Thursday, November 05, 2009

Frost

Last night the air was still and calm, and this morning I had a bit of a frost. It still wasn’t a freeze, and by my cabin thermometer, the temperature never actually reached 32. That could be misleading, though. I took today’s photo just 50 yards or so down the hill from the cabin, and the temperature apparently did dip to 32 near the ground down there. Dewpoints are usually chillier near the ground than a few feet above it. On still nights like last night, that chillier air isn’t moved around by a breeze, and so low-lying little pockets of air can be colder than the surrounding areas.

The temperature at the cabin is also different from what is found down off the mountain. The temperature here is often several degrees cooler than what people in the suburbs report. It’s cooler in summer, and it’s cooler in winter. I can routinely figure on 3-5 degrees, sometimes it’s as much as 7 degrees.

When you have mountains, air pressure is lower as you go higher. The difference in temperature is usually about 5.4 degrees per 1000 feet. As the altitude difference between suburbia and my cabin is only about 700 feet, the temperature difference between there and here should be around 3.5 degrees cooler at the cabin.

But that’s not the whole story. I also don’t have roofs and macadam and street lights around me. Those things can and do raise nearby temperatures. So the 3.5 degree difference in 700 feet should be true if you’re measuring, say, a forest at 300 feet above sea level to a forest at 1000 feet above. When one area is a forest and the other is urbanized, the temperature differences are more pronounced.

That slight temperature difference apparently also accounts for why at least one of the bird species I routinely see at the cabin is different than what people living in similar habitats off the mountain see. But I'll have more on that tomorrow.

1 comment:

Woodswalker said...

The beauties of frost are among the compensations nature provides as her world draws in for winter. Tomorrow conditions should be right here in upstate NY for the plant Frostweed to be extruding its ribbons and clouds of frozen sap vapor. To see photos of this amazing plant and to read Thoreau's journal entry about it, visit this blog: www.watrlily.blogspot.com