Average Weather in December in Irwindale California, United States
Daily high temperatures decrease by 3°F, from 70°F to 67°F, rarely falling below 57°F or exceeding 79°F. The lowest daily average high temperature is 66°F on December 26.
Daily low temperatures decrease by 2°F, from 48°F to 46°F, rarely falling below 39°F or exceeding 54°F. The lowest daily average low temperature is 46°F on December 25.
For reference, on August 22, the hottest day of the year, temperatures in Irwindale typically range from 67°F to 91°F, while on December 25, the coldest day of the year, they range from 46°F to 66°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 Irwindale experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 35% to 40%. The highest chance of overcast or mostly cloudy conditions is 41% on December 28.
The clearest day of the month is December 1, with clear, mostly clear, or partly cloudy conditions 65% of the time.
For reference, on February 22, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 44%, while on September 7, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 91%.
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 Irwindale, the chance of a wet day over the course of December is increasing, starting the month at 11% and ending it at 17%.
For reference, the year's highest daily chance of a wet day is 20% on February 19, and its lowest chance is 0% on July 2.
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 Irwindale is rapidly increasing, starting the month at 1.4 inches, when it rarely exceeds 3.0 inches, and ending the month at 2.4 inches, when it rarely exceeds 6.6 inches or falls below 0.1 inches.
Average Monthly Rainfall in December
Over the course of December in Irwindale, the length of the day is essentially constant. The shortest day of the month is December 21, with 9 hours, 53 minutes of daylight and the longest day is December 1, with 10 hours, 3 minutes of daylight.
Hours of Daylight and Twilight in December
The earliest sunrise of the month in Irwindale is 6:39 AM on December 1 and the latest sunrise is 18 minutes later at 6:57 AM on December 31.
The earliest sunset is 4:42 PM on December 4 and the latest sunset is 10 minutes later at 4:52 PM on December 31.
Daylight saving time is observed in Irwindale 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 5:40 AM and sets 14 hours, 26 minutes later, at 8:06 PM, while on December 21, the shortest day of the year, it rises at 6:53 AM and sets 9 hours, 53 minutes later, at 4:46 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time 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 Irwindale is essentially constant during December, remaining around 0% throughout.
For reference, on August 28, the muggiest day of the year, there are muggy conditions 3% 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 Irwindale is gradually increasing during December, increasing from 7.5 miles per hour to 8.1 miles per hour over the course of the month.
For reference, on December 31, the windiest day of the year, the daily average wind speed is 8.1 miles per hour, while on September 6, the calmest day of the year, the daily average wind speed is 5.2 miles per hour.
The highest daily average wind speed during December is 8.1 miles per hour on December 31.
Average Wind Speed in December
Wind Direction in December
Irwindale 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 Irwindale is gradually decreasing during December, falling by 2°F, from 62°F to 59°F, over the course of the month.
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).
Temperatures in Irwindale are sufficiently warm year round that it is not entirely meaningful to discuss the growing season in these terms. We nevertheless include the chart below as an illustration of the distribution of temperatures experienced throughout the year.
Time Spent in Various Temperature Bands and the Growing Season in December
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 Irwindale are increasing during December, increasing by 194°F, from 5,465°F to 5,659°F, over the course of the month.
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 Irwindale is essentially constant during December, remaining within 0.2 kWh of 3.1 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 3.0 kWh on December 26.
Average Daily Incident Shortwave Solar Energy in December
For the purposes of this report, the geographical coordinates of Irwindale are 34.107 deg latitude, -117.935 deg longitude, and 476 ft elevation.
The topography within 2 miles of Irwindale contains only modest variations in elevation, with a maximum elevation change of 354 feet and an average elevation above sea level of 468 feet. Within 10 miles contains only modest variations in elevation (5,157 feet). Within 50 miles also contains extreme variations in elevation (10,033 feet).
The area within 2 miles of Irwindale is covered by artificial surfaces (81%) and shrubs (19%), within 10 miles by artificial surfaces (64%) and shrubs (29%), and within 50 miles by shrubs (49%) and artificial surfaces (24%).
This report illustrates the typical weather in Irwindale 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 4 weather stations near enough to contribute to our estimation of the temperature and dew point in Irwindale.
For each station, the records are corrected for the elevation difference between that station and Irwindale 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 Irwindale is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Irwindale and a given station.
The stations contributing to this reconstruction are: Mount Wilson (2.8%, 19 kilometers, northwest); Fullerton Municipal Airport (37%, 26 kilometers, south); Ontario International Airport (33%, 27 kilometers, east); and Los Angeles / USC Campus Downtown (27%, 34 kilometers, west).
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , 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 aiports 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.