Tuesday, August 19, 2008

More Lightning

By some odd synchronicity, I have had a bunch of posts on weather, particularly lightning, in the last week or so.

We almost never have lightning here because the air during hot weather is too dry. Hot, humid air is "unstable;" when it rises, it decompresses and cools. As it cools, water condenses. The latent heat of phase transformation heats the rising parcel of air compared to the stationary air around it. So it wants to rise more. This positive feedback process can force a rising column of air to rise up to 30-35 thousand feet, approximately the transition between the troposphere (the lower, mixing layer, where temperatures generally fall with increasing altitude) and the stratosphere (the next higher level, where temperatures generally rise with increasing altitude: this means the stratosphere tends not to mix very much. Colder, lower layers wil not rise into warmer overlying layers). The condensation in the troposphere leads to precipitation, and friction between water droplets leads to electric charges building up, which leads to lightning.

It was clear on Friday and Saturday that the humidities were much higher than normal. Dewpoints were in the low 60's, which, while not unheard of, don't normaly come with the high temperatures we were having at the time. Typically, when we have our hottest weather, the air is moving in from the Eastern Oregon desert, and it's very very dry. (Last summer, I saw what I believe is the lowest relative humidity I've ever experienced: 8%. I could feel my eyes wrinkling up like transparent rasins.)

Saturday night we had what I believe is the best lightning storm I've ever seen in Corvallis. Well, bummer, I thought I had saved the radar images from the two episodes, but all that I've got is the grid outlines. Oh well.
The first storm came through just after midnight; a lot of cloud-to-cloud lightning, but no ground strikes until the storm reached the line of hills about seven miles north of the middle of town. As the main body of the storm passed over, there was a pretty solid downpour that tapered off pretty quickly. It wasn't as intense as a midwest downpour, but good for Western Oregon. The second storm passed over about 1:30. Not as much precipitation, but lots of ground strikes, including one that had to be within a block of me. FLASHBOOM!
Sunday was much, much cooler and cloudier than predicted (The original prediction had been about 90). The Weather Channel claimed we had reached 82, but I had been keeping an eye on the toolbar temperature gadget all day and I never saw higher than 78. It was an enormous relief over Saturday's 100. Sunday night, no storms, but it was flickering and grumbling all night with occasional drizzle. Great sleeping weather! Yesterday and last night we had a few showers, but the lightning was rare. And today we've had consistent drizzle all afternoon- traditional Western Oregon precipitation.
The Oregonian had an article today about the lightning storms Sunday night and yesterday morning- the storms that hit us Saturday night were fairly tight and didn't hit Portland, so they didn't report on those. The highlight of this article is the animation of the lightning strikes over a period of an hour and a half or two Sunday night.
It's difficult to explain why I get so excited about a good storm- especially to those who get them all the time. But while I appreciate the overall temperate nature of our weather here, I often miss the awe-inspiring power that a good storm can put on display. There's also the relief from what, to me, is the physical torture of 90+ temperatures. Our environment is so parched at this time of year, and the smell of moisture and damp ground, of plants almost swooning with relief, is like perfume to me. Then there's the sounds: the gentle sussuration of the rain on leaves is so soothing and relaxing. Like I say, I never sleep better than during a nice rain.
If I can get over my excitement and actually sleep, rather than staying up to absorb every minute of the spectacle that I can.

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