December Weather at Portland International Airport Oregon, United StatesDaily high temperatures decrease by 3°F, from 48°F to 45°F, rarely falling below 36°F or exceeding 55°F. Daily low temperatures decrease by 3°F, from 39°F to 36°F, rarely falling below 27°F or exceeding 47°F. The lowest daily average low temperature is 36°F on December 29. For reference, on August 2, the hottest day of the year, temperatures at Portland International Airport typically range from 60°F to 84°F, while on December 29, the coldest day of the year, they range from 36°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. Vilela, Portugal (5,230 miles away) and Chalais, France (5,257 miles) are the far-away foreign places with temperatures most similar to Portland International Airport (view comparison). CloudsThe month of December at Portland International Airport 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 19, with clear, mostly clear, or partly cloudy conditions 25% of the time. For reference, on January 14, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 76%, while on August 2, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 81%. PrecipitationA wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. At Portland International Airport, the chance of a wet day over the course of December is decreasing, starting the month at 57% and ending it at 52%. For reference, the year's highest daily chance of a wet day is 59% on November 26, and its lowest chance is 5% on August 2. Over the course of December at Portland International Airport, the chance of a day with only rain decreases from 56% to 48%, 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. RainfallTo 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 Portland International Airport is rapidly decreasing, starting the month at 9.6 inches, when it rarely exceeds 15.0 inches or falls below 4.4 inches, and ending the month at 8.1 inches, when it rarely exceeds 13.0 inches or falls below 3.4 inches. SnowfallAs 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 Portland International Airport is increasing, starting the month at 1.1 inches, when it rarely exceeds 4.1 inches, and ending the month at 3.3 inches, when it rarely exceeds 13.7 inches. SunOver the course of December at Portland International Airport, 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, 57 minutes of daylight. The earliest sunrise of the month at Portland International Airport is 7:31 AM on December 1 and the latest sunrise is 20 minutes later at 7:50 AM on December 31. The earliest sunset is 4:26 PM on December 9 and the latest sunset is 10 minutes later at 4:36 PM on December 31. Daylight saving time is observed at Portland International Airport during 2024, but it neither starts nor ends during December, so the entire month is in daylight saving time. For reference, on June 20, the longest day of the year, the Sun rises at 5:21 AM and sets 15 hours, 42 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 4:29 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. MoonThe 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.
HumidityWe 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 Portland 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 1% of the time, while on November 6, the least muggy day of the year, there are muggy conditions 0% of the time. WindThis 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 Portland International Airport is essentially constant during December, remaining within 0.1 miles per hour of 6.1 miles per hour throughout. For reference, on December 2, the windiest day of the year, the daily average wind speed is 6.2 miles per hour, while on August 11, the calmest day of the year, the daily average wind speed is 4.1 miles per hour. The highest daily average wind speed during December is 6.2 miles per hour on December 3. The wind direction at Portland International Airport during December is predominantly out of the south from December 1 to December 13 and the east from December 13 to December 31. Water TemperaturePortland 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 Portland International Airport is essentially constant during December, remaining within 1°F of 50°F throughout. Growing SeasonDefinitions 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 Portland International Airport typically lasts for 8.7 months (265 days), from around March 2 to around November 22, rarely starting before February 10 or after March 26, and rarely ending before October 28 or after December 19. The month of December at Portland 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 33% to 3% 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 Portland International Airport are essentially constant during December, remaining within 4°F of 2,632°F throughout. Solar EnergyThis 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 Portland International Airport 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 25. TopographyFor the purposes of this report, the geographical coordinates of Portland International Airport are 45.589 deg latitude, -122.594 deg longitude, and 20 ft elevation. The topography within 2 miles of Portland International Airport contains only modest variations in elevation, with a maximum elevation change of 292 feet and an average elevation above sea level of 36 feet. Within 10 miles contains only modest variations in elevation (1,243 feet). Within 50 miles contains very significant variations in elevation (11,165 feet). The area within 2 miles of Portland International Airport is covered by artificial surfaces (57%), water (21%), and herbaceous vegetation (17%), within 10 miles by artificial surfaces (61%) and cropland (13%), and within 50 miles by trees (49%) and shrubs (21%). Data SourcesThis report illustrates the typical weather at Portland 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 PointPortland 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 Pearson Airpark, Portland–Troutdale Airport, Scappoose Industrial Airpark, Portland-Hillsboro Airport, Aurora State Airport, Southwest Washington Regional Airport, Mc Minnville Municipal Airport, and McNary Field. Other DataAll 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 © OpenStreetMap contributors. DisclaimerThe 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. 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