Average Weather in Seattle Washington, United States
In Seattle, the summers are short, warm, dry, and partly cloudy and the winters are chilly, wet, and mostly cloudy. Over the course of the year, the temperature typically varies from 37°F to 79°F and is rarely below 28°F or above 88°F.
The warm season lasts for 2.8 months, from June 20 to September 13, with an average daily high temperature above 72°F. The hottest day of the year is August 1, with an average high of 79°F and low of 59°F.
The cool season lasts for 3.6 months, from November 12 to March 1, with an average daily high temperature below 53°F. The coldest day of the year is December 23, with an average low of 37°F and high of 46°F.
Average High and Low Temperature
The figure below shows you a compact characterization of the entire year of hourly average temperatures. The horizontal axis is the day of the year, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature
In Seattle, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.
The clearer part of the year in Seattle begins around June 13 and lasts for 3.8 months, ending around October 8. On August 3, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 75% of the time, and overcast or mostly cloudy 25% of the time.
The cloudier part of the year begins around October 8 and lasts for 8.2 months, ending around June 13. On January 19, the cloudiest day of the year, the sky is overcast or mostly cloudy 72% of the time, and clear, mostly clear, or partly cloudy 28% of the time.
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Seattle varies significantly throughout the year.
The wetter season lasts 6.6 months, from October 11 to April 29, with a greater than 33% chance of a given day being a wet day. The chance of a wet day peaks at 57% on November 18.
The drier season lasts 5.4 months, from April 29 to October 11. The smallest chance of a wet day is 8% on August 5.
Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 56% on November 18.
Daily Chance of Precipitation
To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Seattle experiences extreme seasonal variation in monthly rainfall.
Rain falls throughout the year in Seattle. The most rain falls during the 31 days centered around November 22, with an average total accumulation of 7.7 inches.
The least rain falls around July 30, with an average total accumulation of 0.6 inches.
Average Monthly Rainfall
The sliding 31-day liquid-equivalent quantity of snowfall in Seattle does not vary significantly over the course of the year, staying within 0.1 inches of 0.1 inches throughout.
Average Liquid-Equivalent Monthly Snowfall
The length of the day in Seattle varies significantly over the course of the year. In 2017, the shortest day is December 21, with 8 hours, 26 minutes of daylight; the longest day is June 21, with 15 hours, 59 minutes of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:10 AM on June 15, and the latest sunrise is 2 hours, 48 minutes later at 7:58 AM on November 4. The earliest sunset is at 4:17 PM on December 11, and the latest sunset is 4 hours, 54 minutes later at 9:11 PM on June 25.
Daylight saving time (DST) is observed in Seattle during 2017, starting in the spring on March 12, lasting 7.8 months, and ending in the fall on November 5.
Sunrise & Sunset with Twilight and Daylight Saving Time
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The perceived humidity level in Seattle, as measured by the percentage of time in which the humidity comfort level is muggy, oppressive, or miserable, does not vary significantly over the course of the year, remaining a virtually constant 0% throughout.
Humidity Comfort Levels
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in Seattle experiences mild seasonal variation over the course of the year.
The windier part of the year lasts for 6.3 months, from October 15 to April 24, with average wind speeds of more than 2.2 miles per hour. The windiest day of the year is November 30, with an average hourly wind speed of 2.9 miles per hour.
The calmer time of year lasts for 5.7 months, from April 24 to October 15. The calmest day of the year is August 2, with an average hourly wind speed of 1.5 miles per hour.
Average Wind Speed
The predominant average hourly wind direction in Seattle varies throughout the year.
The wind is most often from the north for 2.4 months, from July 6 to September 19, with a peak percentage of 49% on July 30. The wind is most often from the south for 9.6 months, from September 19 to July 6, with a peak percentage of 59% on January 1.
Seattle is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average water temperature experiences some seasonal variation over the course of the year.
The time of year with warmer water lasts for 3.2 months, from June 24 to September 29, with an average temperature above 54°F. The day of the year with the warmest water is August 18, with an average temperature of 56°F.
The time of year with cooler water lasts for 4.0 months, from December 10 to April 11, with an average temperature below 48°F. The day of the year with the coolest water is February 11, with an average temperature of 46°F.
Average Water Temperature
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy experiences extreme seasonal variation over the course of the year.
The brighter period of the year lasts for 3.4 months, from May 13 to August 24, with an average daily incident shortwave energy per square meter above 5.5 kWh. The brightest day of the year is July 21, with an average of 6.7 kWh.
The darker period of the year lasts for 3.8 months, from October 25 to February 18, with an average daily incident shortwave energy per square meter below 2.1 kWh. The darkest day of the year is December 26, with an average of 0.9 kWh.
Average Daily Incident Shortwave Solar Energy
For the purposes of this report, the geographical coordinates of Seattle are 47.606 deg latitude, -122.332 deg longitude, and 36 ft elevation.
The topography within 2 miles of Seattle contains only modest variations in elevation, with a maximum elevation change of 449 feet and an average elevation above sea level of 135 feet. Within 10 miles contains only modest variations in elevation (1,171 feet). Within 50 miles contains very significant variations in elevation (7,720 feet).
The area within 2 miles of Seattle is covered by artificial surfaces (69%) and water (31%), within 10 miles by artificial surfaces (47%) and water (40%), and within 50 miles by trees (52%) and water (14%).
This report illustrates the typical weather in Seattle, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Seattle.
For each station, the records are corrected for the elevation difference between that station and Seattle according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at Seattle is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Seattle and a given station.
The stations contributing to this reconstruction are: King County International Airport (82%, 9 kilometers, south); Snohomish County Airport (Paine Field) (9%, 34 kilometers, north); and Bremerton National Airport (9%, 35 kilometers, southwest).
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.