Average Weather in December in Portland Oregon, United States
Daily high temperatures decrease by 3°F, from 48°F to 45°F, rarely falling below 36°F or exceeding 55°F. The lowest daily average high temperature is 45°F on December 30.
Daily low temperatures decrease by 3°F, from 38°F to 36°F, rarely falling below 26°F or exceeding 46°F. The lowest daily average low temperature is 36°F on December 30.
For reference, on August 3, the hottest day of the year, temperatures in Portland typically range from 58°F to 84°F, while on December 30, the coldest day of the year, they range from 36°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
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
The month of December in Portland experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 75% throughout the month.
The clearest day of the month is December 20, with clear, mostly clear, or partly cloudy conditions 26% of the time.
For reference, on January 16, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 76%, while on August 3, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 82%.
Cloud Cover Categories in December
0% clear 20% mostly clear 40% partly cloudy 60% mostly cloudy 80% overcast 100%
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Portland, the chance of a wet day over the course of December is rapidly decreasing, starting the month at 58% and ending it at 51%.
For reference, the year's highest daily chance of a wet day is 59% on November 27, and its lowest chance is 4% on August 3.
Over the course of December in Portland, the chance of a day with only rain decreases from 56% 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 in Portland is rapidly decreasing, starting the month at 9.4 inches, when it rarely exceeds 14.7 inches or falls below 4.2 inches, and ending the month at 8.0 inches, when it rarely exceeds 12.9 inches or falls below 3.3 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 in Portland is essentially constant, remaining about 0.2 inches throughout, and rarely exceeding 1.3 inches or falling to 0.0 inches.
Average Monthly Liquid-Equivalent Snowfall in December
Over the course of December in Portland, the length of the day is essentially constant. The shortest day of the month is December 20, with 8 hours, 42 minutes of daylight and the longest day is December 1, with 8 hours, 58 minutes of daylight.
Hours of Daylight and Twilight in December
The earliest sunrise of the month in Portland is 7:30 AM on December 1 and the latest sunrise is 20 minutes later at 7:50 AM on December 31.
The earliest sunset is 4:27 PM on December 10 and the latest sunset is 10 minutes later at 4:37 PM on December 31.
Daylight saving time is observed in Portland during 2021, 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 5:21 AM and sets 15 hours, 41 minutes later, at 9:03 PM, while on December 21, the shortest day of the year, it rises at 7:48 AM and sets 8 hours, 42 minutes later, at 4:30 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time in December
The figure below presents a compact representation of key lunar data for December 2021. 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.
Moon Rise, Set & Phases 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 in Portland is essentially constant during December, remaining around 0% throughout.
For reference, on August 5, the muggiest day of the year, there are muggy conditions 1% 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
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
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 Portland is essentially constant during December, remaining within 0.1 miles per hour of 6.2 miles per hour throughout.
For reference, on December 3, the windiest day of the year, the daily average wind speed is 6.3 miles per hour, while on August 23, the calmest day of the year, the daily average wind speed is 4.2 miles per hour.
The highest daily average wind speed during December is 6.3 miles per hour on December 4.
Average Wind Speed in December
The wind direction in Portland during December is predominantly out of the south from December 1 to December 27 and the east from December 27 to December 31.
Wind Direction in December
Portland 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 in Portland is essentially constant during December, remaining within 1°F of 51°F throughout.
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 in Portland typically lasts for 8.2 months (253 days), from around March 9 to around November 17, rarely starting before February 14 or after April 8, and rarely ending before October 25 or after December 15.
The month of December in Portland 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 25% to 3% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in December
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
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 in Portland are essentially constant during December, remaining within 3°F of 2,487°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 in Portland is essentially constant during December, remaining around 1.0 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 1.0 kWh on December 26.
Average Daily Incident Shortwave Solar Energy in December
For the purposes of this report, the geographical coordinates of Portland are 45.523 deg latitude, -122.676 deg longitude, and 36 ft elevation.
The topography within 2 miles of Portland contains very significant variations in elevation, with a maximum elevation change of 866 feet and an average elevation above sea level of 161 feet. Within 10 miles also contains very significant variations in elevation (1,253 feet). Within 50 miles contains very significant variations in elevation (11,161 feet).
The area within 2 miles of Portland is covered by artificial surfaces (65%), water (19%), and trees (14%), within 10 miles by artificial surfaces (68%) and trees (12%), and within 50 miles by trees (47%) and shrubs (20%).
This report illustrates the typical weather in Portland, 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 Portland.
For each station, the records are corrected for the elevation difference between that station and Portland 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 Portland is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Portland and a given station.
The stations contributing to this reconstruction are: Portland International Airport (69%, 10 kilometers, northeast); Portland-Hillsboro Airport (20%, 21 kilometers, west); and Aurora State Airport (11%, 31 kilometers, south).
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Algorithms 2nd Edition , 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 airports 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.