Friday, July 16, 2010

Classic Willamette Valley Summer Weather

From NWS, this is a snapshot of our weather over the last few days. Mid to late afternoon can be uncomfortably warm for me, but honestly, upper 80's isn't all that bad, and combined with shade, low humidity, and a nice breeze (which is typical), it's actually not bad at all. In the first plot, the red line is temperature, and the green line is dewpoint. Dewpoint is the temperature at which the air would be saturated with respect to its water content; another way to think of it is that it is the air temperature at which water would condense to form a cloud (or fog, at ground level).

Our dewpoints tend to be low during the summer; it has been close to 50 for some time, with a brief rise to near sixty yesterday afternoon. In contrast, the weather station linked to my home town of Athens, Ohio (the airport at Parkersburg, West Virginia) has been reporting dewpoints of 70-74 during the course of the day today, or 20 degrees higher. High dewpoints not only mean higher relative humidity- the way most weather reports relay humidity information- but they also restrict cooling in two important ways. First, when water condenses, either as fog or dew, it releases the heat that went into evaporating it in the first place. So everything else being equal, if water is condensing, the temperature will tend to stay steady; latent heat of condensation will balance out heat being lost through radiation. A related phenomenon is that if the water is condensing as fog, the presence of fog will interfere with heat loss by radiation. Once a fog forms, the temperature will typically stay pretty steady until it burns off or is dispersed by wind. The second issue is that water vapor (vapor, not fog) is the most important greenhouse gas in the atmosphere; it very strongly absorbs and re-emits infrared radiation. So if the dewpoints are high where you live, it won't cool down at night nearly as much as if the dewpoints are low.

So even though the daytime highs are a little warmer than I'd like, I can count on it getting down to around 50 at night. And if I can cool down at night, I'm pretty much good. Our high yesterday was 91; our low was 51, a 40-degree diurnal range- and I've seen ranges over 50 degrees- over a hundred during the day, and in the forties at night. The high at Parkersburg yesterday was 87, and the low was 71 (with fog).

I do not miss Midwest humidity... I miss thunderstorms, but I don't miss the climate it takes to generate them.

1 comment:

pygalgia said...

Monsoons begun around here. Thankfully.