Climate of Seattle

The Pacific Ocean, Puget Sound and Lake Washington serve as moderators of the temperature meaning the city is milder than areas inland during the winter and cooler during the summer.

[20] In an average year, at least 0.01 inches (0.25 mm) of precipitation falls on 150 days, more than nearly all U.S. cities east of the Rocky Mountains.

Annually, total precipitation averages 39.3 in (998 mm), with winter being the wettest season and July the driest month.

At Sea-Tac, rain has fallen in every month since records began there in January 1945, previously in the Seattle City area, the July's of 1896 and 1922 reported no precipitation.

Due to local variations in microclimate, Seattle also receives significantly lower precipitation than some other locations west of the Cascades.

Sixty miles (97 km) to the south of Seattle, the state capital Olympia, which is out of the Olympic Mountains' rain shadow, receives an annual average precipitation of 50 in (1,270 mm).

[13] The city of Bremerton, about 15 mi (24 km) west of downtown Seattle on the other side of the Puget Sound, receives 56.4 in (1,430 mm) of precipitation annually.

[13] One of many exceptions to Seattle's reputation as a damp location occurs in El Niño years, when marine weather systems track as far south as California and less than the usual precipitation falls in the Puget Sound area.

[26] The Seattle area is the cloudiest region of the United States, due in part to frequent storms and lows moving in from the adjacent Pacific Ocean.

Both streams of air originate over the Pacific Ocean; airflow is split by the Olympic Mountains to Seattle's west, then reunited to the east.

[28] Thunderstorms caused by this activity are usually weak and can occur north and south of town, but Seattle itself rarely receives more than occasional thunder and small hail showers.

In December 2007, a strong windstorm brought hurricane force winds and heavy rain, leading to 5 deaths.

It is expected that by this time, frequent flooding will become a problem, with now-annual extreme king tide's becoming monthly or even daily events.

[31] The temperature in Seattle has generally increased steadily and this trend is expected to continue due to anthropogenic warming.

[32] By the end of 2021, the Census Bureau's biannual housing survey found that 53 percent of households in the metropolitan area had air conditioning.