Average Weather at Seattle-Tacoma International Airport Washington, United States
At Seattle-Tacoma International Airport, the summers are short, warm, dry, and partly cloudy and the winters are very cold, wet, and mostly cloudy. Over the course of the year, the temperature typically varies from 37°F to 78°F and is rarely below 28°F or above 89°F.
Based on the tourism score, the best time of year to visit Seattle-Tacoma International Airport for warm-weather activities is from mid July to late August.
The warm season lasts for 2.7 months, from June 23 to September 13, with an average daily high temperature above 72°F. The hottest day of the year is August 3, with an average high of 78°F and low of 58°F.
The cool season lasts for 3.7 months, from November 11 to March 2, with an average daily high temperature below 52°F. The coldest day of the year is December 23, with an average low of 37°F and high of 45°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
At Seattle-Tacoma International Airport, 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 at Seattle-Tacoma International Airport begins around June 14 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 14. On January 19, the cloudiest day of the year, the sky is overcast or mostly cloudy 73% of the time, and clear, mostly clear, or partly cloudy 27% of the time.
Cloud Cover Categories
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days at Seattle-Tacoma International Airport varies significantly throughout the year.
The wetter season lasts 6.8 months, from October 9 to May 1, 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 5.2 months, from May 1 to October 9. The smallest chance of a wet day is 8% on August 4.
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-Tacoma International Airport experiences extreme seasonal variation in monthly rainfall.
Rain falls throughout the year at Seattle-Tacoma International Airport. The most rain falls during the 31 days centered around November 22, with an average total accumulation of 7.9 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 at Seattle-Tacoma International Airport 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 at Seattle-Tacoma International Airport varies significantly over the course of the year. In 2017, the shortest day is December 21, with 8 hours, 27 minutes of daylight; the longest day is June 20, with 15 hours, 58 minutes of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:11 AM on June 15, and the latest sunrise is 2 hours, 46 minutes later at 7:58 AM on November 4. The earliest sunset is at 4:18 PM on December 11, and the latest sunset is 4 hours, 52 minutes later at 9:10 PM on June 25.
Daylight saving time (DST) is observed at Seattle-Tacoma International Airport 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 at Seattle-Tacoma International Airport, 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 at Seattle-Tacoma International Airport experiences mild seasonal variation over the course of the year.
The windier part of the year lasts for 6.2 months, from October 16 to April 24, with average wind speeds of more than 4.2 miles per hour. The windiest day of the year is December 1, with an average hourly wind speed of 5.4 miles per hour.
The calmer time of year lasts for 5.8 months, from April 24 to October 16. The calmest day of the year is August 2, with an average hourly wind speed of 3.0 miles per hour.
Average Wind Speed
The predominant average hourly wind direction at Seattle-Tacoma International Airport varies throughout the year.
The wind is most often from the north for 2.3 months, from July 8 to September 18, with a peak percentage of 42% on July 30. The wind is most often from the south for 9.7 months, from September 18 to July 8, with a peak percentage of 58% on January 1.
Seattle-Tacoma International Airport 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 3.1 months, from June 25 to September 29, with an average temperature above 55°F. The day of the year with the warmest water is August 21, with an average temperature of 57°F.
The time of year with cooler water lasts for 4.0 months, from December 11 to April 11, with an average temperature below 49°F. The day of the year with the coolest water is February 11, with an average temperature of 46°F.
Average Water Temperature
Best Time of Year to Visit
To characterize how pleasant the weather is at Seattle-Tacoma International Airport throughout the year, we compute two travel scores.
The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Seattle-Tacoma International Airport for general outdoor tourist activities is from mid July to late August, with a peak score in the first week of August.
The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Seattle-Tacoma International Airport for hot-weather activities is from mid July to mid August, with a peak score in the first week of August.
For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.
Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.
Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.
Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.
Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
The growing season at Seattle-Tacoma International Airport typically lasts for 9.2 months (279 days), from around February 22 to around November 28, rarely starting before January 23 or after March 17, and rarely ending before November 4 or after December 21.
Time Spent in Various Temperature Bands and the Growing Season
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
Based on growing degree days alone, the first spring blooms at Seattle-Tacoma International Airport should appear around April 17, only rarely appearing before March 28 or after May 8.
Growing Degree Days
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 3.3 months, from May 15 to August 23, with an average daily incident shortwave energy per square meter above 5.6 kWh. The brightest day of the year is July 21, with an average of 6.8 kWh.
The darker period of the year lasts for 3.8 months, from October 26 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-Tacoma International Airport are 47.449 deg latitude, -122.309 deg longitude, and 302 ft elevation.
The topography within 2 miles of Seattle-Tacoma International Airport contains only modest variations in elevation, with a maximum elevation change of 469 feet and an average elevation above sea level of 341 feet. Within 10 miles contains only modest variations in elevation (1,109 feet). Within 50 miles contains very significant variations in elevation (14,360 feet).
The area within 2 miles of Seattle-Tacoma International Airport is covered by artificial surfaces (99%), within 10 miles by artificial surfaces (55%) and water (28%), and within 50 miles by trees (53%) and artificial surfaces (14%).
This report illustrates the typical weather at Seattle-Tacoma International Airport, 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
Seattle-Tacoma International Airport has a weather station that reported reliably enough during the analysis period that we have included it in our network. When available, historical temperature and dew point measurements are taken directly from this weather station. These records are obtained from NOAA's Integrated Surface Hourly data set, falling back on ICAO METAR records as required.
In the case of missing or erroneous measurements from this station, we fall back on records from nearby stations, adjusted according to typical seasonal and diurnal intra-station differences. For a given day of the year and hour of the day, the fallback station is selected to minimize the prediction error over the years for which there are measurements for both stations.
The stations on which we may fall back include but are not limited to King County International Airport, Renton Municipal Airport, Tacoma Narrows Airport, McChord Field Airport (Joint Base Lewis-McChord), Pierce County Airport Thun Field, Madigan Hospital Heliport, Bremerton National Airport, and Snohomish County Airport (Paine Field).
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.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.