Average Weather in December at Seattle-Tacoma International Airport Washington, United States
Daily high temperatures decrease by 2°F, from 47°F to 45°F, rarely falling below 37°F or exceeding 54°F. The lowest daily average high temperature is 45°F on December 23.
Daily low temperatures are around 38°F, rarely falling below 28°F or exceeding 46°F. The lowest daily average low temperature is 37°F on December 23.
For reference, on August 3, the hottest day of the year, temperatures at Seattle-Tacoma International Airport typically range from 58°F to 78°F, while on December 23, the coldest day of the year, they range from 37°F to 45°F.
Average High and Low Temperature in December
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on December. The horizontal axis is the day, 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 December
The month of December at Seattle-Tacoma International Airport experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 72% throughout the month.
The clearest day of the month is December 20, with clear, mostly clear, or partly cloudy conditions 29% of the time.
For reference, on January 19, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 73%, while on August 3, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 75%.
Cloud Cover Categories in December
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. At Seattle-Tacoma International Airport, the chance of a wet day over the course of December is rapidly decreasing, starting the month at 56% and ending it at 49%.
For reference, the year's highest daily chance of a wet day is 57% on November 18, and its lowest chance is 8% on August 11.
Over the course of December at Seattle-Tacoma International Airport, the chance of a day with only rain decreases from 54% to 47%, the chance of a day with mixed snow and rain remains an essentially constant 2% throughout, and the chance of a day with only snow remains an essentially constant 1% throughout.
Probability of Precipitation in December
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during December at Seattle-Tacoma International Airport is rapidly decreasing, starting the month at 7.6 inches, when it rarely exceeds 11.9 inches or falls below 3.3 inches, and ending the month at 6.3 inches, when it rarely exceeds 9.9 inches or falls below 3.2 inches.
Average Monthly Rainfall in December
We report snowfall in liquid-equivalent terms. The actual depth of new snowfall is typically between 5 and 10 times the liquid-equivalent amount, assuming the ground is frozen. As with rainfall, we consider the liquid-equivalent snowfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day liquid-equivalent snowfall during December at Seattle-Tacoma International Airport is essentially constant, remaining about 0.1 inches throughout, and rarely exceeding 0.8 inches or falling below -0.0 inches.
The highest average 31-day liquid-equivalent accumulation is 0.2 inches on December 30.
Average Monthly Liquid-Equivalent Snowfall in December
Over the course of December at Seattle-Tacoma International Airport, the length of the day is essentially constant. The shortest day of the month is December 21, with 8 hours, 27 minutes of daylight and the longest day is December 1, with 8 hours, 44 minutes of daylight.
Hours of Daylight and Twilight in December
The earliest sunrise of the month at Seattle-Tacoma International Airport is 7:36 AM on December 1 and the latest sunrise is 21 minutes later at 7:56 AM on December 31.
The earliest sunset is 4:18 PM on December 11 and the latest sunset is 10 minutes later at 4:28 PM on December 31.
Daylight saving time is observed at Seattle-Tacoma International Airport during 2018, but it neither starts nor ends during December, so the entire month is in standard time.
For reference, on June 21, the longest day of the year, the Sun rises at 5:12 AM and sets 15 hours, 58 minutes later, at 9:10 PM, while on December 21, the shortest day of the year, it rises at 7:54 AM and sets 8 hours, 27 minutes later, at 4:20 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time in December
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 chance that a given day will be muggy at Seattle-Tacoma International Airport is essentially constant during December, remaining around 0% throughout.
For reference, on August 4, the muggiest day of the year, there are muggy conditions 0% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in December
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 is essentially constant during December, remaining within 0.1 miles per hour of 5.3 miles per hour throughout.
For reference, on December 1, the windiest day of the year, the daily average wind speed is 5.4 miles per hour, while on August 2, the calmest day of the year, the daily average wind speed is 3.0 miles per hour.
The highest daily average wind speed during December is 5.4 miles per hour on December 1.
Average Wind Speed in December
Wind Direction in December
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 surface water temperature at Seattle-Tacoma International Airport is gradually decreasing during December, falling by 2°F, from 49°F to 47°F, over the course of the month.
Average Water Temperature in December
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.
The month of December at Seattle-Tacoma International Airport is more likely than not fully outside of the growing season, with the chance that a given day is in the growing season rapidly decreasing from 45% to 2% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in December
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.
The average accumulated growing degree days at Seattle-Tacoma International Airport are essentially constant during December, remaining within 3°F of 2,082°F throughout.
Growing Degree Days in December
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 at Seattle-Tacoma International Airport is essentially constant during December, remaining within 0.1 kWh of 1.0 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 0.9 kWh on December 26.
Average Daily Incident Shortwave Solar Energy in December
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 year round, 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.