Friday morning provided an excellent example of how radiational cooling works.
On clear and calm nights, the day's heat is radiated back into space, with the areas right near the ground, especially over grassy surfaces, becoming the coldest as that's where the most heat is lost and also where the coldest air pools since it is heavier than warm air.
Here are some temperature observations around 7:30 a.m. Friday, courtesy of the UW's Mark Albright:
* It was 34.5 degrees on the rooftop on top of the UW Atmospheric Sciences Building.
* It was 32 degrees at Husky Stadium and also in the Bryant neighborhood, which is north of University Village.
* It was 30 degrees at Sea-Tac Airport
* It was 29.3 degrees on a hillside east of Ballinger Park.
* In Ballinger Park itself (Mountlake Terrace, about 1/4 mile north of Lake Ballinger) it was 26.8 degrees measured at 3 feet above the ground
* But it was a much chillier 19.6 degrees measured at one centimeter above the ground.
So yes, it was over 7 degrees difference just three feet from your toes to waist level.
This kind of stark temperature gradient is of high interest to agricultural interests, because as you can see, even if the ambient air temperature, officially measured at 4-6 feet off the ground, is at or above freezing, it could be well below freezing down at crop level.
Hills and valleys can also affect nighttime temperature, as the heavier cold air pools in valleys. At the same time these observations were taken, an employee at the National Weather Service (Brad Colman -- actually, he's the top dog over at NWS Seattle) found that it was 38 degrees at Squak Mountain (1325 feet), but his car thermometer said it was only 26 degrees at the bottom of a nearby hill at 400 feet.