Average Weather in December in Mercer Island Washington, United States
Daily high temperatures decrease by 2°F, from 48°F to 46°F, rarely falling below 38°F or exceeding 55°F. The lowest daily average high temperature is 46°F on December 23.
Daily low temperatures decrease by 2°F, from 39°F to 37°F, rarely falling below 27°F or exceeding 47°F.
For reference, on August 1, the hottest day of the year, temperatures in Mercer Island typically range from 60°F to 79°F, while on January 1, the coldest day of the year, they range from 37°F to 46°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 in Mercer Island experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 73% throughout the month.
The clearest day of the month is December 20, with clear, mostly clear, or partly cloudy conditions 28% 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 74%.
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 Mercer Island, the chance of a wet day over the course of December is rapidly decreasing, starting the month at 57% and ending it at 50%.
For reference, the year's highest daily chance of a wet day is 59% on November 18, and its lowest chance is 9% on August 10.
Over the course of December in Mercer Island, 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
The average sliding 31-day rainfall during December in Mercer Island is rapidly decreasing, starting the month at 7.9 inches, when it rarely exceeds 12.3 inches or falls below 3.6 inches, and ending the month at 6.6 inches, when it rarely exceeds 10.3 inches or falls below 3.4 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 Mercer Island is essentially constant, remaining about 0.1 inches throughout, and rarely exceeding 0.7 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 in Mercer Island, the length of the day is essentially constant. The shortest day of the month is December 21, with 8 hours, 26 minutes of daylight and the longest day is December 1, with 8 hours, 43 minutes of daylight.
Hours of Daylight and Twilight in December
The earliest sunrise of the month in Mercer Island is 7:36 AM on December 1 and the latest sunrise is 21 minutes later at 7:57 AM on December 31.
The earliest sunset is 4:17 PM on December 11 and the latest sunset is 10 minutes later at 4:27 PM on December 31.
Daylight saving time is observed in Mercer Island 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 5:11 AM and sets 15 hours, 59 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, 26 minutes later, at 4:20 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time in December
The chance that a given day will be muggy in Mercer Island is essentially constant during December, remaining around 0% throughout.
For reference, on August 6, 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
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
The average hourly wind speed in Mercer Island is essentially constant during December, remaining within 0.1 miles per hour of 5.2 miles per hour throughout.
For reference, on December 4, the windiest day of the year, the daily average wind speed is 5.3 miles per hour, while on August 2, the calmest day of the year, the daily average wind speed is 2.7 miles per hour.
The highest daily average wind speed during December is 5.3 miles per hour on December 4.
Average Wind Speed in December
Wind Direction in December
Mercer Island 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 Mercer Island 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
The growing season in Mercer Island typically lasts for 8.7 months (265 days), from around March 1 to around November 21, rarely starting before February 3 or after March 21, and rarely ending before October 30 or after December 18.
The month of December in Mercer Island 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 32% to 3% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in December
The average accumulated growing degree days in Mercer Island are essentially constant during December, remaining within 4°F of 2,344°F throughout.
Growing Degree Days in December
The average daily incident shortwave solar energy in Mercer Island is essentially constant during December, remaining within 0.1 kWh of 0.9 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 0.9 kWh on December 25.
Average Daily Incident Shortwave Solar Energy in December
For the purposes of this report, the geographical coordinates of Mercer Island are 47.571 deg latitude, -122.222 deg longitude, and 52 ft elevation.
The topography within 2 miles of Mercer Island contains only modest variations in elevation, with a maximum elevation change of 374 feet and an average elevation above sea level of 99 feet. Within 10 miles contains only modest variations in elevation (1,991 feet). Within 50 miles contains large variations in elevation (7,864 feet).
The area within 2 miles of Mercer Island is covered by water (41%), artificial surfaces (26%), herbaceous vegetation (19%), and trees (14%), within 10 miles by artificial surfaces (60%) and water (19%), and within 50 miles by trees (53%) and artificial surfaces (14%).
This report illustrates the typical weather in Mercer Island 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 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Mercer Island.
For each station, the records are corrected for the elevation difference between that station and Mercer Island 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 Mercer Island is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Mercer Island and a given station.
The stations contributing to this reconstruction are: King County International Airport (53%, 7 kilometers, southwest); Renton Municipal Airport (43%, 9 kilometers, south); and Snohomish County Airport (Paine Field) (4.1%, 38 kilometers, north).
Time zones for aiports and weather stations are provided by AskGeo.com .