Average Weather in December in Gilroy California, United States
Daily high temperatures decrease by 3°F, from 63°F to 60°F, rarely falling below 54°F or exceeding 69°F. The lowest daily average high temperature is 60°F on December 26.
Daily low temperatures decrease by 2°F, from 41°F to 39°F, rarely falling below 32°F or exceeding 50°F. The lowest daily average low temperature is 39°F on December 31.
For reference, on August 23, the hottest day of the year, temperatures in Gilroy typically range from 55°F to 83°F, while on December 31, the coldest day of the year, they range from 39°F to 60°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 Gilroy experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 47% to 52%.
The clearest day of the month is December 1, with clear, mostly clear, or partly cloudy conditions 53% of the time.
For reference, on February 4, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 54%, while on July 29, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 92%.
Cloud Cover Categories in December
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Gilroy, the chance of a wet day over the course of December is essentially constant, remaining around 24% throughout.
For reference, the year's highest daily chance of a wet day is 30% on February 20, and its lowest chance is -0% on July 22.
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 Gilroy is increasing, starting the month at 2.8 inches, when it rarely exceeds 5.9 inches or falls below 0.4 inches, and ending the month at 3.4 inches, when it rarely exceeds 7.2 inches or falls below 0.3 inches.
Average Monthly Rainfall in December
Over the course of December in Gilroy, the length of the day is essentially constant. The shortest day of the month is December 21, with 9 hours, 37 minutes of daylight and the longest day is December 1, with 9 hours, 49 minutes of daylight.
Hours of Daylight and Twilight in December
The earliest sunrise of the month in Gilroy is 7:00 AM on December 1 and the latest sunrise is 18 minutes later at 7:19 AM on December 31.
The earliest sunset is 4:49 PM on December 6 and the latest sunset is 10 minutes later at 4:59 PM on December 31.
Daylight saving time is observed in Gilroy 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 5:46 AM and sets 14 hours, 42 minutes later, at 8:29 PM, while on December 21, the shortest day of the year, it rises at 7:15 AM and sets 9 hours, 37 minutes later, at 4:53 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 Gilroy is essentially constant during December, remaining around 0% throughout.
For reference, on September 17, 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
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 Gilroy is essentially constant during December, remaining within 0.1 miles per hour of 5.9 miles per hour throughout.
For reference, on June 6, the windiest day of the year, the daily average wind speed is 8.0 miles per hour, while on November 1, the calmest day of the year, the daily average wind speed is 5.6 miles per hour.
The highest daily average wind speed during December is 6.0 miles per hour on December 13.
Average Wind Speed in December
Wind Direction in December
Gilroy 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 Gilroy is essentially constant during December, remaining around 55°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 Gilroy typically lasts for 11 months (333 days), from around January 18 to around December 17, rarely starting after March 6, or ending before November 21.
During December in Gilroy, the chance that a given day is within the growing season is rapidly decreasing falling from 76% to 39% over the course of the month.
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 Gilroy are gradually increasing during December, increasing by 81°F, from 3,404°F to 3,485°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 Gilroy is essentially constant during December, remaining within 0.1 kWh of 2.5 kWh throughout.
The lowest average daily incident shortwave solar energy during December is 2.3 kWh on December 26.
Average Daily Incident Shortwave Solar Energy in December
For the purposes of this report, the geographical coordinates of Gilroy are 37.006 deg latitude, -121.568 deg longitude, and 210 ft elevation.
The topography within 2 miles of Gilroy contains only modest variations in elevation, with a maximum elevation change of 449 feet and an average elevation above sea level of 209 feet. Within 10 miles contains only modest variations in elevation (2,740 feet). Within 50 miles contains large variations in elevation (5,046 feet).
The area within 2 miles of Gilroy is covered by artificial surfaces (64%) and cropland (29%), within 10 miles by grassland (43%) and cropland (23%), and within 50 miles by grassland (30%) and shrubs (17%).
This report illustrates the typical weather in Gilroy 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 Gilroy.
For each station, the records are corrected for the elevation difference between that station and Gilroy 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 Gilroy is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Gilroy and a given station.
The stations contributing to this reconstruction are: South County Airport Of Santa Clara County (61%, 9 kilometers, north); Hollister Municipal Airport (22%, 18 kilometers, southeast); and Watsonville Municipal Airport (17%, 21 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.
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.