It is a cold, clear winter night. No fronts or storms are coming in. What time will there be the coldest temperatures on average?
Ten minutes before sunrise?
Some time during the 1/2 hr after sunrise.
None of the above.
The answer, believe it or not, is during the 1/2 hr after sunrise.
Here is an example during the past week (31 December) at the UW. Sunrise was at 7:57 AM, and the coldest temperature was few minutes later (click for larger image)
So why are the lowest temperatures so late? Its all about energy in and energy out. When more energy is leaving the surface then coming in, the temperature cools. More energy in than leaving, warming.
There are two main energy streams at the surface. First, at all times of the day the surface is emitting radiation, mainly in the infrared. The warmer the surface, the more infrared it emits. Then there is solar radiation coming in during the day. There can also be heat conducted from the underlying surface, but lets neglect that here (and the temperatures shown above were from a temperature sensor above the ground).
At night, after sunset, there is no solar heating, so the infrared cooling to space cools the surface . The cooling is often more rapid at first but continues through the night since the infrared loss to space continues. So the lowest temperature SHOULD wait at least until sunrise. However, cooling can continue for a short while AFTER sunrise. Why? Because initially the sun is very low in the sky and the solar radiation is very weak, weaker than the infrared loss to space. As long as this is true, the temperatures will continue to fall, EVEN THOUGH THE SUN IS UP!
Anyway, a nice weather factoid to amaze your friends and concern your enemies.
There was a nice rain shadow today for the Puget Sound lowlands (see radar image below ). The winds were relatively westerly so the rain shadow rotated over Seattle and the nearby lowlands. Those smug folks in Sequim and Port Townsend got rain today due to the rotated winds (they get the rainshadow during the more normal southerly or southwesterly flow).
By the way, I would not be surprised if some of the Sequim denizens were originally from southern CALIFORNIA ! In fact, I frequently get calls from Californians looking to retire in Washington, asking if Sequim is really about as dry as LA. After I tell them about the cacti and the irrigation festival, they are ready to pack their bags! When I describe the lavender farms and nearby Hurricane Ridge, they are rushing to call the moving van. But then I mention the winds....
Ah yes...the cold is on for this weekend. And I would be surprised if someone in the lowlands doesn't get a bit of snow. But no big snowstorm at this point. But more on that later.
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