Daily high temperatures decrease by 5°F, from 40°F to 35°F, rarely falling below 24°F or exceeding 51°F. The lowest daily average high temperature is 35°F on December 23.
Daily low temperatures decrease by 3°F, from 26°F to 23°F, rarely falling below 9°F or exceeding 36°F. The lowest daily average low temperature is 23°F on December 23.
For reference, on August 1, the hottest day of the year, temperatures in Ahtanum typically range from 56°F to 89°F, while on December 23, the coldest day of the year, they range from 23°F to 35°F.
Average High and Low Temperature in December
The daily average high (red line) and low (blue line) temperature, with 25th to 75th and 10th to 90th percentile bands. The thin dotted lines are the corresponding average perceived temperatures.
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 average hourly temperature, color coded into bands: 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 shaded overlays indicate night and civil twilight.
The month of December in Ahtanum experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 62% throughout the month.
The clearest day of the month is December 6, with clear, mostly clear, or partly cloudy conditions 39% of the time.
For reference, on January 16, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 65%, 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
The percentage of time spent in each cloud cover band, categorized by the percentage of the sky covered by clouds: clear < 20% < mostly clear < 40% < partly cloudy < 60% < mostly cloudy < 80% < overcast.
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Ahtanum, the chance of a wet day over the course of December is gradually decreasing, starting the month at 38% and ending it at 34%.
For reference, the year's highest daily chance of a wet day is 38% on December 1, and its lowest chance is 4% on August 5.
Over the course of December in Ahtanum, the chance of a day with only rain decreases from 20% to 11%, the chance of a day with mixed snow and rain remains an essentially constant 9% throughout, and the chance of a day with only snow increases from 7% to 13%.
Probability of Precipitation in December
The percentage of days in which various types of precipitation are observed, excluding trace quantities: rain alone, snow alone, and mixed (both rain and snow fell in the same day).
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 Ahtanum is decreasing, starting the month at 2.1 inches, when it rarely exceeds 4.3 inches or falls below 0.4 inches, and ending the month at 1.4 inches, when it rarely exceeds 3.0 inches or falls below 0.1 inches.
Average Monthly Rainfall in December
The average rainfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average liquid-equivalent snowfall.
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 Ahtanum is gradually increasing, starting the month at 0.7 inches, when it rarely exceeds 1.6 inches or falls below 0.1 inches, and ending the month at 1.2 inches, when it rarely exceeds 2.4 inches or falls below 0.2 inches.
The highest average 31-day liquid-equivalent accumulation is 1.2 inches on December 30.
Average Monthly Liquid-Equivalent Snowfall in December
The average liquid-equivalent snowfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average rainfall.
Over the course of December in Ahtanum, the length of the day is essentially constant. The shortest day of the month is December 21, with 8 hours, 34 minutes of daylight and the longest day is December 1, with 8 hours, 51 minutes of daylight.
Hours of Daylight and Twilight in December
The number of hours during which the Sun is visible (black line). From bottom (most yellow) to top (most gray), the color bands indicate: full daylight, twilight (civil, nautical, and astronomical), and full night.
The earliest sunrise of the month in Ahtanum is 7:26 AM on December 1 and the latest sunrise is 21 minutes later at 7:46 AM on December 31.
The earliest sunset is 4:14 PM on December 10 and the latest sunset is 10 minutes later at 4:24 PM on December 31.
Daylight saving time is observed in Ahtanum 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 5:09 AM and sets 15 hours, 50 minutes later, at 8:59 PM, while on December 21, the shortest day of the year, it rises at 7:43 AM and sets 8 hours, 34 minutes later, at 4:17 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time in December
The solar day over the course of December. From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray.
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 Ahtanum is essentially constant during December, remaining around 0% throughout.
For reference, on July 16, 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
The percentage of time spent at various humidity comfort levels, categorized by dew point: 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 Ahtanum is essentially constant during December, remaining within 0.2 miles per hour of 6.0 miles per hour throughout.
For reference, on March 29, the windiest day of the year, the daily average wind speed is 7.1 miles per hour, while on December 20, the calmest day of the year, the daily average wind speed is 5.8 miles per hour.
The lowest daily average wind speed during December is 5.8 miles per hour on December 21.
Average Wind Speed in December
The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.
The hourly average wind direction in Ahtanum throughout December is predominantly from the west, with a peak proportion of 43% on December 1.
Wind Direction in December
The percentage of hours in which the mean wind direction is from each of the four cardinal wind directions (north, east, south, and west), excluding hours in which the mean wind speed is less than 1 mph. The lightly tinted areas at the boundaries are the percentage of hours spent in the implied intermediate directions (northeast, southeast, southwest, and northwest).
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 Ahtanum typically lasts for 4.9 months (150 days), from around May 8 to around October 5, rarely starting before April 18 or after May 26, and rarely ending before September 18 or after October 24.
The month of December in Ahtanum is reliably fully outside of the growing season.
Time Spent in Various Temperature Bands and the Growing Season in December
The percentage of time spent in various temperature bands: 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 black line is the percentage chance that a given day is within 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.
The average accumulated growing degree days in Ahtanum are essentially constant during December, remaining within 1°F of 2,712°F throughout.
Growing Degree Days in December
The average growing degree days accumulated over the course of December, with 25th to 75th and 10th to 90th percentile bands.
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 Ahtanum is essentially constant during December, remaining within 0.1 kWh of 1.3 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 1.2 kWh on December 26.
Average Daily Incident Shortwave Solar Energy in December
The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.
For the purposes of this report, the geographical coordinates of Ahtanum are 46.560 deg latitude, -120.622 deg longitude, and 1,286 ft elevation.
The topography within 2 miles of Ahtanum contains only modest variations in elevation, with a maximum elevation change of 305 feet and an average elevation above sea level of 1,256 feet. Within 10 miles contains only modest variations in elevation (2,467 feet). Within 50 miles contains large variations in elevation (11,900 feet).
The area within 2 miles of Ahtanum is covered by cropland (86%), within 10 miles by cropland (44%) and shrubs (39%), and within 50 miles by shrubs (55%) and trees (26%).
This report illustrates the typical weather in Ahtanum 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
There are 2 weather stations near enough to contribute to our estimation of the temperature and dew point in Ahtanum.
The estimated value at Ahtanum is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Ahtanum and a given station.
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.
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.