Saturday, May 8, 2010
Mountain Convection and A Strange Whirlwind
During the past few days we have had marvelous, and sometimes impressive, convection over our mountains (as seen in the picture above from Dale Ireland's webcam). The mornings have started out clear or close to it and then by mid-morning puffy white cumulus clouds formed over the Olympics and Cascades. By early afternoon some have grown into towering cumulus congestus and finally into fully formed cumulonimbus with showers and even some lightning.
Dale Ireland has a wonderful high definition web cam aimed at the western Olympics and here is the daily animation for Friday (http://www.drdale.com/lapse/lapse100507.mov).
Take a look at the visible satellite mixture for Friday morning and then at 5 PM. It is not hard to see the building convection from space (35,000 km above the planet!). The convection on Friday was most intense over the western Cascade slopes and the cool down rush of air from the storms pushed westward towards the lowlands, spawning some modest convection at its leading edge.
So why did we see this convection? Convection is the result of vertical instability of the atmosphere-- under the right conditions, when air given a push upwards, it keeps on going for while. Buoyant plumes of air, warmer than their surroundings, move upwards to 10, 20, or even 30 thousand feet in some locations. The atmosphere was close but not quite unstable throughout the region, but over the mountains the instability could be released.
Why is that?
As mountains are heated by the sun, air moves up their slopes and converges near the mountain crests. The two upward air stream create a strong upward current, which can give air parcel enough of a kick upwards to make the air parcel lift enough to become saturated and for instability to be released (I am skipping the details here, but you get the idea...upward motions on the mountains helps cause cumulus to form).
Anyway, things will calm down tomorrow...a perfect Mother's Day..highs in the mid 60s.
And now the fun part of the blog.
A "whirlwind" hit Genesee Park in Seattle this afternoon destroying a tent at a bicycle swap meet and sending a collection of papers into the sky! King TV was so impressed that is started the 10 PM news with it (here is their coverage at 5 PM: http://www.king5.com/news/local/Whirlwind-blows-through-Seattle-park-93204524.html) However, Jim Guy of KING TV was careful not to call this incident a possible tornado, as some outlets implied.
This was not a tornado. And there were no clouds overhead. Sounds like a big, dust devil type circulation. Strong heating at the surface and cool air aloft helps create strong thermals, especially over a nice open field. Such thermals can develop some spin and considerable speed (I suspect most of you have seen dust devils at some time in your life). Anyway, I believe that is what happened. Not the land spout that some have claimed. Fun, but not dangerous.
PS: Here is a video of a dust devil. Warning! An individual in the video uses" colorful" language at the end.
Posted by Cliff Mass at 9:31 PM