WASHINGTON — Monday morning temperatures were some of the lowest we’ve seen so far this fall, with a frigid 27 degrees in Shelton and even a frosty 37 degrees at Boeing Field in Seattle.
When skies clear and winds stay calm, it’s a perfect setup for radiational cooling, the process by which the Earth’s surface loses heat to space overnight. The cooler the ground gets, the cooler the air just above it becomes.
Add clouds to the mix, and temperatures don’t fall nearly as much because clouds act like a blanket, trapping some of that heat near the surface.
While clouds come and go, what we can’t stop is the march of time. We continue to lose daylight as fall progresses, and that loss happens faster at our northern latitude than it does farther south.
On Monday, Oct. 12, Seattle has just over 11 hours of daylight. With daylight shrinking by a little more than three minutes per day, we’ll dip below 11 hours on Tuesday. We’re headed toward our annual minimum of 8 hours, 25 minutes of daylight on the first day of winter, Dec. 21.
Hard to believe that just a few months ago, at the start of summer, we had nearly 16 hours of daylight. Now that’s just a memory.
The shrinking daylight also means less opportunity for daytime warming. As a result, it becomes increasingly difficult to reach 70 degrees in Seattle by mid-to-late October.
We may get one more shot later this week, however, with upper 60s and 70s looking common across the Western Washington lowlands Thursday and Friday before cooler air and increasing rain chances return to kick off the weekend.