Climate of Seattle

File:Climatological Data for SEATTLE TACOMA AIRPORT, WA - June 2021.pdf in June 2021. Seattle recorded its highest temperature ever on June 28, reaching {{Convert|108|F}}.{{Cite web|url=https://www.nytimes.com/2021/06/27/us/heat-wave-seattle-portland.html|title=Pacific Northwest Heat Wave Shatters Temperature Records|work=The New York Times|access-date=2023-09-11|archive-url=https://web.archive.org/web/20230911122727/https://www.nytimes.com/2021/06/27/us/heat-wave-seattle-portland.html|archive-date=2023-09-11}}]]

The city's regime of temperature features small seasonal swings, due to its proximity to the ocean. 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. Extreme heatwaves are rare, as are cold temperatures.

Hot temperature extremes are enhanced by dry, compressed wind from the west slopes of the Cascades,{{cite web|title=What is offshore flow?|url=http://komonews.com/weather/faq/what-is-offshore-flow |archive-url=https://web.archive.org/web/20160126002256/http://komonews.com/weather/faq/what-is-offshore-flow |url-status=live |archive-date=January 26, 2016|first=Scott|last=Sistek|date=October 4, 2006}} while cold temperatures are generated mainly from the Fraser Valley in British Columbia.{{cite web|title=What are the different snow scenarios?|url=http://www.komonews.com/weather/faq/4307422.html#1a|date=December 25, 2015|access-date=April 24, 2021|archive-date=September 5, 2015|archive-url=https://web.archive.org/web/20150905061244/http://www.komonews.com/weather/faq/4307422.html#1a|url-status=dead}} Records are taken from the Seattle City area from 1894 to 1944 and at Sea-Tac Airport from 1945.

In an average year, the temperature will usually be between {{convert|21|F}} and {{convert|94|F}} with temperatures greatly exceeding these values being uncommon. Temperatures above {{convert|97|F}} and below {{convert|15|F}} are very rare, with the last occurrences being June 28, 2021 and November 24, 2010, respectively.

{{Weather box

|location = Seattle (Seattle-Tacoma International Airport), 1991–2020 normals

|single line = y

|collapsed = Y

|Jan high F = 48.0

|Feb high F = 50.3

|Mar high F = 54.2

|Apr high F = 59.3

|May high F = 66.3

|Jun high F = 71.1

|Jul high F = 77.4

|Aug high F = 77.6

|Sep high F = 71.6

|Oct high F = 60.5

|Nov high F = 52.1

|Dec high F = 47.0

|year high F = 61.3

|Jan mean F = 42.8

|Feb mean F = 44.0

|Mar mean F = 47.1

|Apr mean F = 51.3

|May mean F = 57.5

|Jun mean F = 62.0

|Jul mean F = 67.1

|Aug mean F = 67.4

|Sep mean F = 62.6

|Oct mean F = 53.8

|Nov mean F = 46.5

|Dec mean F = 42.0

|year mean F = 53.7

|Jan low F = 37.7

|Feb low F = 37.7

|Mar low F = 39.9

|Apr low F = 43.3

|May low F = 48.7

|Jun low F = 53.0

|Jul low F = 56.8

|Aug low F = 57.2

|Sep low F = 53.6

|Oct low F = 47.0

|Nov low F = 40.9

|Dec low F = 37.1

|year low F = 46.1

|Jan avg record high F = 57.0

|Feb avg record high F = 59.1

|Mar avg record high F = 66.4

|Apr avg record high F = 74.3

|May avg record high F = 81.9

|Jun avg record high F = 85.8

|Jul avg record high F = 91.2

|Aug avg record high F = 89.9

|Sep avg record high F = 84.1

|Oct avg record high F = 72.0

|Nov avg record high F = 61.6

|Dec avg record high F = 56.8

|year avg record high F= 94.1

|Jan avg record low F = 26.1

|Feb avg record low F = 27.3

|Mar avg record low F = 31.3

|Apr avg record low F = 35.6

|May avg record low F = 40.6

|Jun avg record low F = 46.6

|Jul avg record low F = 51.5

|Aug avg record low F = 51.7

|Sep avg record low F = 45.8

|Oct avg record low F = 36.8

|Nov avg record low F = 29.2

|Dec avg record low F = 25.4

|year avg record low F= 21.5

|source 1= NOAA

}}

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NOTE: in the tables below, all numbers before 1945 come from locations in downtown Seattle, which tends to be somewhat warmer than the current official location of Sea-Tac airport. To distinguish these older numbers we mark them below in italics.

File:Cherry St after the big snowfall of 1880, looking northeast, Seattle, January 10, 1880 (PEISER 144).jpeg

The city sees frequent, though light rainfall between October and May, with rainfall becoming lighter and sparser between June and September. With many more "rain days" than other major American cities, Seattle has a well-earned reputation for frequent rain.{{cite web|url=http://www.komonews.com/weather/faq/4306627.html|title=What Is The Olympic Rain Shadow?|publisher=KOMOTV.com|access-date=September 28, 2007|date=October 4, 2006|archive-date=November 6, 2015|archive-url=https://web.archive.org/web/20151106104412/http://www.komonews.com/weather/faq/4306627.html|url-status=dead}} In an average year, at least {{convert|0.01|in|mm}} of precipitation falls on 150 days, more than nearly all U.S. cities east of the Rocky Mountains. In November, Seattle averages more rainfall than any other U.S. city of more than 250,000 people; it also ranks highly in winter precipitation. Conversely, the city receives some of the lowest precipitation amounts of any large city from June to September. Seattle is one of the five rainiest major U.S. cities as measured by the number of days with precipitation.{{cite web|url=http://www1.ncdc.noaa.gov/pub/data/ccd-data/prge0112.txt|title=Mean Number of Days with Precipitation 0.01 Inch or More|publisher=NOAA Satellites and Information|url-status=dead|archive-url=https://web.archive.org/web/20130928081654/http://www1.ncdc.noaa.gov/pub/data/ccd-data/prge0112.txt|archive-date=September 28, 2013}} However, because Seattle often has merely a light drizzle falling from the sky for many days, it actually receives significantly less rainfall (or other precipitation) overall than many other U.S. cities like New York City, Miami, or Houston. Seattle experiences its heaviest rainfall during November, December, and January, receiving roughly half of its annual rainfall (by volume) during this period. In late fall and early winter, atmospheric rivers (also known as "Pineapple Express" systems), strong frontal systems, and Pacific low-pressure systems are common. Light rain and drizzle are the predominant forms of precipitation during the remainder of the year. For instance, on average, less than {{convert|1.6|in|mm|0|abbr=on}} of rain falls in July and August combined when rain is less common.

Annually, total precipitation averages {{convert|39.3|in|mm|0|abbr=on}}, 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. Long stretches of little precipitation can occur. No measurable precipitation, greater than {{convert|0.01|in}}, fell between June 18 and August 13, 2017.{{cite web|url= https://www.seattleweatherblog.com/rain/seattles-record-dry-streak-ends-55-days/|title = It's over: Seattle's record dry streak ends at 55 days| date=13 August 2017 |publisher = Seattle Weather Blog|access-date = June 25, 2021}} The city also sees snow, primarily in winter, but sometimes in the late autumn and early spring. Snowfall averages {{convert|6.3|in|cm|1|abbr=on}} per year but is highly variable between winter seasons. The most rainfall in 24 hours was {{convert|5.02|in|1|abbr=on}} on October 20, 2003, and the most snowfall was {{convert|21.5|in|cm|abbr=on}} on February 2, 1916. Seattle typically receives some snowfall on an annual basis but heavy snow is rare. Average annual snowfall, as measured at Sea-Tac Airport, is {{convert|6.3|in|cm|1}}. From winter season to winter season, amounts can be extremely variable.

Due to local variations in microclimate, Seattle also receives significantly lower precipitation than some other locations west of the Cascades. Around {{convert|80|mi|-1|abbr=on}} to the west, the Hoh Rain Forest in Olympic National Park on the western flank of the Olympic Mountains receives an annual average precipitation of {{convert|142|in|mm|abbr=on}}. {{convert|60|mi|km|spell=In}} to the south of Seattle, the state capital Olympia, which is out of the Olympic Mountains' rain shadow, receives an annual average precipitation of {{convert|50|in|mm|sigfig=3|abbr=on}}. The city of Bremerton, about {{convert|15|mi|abbr=on}} west of downtown Seattle on the other side of the Puget Sound, receives {{convert|56.4|in|mm|sigfig=3|abbr=on}} of precipitation annually.

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.{{cite news|url=http://seattletimes.nwsource.com/html/localnews/2003297665_webnino10.html|title=El Niño could cause Northwest drought, mild winter elsewhere, forecasters say|work=The Seattle Times|date=October 10, 2006|author=Randolph E. Schmid|access-date=November 1, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070217135735/http://seattletimes.nwsource.com/html/localnews/2003297665_webnino10.html|archive-date=February 17, 2007}} However, the El Nino of 2015-2016 caused an increase in rainfall. Since the region's water comes from mountain snow packs during the dry summer months, El Niño winters can not only produce substandard skiing but can result in water rationing and a shortage of hydroelectric power the following summer.{{cite news|url=http://seattletimes.nwsource.com/html/localnews/2002162116_snowpack27m.html|title=Lack of snow may take toll|work=The Seattle Times|author=Nick Perry|date=February 23, 2005|access-date=November 1, 2007|url-status=dead|archive-url=https://web.archive.org/web/20080722203738/http://seattletimes.nwsource.com/html/localnews/2002162116_snowpack27m.html|archive-date=July 22, 2008}}

===Averages===

{{Weather box

|name = Seattle weatherbox

|location = Seattle (SeaTac International Airport), 1991–2020 normals

|single line = y

|Jan precipitation inch = 5.78

|Feb precipitation inch = 3.76

|Mar precipitation inch = 4.17

|Apr precipitation inch = 3.18

|May precipitation inch = 1.88

|Jun precipitation inch = 1.45

|Jul precipitation inch = 0.60

|Aug precipitation inch = 0.97

|Sep precipitation inch = 1.61

|Oct precipitation inch = 3.91

|Nov precipitation inch = 6.31

|Dec precipitation inch = 5.72

|year precipitation inch=39.34

|Jan snow inch = 2.5

|Feb snow inch = 2.2

|Mar snow inch = 0.9

|Apr snow inch = 0

|May snow inch = 0

|Jun snow inch = 0

|Jul snow inch = 0

|Aug snow inch = 0

|Sep snow inch = 0

|Oct snow inch = 0

|Nov snow inch = 0.5

|Dec snow inch = 2.1

|year snow inch=

|unit precipitation days = 0.01 in

|unit snow days = 0.1 in

|Jan precipitation days = 18.7

|Feb precipitation days = 15.9

|Mar precipitation days = 17.1

|Apr precipitation days = 15.0

|May precipitation days = 11.3

|Jun precipitation days = 9.2

|Jul precipitation days = 4.7

|Aug precipitation days = 4.9

|Sep precipitation days = 8.3

|Oct precipitation days = 14.3

|Nov precipitation days = 18.4

|Dec precipitation days = 18.4

|year precipitation days=156.2

|Jan snow days = 2.3

|Feb snow days = 2.0

|Mar snow days = 0.9

|Apr snow days = 0

|May snow days = 0

|Jun snow days = 0

|Jul snow days = 0

|Aug snow days = 0

|Sep snow days = 0

|Oct snow days = 0

|Nov snow days = 0.2

|Dec snow days = 1.5

|year snow days= 4.7

|source 1= NOAA{{cite web |url = https://w2.weather.gov/climate/xmacis.php?wfo=sew |title = NowData - NOAA Online Weather Data |publisher = National Oceanic and Atmospheric Administration |access-date = June 25, 2021}}

}}

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File:Seattle Skyline-.jpg

The city generally experiences cloudy conditions, with clear days occurring infrequently. As a result of the city's latitude, it experiences a moderate difference in daylight hours between summer and winter, though is not subject to the extremes of cities further north.{{cite web |url=https://www.weather-us.com/en/washington-usa/seattle-climate |publisher=Weather Atlas|title=Monthly weather forecast and climate Seattle, WA|access-date=June 25, 2021}} 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. Seattle is cloudy 201 days out of the year and partly cloudy 93 days. (Official weather and climatic data is collected at Seattle–Tacoma International Airport, located about {{convert|19|km|mi|abbr=on}} south of downtown in the city of SeaTac, which is at a higher elevation, and records more cloudy days and fewer partly cloudy days per year.)

{{Weather box

|location = Seattle

|collapsed = y

|Jan sun = 69.8 |Jan percentsun = 25

|Feb sun = 108.8 |Feb percentsun = 38

|Mar sun = 178.4 |Mar percentsun = 48

|Apr sun = 207.3 |Apr percentsun = 51

|May sun = 253.7 |May percentsun = 54

|Jun sun = 268.4 |Jun percentsun = 56

|Jul sun = 312.0 |Jul percentsun = 65

|Aug sun = 281.4 |Aug percentsun = 64

|Sep sun = 221.7 |Sep percentsun = 59

|Oct sun = 142.6 |Oct percentsun = 42

|Nov sun = 72.7 |Nov percentsun = 26

|Dec sun = 52.9 |Dec percentsun = 20

|year percentsun = 49

| Jan uv =1

| Feb uv =2

| Mar uv =3

| Apr uv =5

| May uv =6

| Jun uv =7

| Jul uv =7

| Aug uv =6

| Sep uv =5

| Oct uv =3

| Nov uv =1

| Dec uv =1

| Jan light = 9.0

| Feb light = 10.3

| Mar light = 12.0

| Apr light = 13.7

| May light = 15.2

| Jun light = 15.9

| Jul light = 15.5

| Aug light = 14.2

| Sep light = 12.5

| Oct light = 10.8

| Nov light = 9.3

| Dec light = 8.5

| year light=

|source 1= NOAA (relative humidity and sun 1961–1990){{cite web |url = http://www.nws.noaa.gov/climate/xmacis.php?wfo=okx |title = NowData - NOAA Online Weather Data |publisher = National Oceanic and Atmospheric Administration |access-date = 2018-04-10}}

|source 2= Weather Atlas

}}

The Puget Sound Convergence Zone is an important feature of Seattle's weather. In the convergence zone, air arriving from the north meets air flowing in from the south. 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. When the air currents meet, they are forced upward, resulting in convection.{{cite web|url=http://komonews.com/weather/faq/what-is-a-puget-sound-convergence-zone|title=What is a Puget Sound Convergence Zone?|publisher=KOMO |archive-date=January 26, 2016 |archive-url=https://web.archive.org/web/20160126002111/http://komonews.com/weather/faq/what-is-a-puget-sound-convergence-zone|first=Scott|last=Sistek|date=December 17, 2015 |url-status=live}} 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. The Hanukkah Eve Wind Storm in December 2006 is an exception that brought heavy rain and winds gusting up to {{convert|69|mph|km/h|abbr=on|0}}, an event that was not caused by the Puget Sound Convergence Zone and was widespread across the Pacific Northwest. In December 2007, a strong windstorm brought hurricane force winds and heavy rain, leading to 5 deaths.{{cite web|url=http://www.ncdc.noaa.gov/sotc/national/2007/12|title=State of the Climate – National Overview – December 2007|date=January 2008|publisher=National Climatic Data Center|access-date=July 3, 2011}}{{cite web|title=5 Dead in Washington Storm|url=http://www.kirotv.com/weather/14758195/detail.html|archive-url=https://web.archive.org/web/20081122071056/http://www.kirotv.com/weather/14758195/detail.html|archive-date=November 22, 2008|publisher=Kiro TV News|access-date=January 24, 2009}}

Being a coastal city, Seattle may experience significant effects from rising sea levels. The sea has risen by {{convert|6|in|cm}} in the past century, and is expected to rise {{convert|28|in|cm}} by 2100 and {{convert|47|in|cm}} by 2150. 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.{{cite web|url=http://www.seattle.gov/utilities/protecting-our-environment/community-programs/climate-change/projected-changes |publisher=Seattle.gov|title=Projected Climate Changes |access-date=June 25, 2021}}

The temperature in Seattle has generally increased steadily and this trend is expected to continue due to anthropogenic warming. For the last century, the average has increased roughly 0.3 degrees Fahrenheit each decade. By the end of the century, it is predicted that there will be on average around two weeks of {{convert|90|F|C}} days each year.

Homes in the area have historically not used air conditioning due to the temperate summer climate; the United States Census Bureau found that 31 percent of households in the Seattle metropolitan area used air conditioning. Several major heat waves in the 2010s and 2020s, including the June 2021 heat dome that set record temperatures in the state, led to a large increase in air conditioning use.{{cite news |last=Weinberger |first=Hannah |date=December 29, 2022 |title=Seattle is no longer the U.S.'s least air conditioned big city |url=https://crosscut.com/environment/2022/12/seattle-no-longer-uss-least-air-conditioned-big-city |work=Crosscut.com |accessdate=May 14, 2024}} 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.{{cite news |last=Ryan |first=John |date=May 14, 2024 |title=Seattle is now an air conditioning town |url=https://www.kuow.org/stories/seattle-is-now-an-air-conditioning-town |publisher=KUOW |accessdate=May 14, 2024}}

Precipitation in the city has increased slightly, but this trend is expected to continue. Extreme rainfall events have become more frequent over the previous years and this trend is also expected to continue.

{{Seattle weatherbox}}

{{Graph:Weather monthly history

| table=ncei.noaa.gov/weather/Seattle.tab

| title=Seattle, WA weather

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