Weather Blog

Why does it stay warmer in Seattle at night?

Why does it stay warmer in Seattle at night?
Seattle has a reputation for being a rather cool town, both literally and figuratively, but at night, it's the region's electric blanket.  The city frequently has one of the warmest low temperatures of the surrounding areas.

Why is that? Just like if you huddle up a bunch of people together, you'd be warmer as a group, Seattle stays warmer because there are more people here -- although body temperature has nothing to do with it, it's the infrastructure of huddling all those people here that keep us warm.

Concrete and asphalt do a very good job of absorbing and holding in heat, where as trees and vegetation don't hold the day's heat nearly as well.

Thus, urban areas typically stay warmer at night because all the buildings and roads and whatnot double as a space heater.

You can see this clearly in Saturday morning's statistics, which was the coldest night of the season so far. Seattle "only" dropped down to 37 degrees at Sea-Tac Airport, and 38 degrees at the NOAA weather office at Sand Point. Bellevue also only got down to 38.

But look at some of these smaller cities: Bremerton was at 27, Shelton was at 25, Arlington was 27, and Olympia was 27.  But the big note was Bellingham at 26.

According to UW research meteorologist Mark Albright, that 26 degree reading was the coldest temperature recorded for so early in the season, and it broke it by two weeks. The previous earliest low that cold was on Oct. 25, 1954 when it also hit 26.

There can be a few other factors that determine low temperature aside from city infrastructure. Immediate proximity to water can keep temperatures warmer in the winter around here since the water temperature is typically in the 50s.

Also, being in a valley (or, having a reporting station in a valley or at least in a dip in the terrain) can make for an artificially cold reading because cold air is heavier than warm air and pools in dips and valleys.

This might explain Olympia, which typically records some of the coldest low temperatures despite having at least some semblance of a city, although certainly not to the extent of Seattle, Tacoma and Everett. Olympia also doesn't get as much moderating effect from the Sound since the water is a small part of their city and maybe their recording thermometer at the airport is in a slight valley?

If anyone who lives there wants to send me some of their overnight readings so I can compare to the airport, that'd make for a neat exercise.

And finally, while I've still got you here, another interesting weekend tidbit from Mark Albright: Boise got 1.7 inches of snow on Friday night, Oct. 10. That was their earliest measurable snow in their recorded history, which began in 1898. It beat the record by two days.