December Weather 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 22.

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 22.

For reference, on August 2, the hottest day of the year, temperatures at Seattle-Tacoma International Airport typically range from 58°F to 78°F, while on December 22, the coldest day of the year, they range from 37°F to 45°F.

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.

Paris, France (5,011 miles away) is the far-away foreign place with temperatures most similar to Seattle-Tacoma International Airport (view comparison).

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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 19, 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 2, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 75%.

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 55% and ending it at 49%.

For reference, the year's highest daily chance of a wet day is 57% on November 17, and its lowest chance is 8% on August 10.

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.

Rainfall

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 7.5 inches, when it rarely exceeds 11.8 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.

Snowfall

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day snowfall during December at Seattle-Tacoma International Airport is increasing, starting the month at 0.7 inches, when it rarely exceeds 2.1 inches or falls below -0.0 inches, and ending the month at 2.0 inches, when it rarely exceeds 7.7 inches or falls below -0.0 inches.

The highest average 31-day accumulation is 2.0 inches on December 29.

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, 43 minutes of daylight.

The earliest sunrise of the month at Seattle-Tacoma International Airport is 7:36 AM on December 1 and the latest sunrise is 20 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 2024, but it neither starts nor ends during December, so the entire month is in standard time.

For reference, on June 20, the longest day of the year, the Sun rises at 5:12 AM and sets 15 hours, 58 minutes later, at 9:09 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 4:21 PM.

The figure below presents a compact representation of the sun's elevation (the angle of the sun above the horizon) and azimuth (its compass bearing) for every hour of every day in the reporting period. The horizontal axis is the day of the year and the vertical axis is the hour of the day. For a given day and hour of that day, the background color indicates the azimuth of the sun at that moment. The black isolines are contours of constant solar elevation.

The figure below presents a compact representation of key lunar data for December 2024. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases. The label associated with each bar indicates the date and time that the phase is obtained, and the companion time labels indicate the rise and set times of the Moon for the nearest time interval in which the moon is above the horizon.

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 3, 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.

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 November 30, the windiest day of the year, the daily average wind speed is 5.4 miles per hour, while on August 1, the calmest day of the year, the daily average wind speed is 3.0 miles per hour.

The hourly average wind direction at Seattle-Tacoma International Airport throughout December is predominantly from the south, with a peak proportion of 61% on December 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 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.

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 9.1 months (279 days), from around February 22 to around November 27, rarely starting before January 23 or after March 16, and rarely ending before November 3 or after December 20.

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 43% to 2% over the course of the month.

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.

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 25.