Average Weather in Alum Rock California, United States
In Alum Rock, the summers are long, warm, arid, and mostly clear and the winters are cold, wet, and partly cloudy. Over the course of the year, the temperature typically varies from 42°F to 85°F and is rarely below 34°F or above 95°F.
Based on the tourism score, the best time of year to visit Alum Rock for warm-weather activities is from mid June to late September.
The warm season lasts for 3.9 months, from June 9 to October 6, with an average daily high temperature above 79°F. The hottest day of the year is July 18, with an average high of 85°F and low of 61°F.
The cool season lasts for 3.0 months, from November 24 to February 23, with an average daily high temperature below 64°F. The coldest day of the year is January 2, with an average low of 42°F and high of 59°F.
Average High and Low Temperature
The figure below shows you a compact characterization of the entire year of hourly average temperatures. The horizontal axis is the day of the year, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature
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
Mariscala, Uruguay (6,554 miles away); Aveiras de Cima, Portugal (5,657 miles); and Stellenbosch, South Africa (10,244 miles) are the far-away foreign places with temperatures most similar to Alum Rock (view comparison).
In Alum Rock, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.
The clearer part of the year in Alum Rock begins around May 23 and lasts for 5.1 months, ending around October 26. On July 29, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 92% of the time, and overcast or mostly cloudy 8% of the time.
The cloudier part of the year begins around October 26 and lasts for 6.9 months, ending around May 23. On January 11, the cloudiest day of the year, the sky is overcast or mostly cloudy 56% of the time, and clear, mostly clear, or partly cloudy 44% of the time.
Cloud Cover Categories
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. The chance of wet days in Alum Rock varies throughout the year.
The wetter season lasts 5.2 months, from November 4 to April 10, with a greater than 15% chance of a given day being a wet day. The chance of a wet day peaks at 30% on February 20.
The drier season lasts 6.8 months, from April 10 to November 4. The smallest chance of a wet day is 0% on July 19.
Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 30% on February 20.
Daily Chance of Precipitation
To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Alum Rock experiences significant seasonal variation in monthly rainfall.
The rainy period of the year lasts for 7.4 months, from October 3 to May 15, with a sliding 31-day rainfall of at least 0.5 inches. The most rain falls during the 31 days centered around February 15, with an average total accumulation of 3.9 inches.
The rainless period of the year lasts for 4.6 months, from May 15 to October 3. The least rain falls around July 30, with an average total accumulation of 0.0 inches.
Average Monthly Rainfall
The length of the day in Alum Rock varies significantly over the course of the year. In 2021, the shortest day is December 21, with 9 hours, 35 minutes of daylight; the longest day is June 20, with 14 hours, 45 minutes of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:46 AM on June 13, and the latest sunrise is 1 hour, 52 minutes later at 7:37 AM on November 6. The earliest sunset is at 4:49 PM on December 5, and the latest sunset is 3 hours, 43 minutes later at 8:32 PM on June 27.
Daylight saving time (DST) is observed in Alum Rock during 2021, starting in the spring on March 14, lasting 7.8 months, and ending in the fall on November 7.
Sunrise & Sunset with Twilight and Daylight Saving Time
The figure below presents a compact representation of key lunar data for 2021. 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.
Moon Rise, Set & Phases
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 perceived humidity level in Alum Rock, as measured by the percentage of time in which the humidity comfort level is muggy, oppressive, or miserable, does not vary significantly over the course of the year, remaining a virtually constant 0% throughout.
Humidity Comfort Levels
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 Alum Rock experiences mild seasonal variation over the course of the year.
The windier part of the year lasts for 4.7 months, from April 3 to August 25, with average wind speeds of more than 7.1 miles per hour. The windiest day of the year is June 6, with an average hourly wind speed of 8.3 miles per hour.
The calmer time of year lasts for 7.3 months, from August 25 to April 3. The calmest day of the year is October 21, with an average hourly wind speed of 5.9 miles per hour.
Average Wind Speed
The predominant average hourly wind direction in Alum Rock varies throughout the year.
The wind is most often from the west for 9.2 months, from February 8 to November 15, with a peak percentage of 92% on August 10. The wind is most often from the north for 2.8 months, from November 15 to February 8, with a peak percentage of 32% on January 1.
Alum Rock 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 water temperature experiences some seasonal variation over the course of the year.
The time of year with warmer water lasts for 2.5 months, from August 1 to October 19, with an average temperature above 57°F. The day of the year with the warmest water is September 15, with an average temperature of 58°F.
The time of year with cooler water lasts for 5.7 months, from December 21 to June 12, with an average temperature below 54°F. The day of the year with the coolest water is May 3, with an average temperature of 53°F.
Average Water Temperature
Best Time of Year to Visit
To characterize how pleasant the weather is in Alum Rock throughout the year, we compute two travel scores.
The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Alum Rock for general outdoor tourist activities is from mid June to late September, with a peak score in the last week of July.
The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Alum Rock for hot-weather activities is from late June to early September, with a peak score in the last week of July.
For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.
Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.
Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.
Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.
Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.
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).
While it does not do so every year, freezing temperatures are seen in Alum Rock over some winters. The day least likely to be in the growing season is January 2, with a 63% chance.
Time Spent in Various Temperature Bands and the Growing Season
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
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.
Based on growing degree days alone, the first spring blooms in Alum Rock should appear around January 30, only rarely appearing before January 20 or after February 13.
Growing Degree Days
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 experiences extreme seasonal variation over the course of the year.
The brighter period of the year lasts for 3.5 months, from May 7 to August 25, with an average daily incident shortwave energy per square meter above 7.2 kWh. The brightest day of the year is June 27, with an average of 8.5 kWh.
The darker period of the year lasts for 3.5 months, from November 4 to February 18, with an average daily incident shortwave energy per square meter below 3.5 kWh. The darkest day of the year is December 26, with an average of 2.2 kWh.
Average Daily Incident Shortwave Solar Energy
For the purposes of this report, the geographical coordinates of Alum Rock are 37.366 deg latitude, -121.827 deg longitude, and 148 ft elevation.
The topography within 2 miles of Alum Rock contains very significant variations in elevation, with a maximum elevation change of 1,017 feet and an average elevation above sea level of 224 feet. Within 10 miles contains very significant variations in elevation (3,885 feet). Within 50 miles also contains extreme variations in elevation (4,285 feet).
The area within 2 miles of Alum Rock is covered by artificial surfaces (88%), within 10 miles by artificial surfaces (50%) and grassland (31%), and within 50 miles by water (24%) and grassland (23%).
This report illustrates the typical weather in Alum Rock, 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 Alum Rock.
For each station, the records are corrected for the elevation difference between that station and Alum Rock 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 Alum Rock is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Alum Rock and a given station.
The stations contributing to this reconstruction are: Reid-Hillview of Santa Clara County Airport (76%, 3.8 kilometers, south); Norman Y. Mineta San José International Airport (21%, 9 kilometers, west); Livermore Municipal Airport (2.5%, 36 kilometers, north); and Modesto City-County Airport (0.8%, 83 kilometers, east).
All 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 © 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.