Congratulations to Ron Martin for winning yesterday's cookie contest. As you may recall, I offered cookies in exchange for equations showing the increase in wind power when the temperature drops. According to Ron, 40 degree wind has 10 percent more power than 90 degree wind. That jibes well with my observations, so he gets the cookies. Arthur took the time to write in with an observation, which is something I'd thought of as well. As temperature drops, air gets denser (particles move closer together). We both think that an anemometer is not actually measuring wind speed. It is measuring the total momentum (mass X velocity) of air particles that hit the blades of the measuring device. So, as air gets colder, and thus denser, more air particles hit the blades of the anemometer, thus increasing the wind reading in colder temperatures. If this is true, then a 23mph reading in cold air is actually equal to a 23 mile per hour reading in hot air (wind power, according to the anemometer, is identical for both readings). Anyone who can shed light on that (other than Ron and Arthur, who already get cookies) will get cookies as well. Anyway, here are Ron's winning calculations:
Pretty sure wind power is directly proportional to the air density (I can
look this up). And, as you know from your chemistry, the ideal gas law is
PV = nRT (absolute temperature of course)
and n = m/(MW)
so, density = m/V = [P(MW)]/[RT]
To compare power at 2 different temperatures, just take the ratio:
Power (at T1) = Power (at T2) x [T2/T1]
if T1 is 40F and T2 is 90F, we get
Power (at 40F) = Power (at 90F) x [(90+459.7)/(40+459.7)]
Power (at 40F) = Power (at 90F) x 1.1
(about a 10% increase)
I know that's just what you wanted first thing in the morning!
The sun is shining and east winds are picking up already! We're seeing a .09 gradient to start the day, nothing to write home about, but this will increase as the day goes on. High pressure builds over eastern Oregon and a shortwave drops into the wheat belt, intensifying high pressure east of the Gorge. With the heat low centered on the Oregon Coast (making for warm, sunny surfing), we've got a perfect setup for easterlies (also known as the exact opposite of what we'd like for west winds). Yes, I can hear the excited murmers already. Winds climb to 23-27 by mid afternoon (I know, so much for the nuclear easterly event. At least you guys are less likely to lynch me for mis-predicting strong easterlies), and will actually increase overnight. Strongest winds buffet Gorge trees in the wee hours. If you're driving home from Portland in a small car after partying tonight, you'll have an exciting ride!
This means you should get out early tomorrow (and maybe skip the school-night partying). Easterlies rule once again, with winds blowing in the 28-32 range. We'll see a "typical" easterly day tomorrow, with winds dying out around 1-2pm. Don't get caught on the water - easterlies die fast, and it's a long swim in at Stevenson!
Models present a slightly confused picture on Tuesday. They can't decide where the heat low is headed. For now, I'll say light east winds in the morning, possibly shifting top perfect wakeboarding weather by afternoon.
Okay. You kids have a good time on the water today. Have fun!
Print | posted @ Sunday, September 09, 2007 8:17 AM