get strong winds, there's a low frequency rumble that is loud and powerful. I've never heard it like this in any other place I've ever lived. It's actually a little bit unnerving because I can really sense the incredible power.

Is it just because I'm closer to the stronger upper level winds that the sound effect is stronger?

Scrapycandy said...

Is it possible (based on history) that we could have snow in February, in the lowlands?

Josh said...

Scrapycandy-

February of 1989. Everyone was about write off winter. Then an push of Arctic air on Feb 1st.

http://www.komonews.com/weather/blogs/scott/113555249.html