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.
ReplyDeleteThis must be radiative cooling
DeleteI 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.
ReplyDeleteI suspect there are two reasons Snoqualmie Pass gets more snow than Mt. Si. Snow forms 1,000's of feet above the ground and then falls to earth. The air is forced higher and higher as it tries to cross the Cascades, so it snows (and rains) more as the air moves to the crest of the Cascades. Snoqualmie Pass gets more than 110 inches of precipitation a year. And North Bend gest about 60 inches of precipitation a year. The other reason is that Snoqualmie Pass is kept colder due to east winds at the pass level. So the freezing level in Puget Sound can be 1000's of feet above the pass level. If you have ever spent much time at Snoqualmie Pass you will notice that it can turn from snow to rain in a very short time as the warm air finally pushes the cold air at the pass level out of the way.
DeleteI've been hoping you would write an article about this for some time. I live several hundred feet above the valley in Leavenworth. I have been confused how we can easily be 10 degrees warmer than the valley sometimes but then 5 degrees cooler at other times. I understand this concept of cool air sinking but how does that interplay with temperatures decreasing with elevation? Is it relative to the time of day, at what point does the elevation overcome the rising warm air?
ReplyDeleteAll things being equal air cools at 5.4 degrees F per 1000 feet of elevation. By radiative cool near the surface can cause the air near the ground to cool much more than the surrounding air. As Cliff pointed out, that cooler air sinks. Particularly in the winter when the sun is low in the sky and the days are short a lot of air at the surface can be cooled causing a large pool of cool air in a valley. I've noticed this often while skiing at Crystal Mountain. The summit at 6,800 feet can be 10 to 20 degrees warmer than the base area at dawn. This all changes as the say warms up and the air begins to mix. Understand the air was always warm aloft. It's temperature did not change. It is the surface temperature that changed. This phenomenon has a corollary. In the early morning the sky can be clear in the mountains. As the surface heats up, particularly the the upper slopes catching a lot of sun, the air is warmed are it rises. The adiabatic cooling causes this warmed air to cool slight as it goes up. If their is some moisture in the air fog or clouds will form. When water vapor turns to liquid it releases heat which warms up the are relative to the surrounding air so it it now light and continues to go up, and continue to cool (still warmer than the surrounding air). As it cools more moisture is turn to fog or clouds. Sometimes we see large clouds over the mountains in the afternoon while it is clear in the flatlands. Sometimes by the middle of the day the warmer air gets into a layer that is windy and mixes really well making the after moisture below the threshold to turn to fog or clouds. Then you have a pleasnt clear afternoon.
DeleteThis morning I had 17F degree difference. The valley at around elevation of 250ft bottomed out at 57 while at ~1200ft, I bottomed out at 74 just prior to sunrise.
ReplyDeleteWe’re seeing some pretty remarkable macroscale temperature variations in Whatcom County with this heat wave. Panorama Dome, the high point of the Mount Baker Ski Area at just over 5000’ AMSL, reported a morning minimum temperature of 66F on 8/12 while thousands of feet below along the North Fork Nooksack River minimum temps were ~10F cooler.
ReplyDeleteAt my location in Bellingham, yesterday had the highest daily average temperature (74.4F) since the freak heat burst of 8/12/2021, when KBLI recorded its first 100F temperature. The daily maximum temperature of 91.8F at my location was also the highest daily max temp since the aforementioned heat burst as well as the first instance since that event that the temperature reached the 90F mark. Fortunately, there was better overnight cooling than last night and the current forecast still indicates that we’re on track for a whole August’s worth of precipitation toward the end of the week!
Great post!
ReplyDeleteI just noticed this phenomenon on my bike ride this morning around Green Lake and up through Woodlawn Park. I started on the east side of Green Lake, and when I got to the little putt putt golf course, it is shaded and they had been watering it early in the morning. It felt like 10 degrees cooler! I assume it is acting like a giant swamp cooler.
There's always some radiational cooling on a calm clear night but I believe there was major enhancement from katabatic flow. You can see using Caltopo map that Marmot Lake lies in a bowl plus there was cool outflow descending from the even larger Hart Lake bowl above it.
ReplyDeleteSeen the same effect at Royal Basin, elevation 5,200 feet and surrounded by 6,500 to 7,500 feet ridges. Regular morning frost in July while Seattle was enjoying 85 degree afternoons.