Why do clouds mess up Gorge winds?

[bobgatpdx1]

I've been hearing this pearl of wisdom for many years - today it was certainly true at the Hatch. Was blowing OK until some clouds drifted over the river and sure enough the wind died.

But WHY does this happen?

The Gorge is driven by pressure gradients - I typically look at the PDX-DLS gradient to be 0.06 or greater for good winds in the Corridor. A cloud passing overhead doesn't change the overall gradient, so why does it shut the wind down?

Discuss!
BobG

[isobars]

If the clouds include a squall or a dry downdraft causing the wind, the wind will slam shut as the downdraft passes overhead. In the former case, that first raindrop is your warning to get yer butt ashore immediately, as the downdraft is at your door.

To summarize whole chapters from the many micro-weather books written by Alan Watts for sailors ... If the surface winds are caused by overhead winds mixing down to the surface, each clear area heats up the surface below it, causing a rising bubble of surface air. That bubble displaces a higher-altitude chunk of air, which is moving horizontally already -- very rapidly if the jet stream is anywhere nearby. The larger/longer-lasting that clear area is, the steadier the mixing. Vertical stability/instability play a large role in this process. Big puffy clouds, whether white or dark, can indicate localized vertical air displacement which can impact wind quality by "grabbing" chunks of fast-moving and throwing them at us randomly to create gusty conditions, while stratified clouds -- boring, featureless layers of gray with or without rain -- indicate limited vertical mixing, in which case it can blow quite steadily if a gradient provides the push. This is when a visored helmet can make life a lot easier, as wind-driven rain stings but the wind quality can make it worthwhile.

Or clouds can make a front visible, in which case we can expect gusty SW winds ahead of a frontal cloud line, a dead zone as the front passes overhead, and clearing winds from W or NW behind the cloud line. (Note: "expecting" wind and getting it are obviously not the same thing.)

If the cloud is distinct, mushroom shaped, and glowing, rig your 2.0 and get ready for a millisecond of honest-to-God nuclear wind.

Mike \m/

[ericandholly]

Chicken or the egg? Do clouds mess up wind, or are clouds evidence of wind that is messed up?

[Bond1]

[einhorn]

Don't know if the clouds cause or are caused by the changing conditions, and don't really care. I can't see pressure changes, but I can see clouds, and when I see them, it's enough to know to pay more attention to possible changes in the wind.

[isobars]

On the flip side, many people overanalyze and react to every stinking cloud they see. There's a happy medium. If clouds meant just a little bit less than they do, I'd have made a T shirt decades ago that read, "Clouds Don't Mean $#!+".

Mike \m/

[bobgatpdx1]

Here's a link to the Alan Watts books that isobars referenced:

http://www.sheridanhouse.com/meteorology.html

Sounds like this would be useful for any windsurfer.
BobG

PS: Would still like to hear a comment from iW weather geeks on this topic.

[gmclean]

i think it depend a lot on the type of clouds and the overall weather pattern.

Low level cumulus in westerly flow indicate vertical motion. In general if the air is rising it cools and condenses so low level clouds usually indicate an updraft which makes for turbulent gusty conditions. The wind can decouple from the surface in these conditions so wind on the river drops but is still strong aloft.

If I remember correctly, the wind on Saturday was a mix of westerly flow aloft mixing to the surface and surface pressure gradients. This is common with a trough over the region which sets up cold air aloft and unstable conditions. Clouds coming through the central gorge (Hood River) in westerly flow would indicate updrafts, gusty conditions and potential for decoupling.

Of course the standard meteorological caveat is that this is a generalization and YMMV

[ericandholly]

I got decoupled rather often in high school. Happily married now, though.

What's up George? Long time no see.

Eric

[gwandsh]

I noted on Sunday that the wind was extremely flukey in the eastern gorge. The clouds seemed to be higher and wispier, not the thicker Cumulus type.

It was really odd, because the sun would occassionally poke out, and in the past I would expect this to coincide with a big wind build (and sometimes a direction change). But for some reason the wind speed seemed disconnected from the cloud density.

When somebody hit the switch and it dropped from 20+ to 10 in a few minutes, the day looked pretty much as it had for a few hours.