Tuesday, December 8, 2009


(click the pic for a link to current data) A week ago, it was foggy, cold, and overcast... and we were all looking forward to partly sunny on Wednesday. We knew it was going to get cold at night, but we really weren't expecting it to get this cold, and stay near freezing during the day. When I came in this morning, about 8:00, it was 12 degrees F. Which was up a degree from the overnight low of 11. Note that dewpoint, the green line has been in single digits for nearly two days now. That's not unheard of in Corvallis, but it's rare.

Dewpoint is defined as the temperature at which a mass of air would be saturated with water vapor. Warmer than that, and the air would be able to hold more vapor, cooler than that, and some of the vapor will condense to liquid (or sublimate to frost, if below freezing). If you enjoy watching the weather, this is an unappreciated stat. In winter here, moderate dewpoints in the upper twenties to thirties, effectively set a lower boundary on how cold it's going to get at night, even if the sky is clear. Suppose the dewpoint is 30 degrees... when the air cools to that temperature, fog forms. This has two consequences: first, condensation of water vapor releases a tremendous amount of heat, so as the fog forms, the air maintains fairly steady temperature. Heat is being lost as radiation, but it is balanced by heat released as water changes phase. Second, fog is very effective at trapping radiated heat, so it tends to shut down heat loss.

In short, reaching an air temperature equal to dewpoint locks up heat loss at night. When we cool to our typical winter dewpoints (according to Weather Underground, 20 degrees average for December in Salem; sounds low to me), it stops getting colder.

I'm sure there are plenty of people in colder climates, some of them not all that far from my favorite coffee shop. I heard that Millican, Oregon reported a temperature of -38 this morning. But I'm not used to this, and sunshine notwithstanding, I don't like it.

No break until Friday. When NOAA sez we're likely to get freezing rain. Oh. Frakking. Boy.

1 comment:

pygalgia said...

Buddy, I got 30" of snow. And it's 8 degrees outside. Not forecast to get above freezing any time in the 10 day forecast.
I think I'll show up on your doorstep.