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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.

Temperature

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 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 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

Average Hourly Temperature in Alum RockJanFebMarAprMayJunJulAugSepOctNovDec12 AM4 AM8 AM12 PM4 PM8 PM12 AMwarmcoldcoolcomfortablevery cold
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.

Clouds

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

Cloud Cover Categories in Alum RockclearercloudiercloudierJanFebMarAprMayJunJulAugSepOctNovDec0%100%10%90%20%80%30%70%40%60%50%50%60%40%70%30%80%20%90%10%100%0%Jul 2992%Jul 2992%Jan 1144%Jan 1144%May 2367%May 2367%Oct 2668%Oct 2668%clearovercastpartly cloudymostly clearmostly cloudy
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.

Precipitation

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

Daily Chance of Precipitation in Alum RockwetwetdryJanFebMarAprMayJunJulAugSepOctNovDec0%10%20%30%40%50%60%70%80%90%100%Feb 2030%Feb 2030%Jul 190%Jul 190%Jan 126%Jan 126%Nov 415%Nov 415%Apr 1015%Apr 1015%rain
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).

Rainfall

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 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.

Sun

The length of the day in Alum Rock varies significantly over the course of the year. In 2017, the shortest day is December 21, with 9 hours, 35 minutes of daylight; the longest day is June 20, with 14 hours, 44 minutes of daylight.

Hours of Daylight and Twilight

Hours of Daylight and Twilight in Alum RockJanFebMarAprMayJunJulAugSepOctNovDec0 hr24 hr4 hr20 hr8 hr16 hr12 hr12 hr16 hr8 hr20 hr4 hr24 hr0 hr12 hr, 7 minMar 2012 hr, 7 minMar 2014 hr, 44 minJun 2014 hr, 44 minJun 2012 hr, 8 minSep 2212 hr, 8 minSep 229 hr, 35 minDec 219 hr, 35 minDec 21nightnightday
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 is at 5:46 AM on June 13, and the latest sunrise is 1 hour, 49 minutes later at 7:35 AM on November 4. The earliest sunset is at 4:49 PM on December 6, 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 2017, starting in the spring on March 12, lasting 7.8 months, and ending in the fall on November 5.

Sunrise & Sunset with Twilight and Daylight Saving Time

Sunrise & Sunset with Twilight and Daylight Saving Time in Alum RockJanFebMarAprMayJunJulAugSepOctNovDec12 AM2 AM4 AM6 AM8 AM10 AM12 PM2 PM4 PM6 PM8 PM10 PM12 AMJun 135:46 AMJun 135:46 AM8:32 PMJun 278:32 PMJun 27Dec 64:49 PMDec 64:49 PM7:35 AMNov 47:35 AMNov 4Mar 12DSTMar 12DSTDSTNov 5DSTNov 5daynightnightnightnightSolarMidnightSolarMidnightSolarNoonSunriseSunset
The solar day over the course of the year 2017. 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. The transitions to and from daylight saving time are indicated by the 'DST' labels.

Humidity

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

Humidity Comfort Levels in Alum RockJanFebMarAprMayJunJulAugSepOctNovDec0%10%20%30%40%50%60%70%80%90%100%Feb 260%Feb 260%Sep 120%Sep 120%drydrycomfortablecomfortablehumidhumid
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.

Wind

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

Average Wind Speed in Alum RockwindyJanFebMarAprMayJunJulAugSepOctNovDec0 mph1 mph2 mph3 mph4 mph5 mph6 mph7 mph8 mph9 mph10 mph11 mph12 mphJun 68.3 mphJun 68.3 mphOct 215.9 mphOct 215.9 mphApr 37.1 mphApr 37.1 mphAug 257.1 mphAug 257.1 mph
The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.

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.

Wind Direction

Wind Direction in Alum RockNWNJanFebMarAprMayJunJulAugSepOctNovDec0%100%20%80%40%60%60%40%80%20%100%0%westnortheast
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).

Water Temperature

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

The daily average water temperature (purple line), with 25th to 75th and 10th to 90th percentile bands.

Solar Energy

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

Average Daily Incident Shortwave Solar Energy in Alum RockbrightdarkdarkJanFebMarAprMayJunJulAugSepOctNovDec0 kWh1 kWh2 kWh3 kWh4 kWh5 kWh6 kWh7 kWh8 kWh9 kWh10 kWhJun 278.5 kWhJun 278.5 kWhDec 262.2 kWhDec 262.2 kWhMay 77.2 kWhMay 77.2 kWhAug 257.2 kWhAug 257.2 kWhNov 43.5 kWhNov 43.5 kWhFeb 183.5 kWhFeb 183.5 kWh
The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.

Topography

For the purposes of this report, the geographical coordinates of Alum Rock are 37.366 deg latitude, -121.827 deg longitude, and 157 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%).

Data Sources

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: Norman Y. Mineta San José International Airport (80%, 9 kilometers, west); Livermore Municipal Airport (10%, 36 kilometers, north); Watsonville Municipal Airport (7%, 48 kilometers, south); and Modesto City-County Airport (3.0%, 83 kilometers, east).

Other Data

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.

Disclaimer

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.