Have you ever experienced an unexpected breeze of cool air during an evening stroll after a warm day--like yesterday or today?
On such evenings, temperatures can remain warm on hilltops or broad, level areas, but on lower portions of slopes or downstream of terrain gaps, temperatures can be significantly cooler.
Even during the day, it can be much cooler beneath vegetated slopes, particularly shaded ones. We are talking about 1-10°F cooler.
Downslope flow 101
Cooler air is denser (and heavier) than warmer air and tends to settle into lower elevations (see figure). There is a fancy term for this: katabatic flow.
If you ever want to experience daytime slope cooling, on a warm day take a ride north from the University of Washington on the Burke Gilman Trail during the afternoon.
Temperatures can be toasty south of the 45th Street viaduct, but north of there, the trail is adjacent to the east-facing vegetation slope (see picture) below, and temperatures are often 5-10F cooler. I have measured this with a thermometer.
Moist, vegetated slopes are particularly powerful cooling machines because evaporation from moist surfaces is stunningly effective at cooling, particularly when the air is relatively dry.
It takes 540 calories of heat (from the sun) to evaporate one gram of water. This is why evaporative coolers can be so effective.
Being in a shaded stream valley can be particularly cool. There is such a location on the Burke Gilman Trail in north Seattle, just south of NE 55th Street (see map and picture below). I call it "Chilly Valley" and it is often iced up in winter when every place else is clear--frequently 5-10°F cooler than areas to the north or south.
During warm nights, the higher elevation view homes are often much warmer than the low-rent locations to the south.
For example, walking around with a thermometer in the neighborhood below, temperatures were 4°F warmer at the top of the slope than at the bottom.
It may seem nerdy to walk around with a thermometer, but it sure can be interesting. 😊
Thanks, Cliff. I walk through Chilly Valley frequently, enjoying the shade trees and cool temps along the trail. On these warm days, the contrast is delightful!
ReplyDeleteI used to notice effects like this when riding my motorcycle through the coast range of California. Sometimes when I dropped down into a dip in the road, I experienced cool, refreshing air. Often there was a smell of rosemary that I hadn't noticed elsewhere. I still think about it when I smell rosemary. Do you think it's possible for these gradients to affect vegetation?
ReplyDeleteThis brings to mind a backpacking trip I made to LaCrosse Basin (ONP) I made in late August 2016. We were camped at an elevation of 5,000 feet in warm weather. We broke camp early in the morning with my thermometer indicating 55F. We descended 700 feet in elevation to Marmot Lake which lies in a small basin below Hart Lake. The outlet stream of Hart Lake feeds Marmot Lake. We were surprised to see a light frost at Marmot Lake meaning there was a 20 degree plus drop in temperature over a very short distance.
ReplyDeleteI live in exactly that type of location. Vegetated hills to the south, east and west of me with a flat grassy area where our house located. The cooling is especially noticeable in the winter where we will consistently have all day frost when areas around us have melted out. My only complaint is it doesn't help our heating bill!
ReplyDeleteI often notice this driving down from Preston towards Fall City, or from Snoqualmie Falls back in that direction. I wonder, Dr. Mass, if you could explain another local phenomenon out here in the valley -- why Mt. Si seems to get so much less snow at 4000+ feet, than Snoqualmie Pass at 3000 feet. I know part of it is this side faces the sun and the snow melts, but I don't think that's all of it.
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