Why is Olympia so much colder than Tacoma? (Seattle, Bellingham: new construction, university)

[belmont22]

In December Tacoma averages a low of 38 and a high of 45, while Olympia averages a low of 32 and a high of 43 and many more days and nights below freezing.

Why are their winter climates so different? Olympia is far colder than any of the other cities in Oregon/Washington on the I-5 corridor except for Bellingham and Mount Vernon which are about the same.

[Inkpoe]

Originally Posted by belmont22

In December Tacoma averages a low of 38 and a high of 45, while Olympia averages a low of 32 and a high of 43 and many more days and nights below freezing.

Why are their winter climates so different? Olympia is far colder than any of the other cities in Oregon/Washington on the I-5 corridor except for Bellingham and Mount Vernon which are about the same.

Simple. Tacoma benefits from being in (fringe end of) the rain shadow which provides less rain and some stability in ("better") weather.

[Hemlock140]

When you look a map, Tacoma has a lot of water to the west of it, where the wind comes from. That keeps it warmer, in addition to the south end of the Olympic Mountains diverting some storms slightly south to . . Olympia. The nearest water west of Olympia is way over at Grays Harbor, so the air has a chance to cool off before it gets there.

[Olympian2]

The explanations given so far are certainly reasonable, but I decided to ask an old friend of mine - Jeeves - about this: Ask Jeeves - What's Your Question?

Jeeves remembers a fellow who attributes the Olympia Chill to the lack of a westward body of water and more: Why Does It Get Colder In Olympia At Night? | F.A.Q. | Seattle News, Weather, Sports, Breaking News | KOMO News

Ah, but then Jeeves knows of a fellow who finds this particular question worthy of in depth study: Cliff Mass Weather Blog: The Olympia Mystery

You'll note that the second fellow also noted that Olympia is also one of the warmest places in Western Washington. I've noticed that on forecast maps myself. Anecdotally, in a clockwise drive around the OlyPen which began at University Mazda in Seattle, I felt and noticed distinct warming on the car's thermometer as I approached and drove through Olympia. Hoquiam was the coolest location until I arrived at the beaches in the Olympic National Park.

[pnwguy2]

Nobody has mentioned “heat islands”, basically how a denser city will produce more heat than smaller or more spread out cities. Seattle has become slightly warmer, atleast in the core due to all the new construction. While this has not really occurred in Tacoma, that city is large enough to produce somewhat of a heat island.

[Thealpinist]

Originally Posted by pnwguy2

Nobody has mentioned “heat islands”, basically how a denser city will produce more heat than smaller or more spread out cities. Seattle has become slightly warmer, atleast in the core due to all the new construction. While this has not really occurred in Tacoma, that city is large enough to produce somewhat of a heat island.

Heat islands exist but that is not the reason Olympia is cold. Statistically speaking Olympia has always been colder in winter. Olympia has recorded lows below zero in winter. It mostly has to do with the location.

1) Tacoma is much closer to Puget Sound, which keeps temperatures warmer in winter
2) Olympia tends to get more moisture in winter. When it does snow, it gets higher snow totals in winter as well. Because there is no shadowing from the Olympics the Olympia area can easily get 12+ inch snow totals when Seattle/tacoma get just a few inches. This allows for better radiational cooling at night.
3) Topography. Mainly because Olympia is in the Nisqually Basin and cold air immediately sinks off the Mountain ranges.

I’m going to guess that Puget Sound has the biggest impact on temps though. Really keeps the temps in Seattle/Tacoma a bit more moderate.

[pnwguy2]

I never said it was a source of colder temps, just a contributor. Indeed being closer to the Sound tends to moderate temps. Olympia can get cold and even can have more snow than Seattle or Tacoma.

[fruityfred11]

Olympia's official recording station at the airport is a known cold spot well outside of the urban heat island and away from the water, that insulates extremely well and is actually in Tumwater. That's the main reason they appear so much colder. The airport numbers can be a little misleading as the Olympia city proper will often be 3 to 5 degrees warmer on a clear night.

Otherwise it's the same climate as Tacoma, just a little wetter during the winter months as they are indeed less shadowed by the Olympics. This can sometimes lead to heavier snowfall as well during a cold setup, but it isn't always the case. I doubt you would notice a significant difference living in the two locations.