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Average Weather in September in Alta Sierra California, United States

In Alta Sierra, the month of September is characterized by falling daily high temperatures, with daily highs decreasing by 7°F, from 86°F to 79°F over the course of the month, and rarely exceeding 93°F or dropping below 67°F.

Daily low temperatures decrease by 6°F, from 64°F to 58°F, rarely falling below 48°F or exceeding 71°F.

For reference, on July 28, the hottest day of the year, temperatures in Alta Sierra typically range from 67°F to 89°F, while on December 27, the coldest day of the year, they range from 39°F to 52°F.

Average High and Low Temperature in September

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 hourly average temperatures for the quarter of the year centered on September. 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 September

Average Hourly Temperature in September in Alta Sierra1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303012 AM4 AM8 AM12 PM4 PM8 PM12 AMAugOctcoldcomfortablewarmhotcool
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.


The month of September in Alta Sierra experiences increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 13% to 22%.

The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 87% of the time.

For reference, on February 26, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 60%, while on July 29, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 90%.

Cloud Cover Categories in September

Cloud Cover Categories in September in Alta Sierra181522291122334455667788991010111112121313141415151616171718181919202021212222232324242525262627272828292930300%100%10%90%20%80%30%70%40%60%50%50%60%40%70%30%80%20%90%10%100%0%AugOctSep 187%Sep 187%Sep 3078%Sep 3078%Sep 1186%Sep 1186%Sep 2182%Sep 2182%clearovercastmostly clearmostly cloudypartly 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.


A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Alta Sierra, the chance of a wet day over the course of September is increasing, starting the month at 2% and ending it at 8%.

For reference, the year's highest daily chance of a wet day is 35% on February 28, and its lowest chance is 0% on July 21.

Probability of Precipitation in September

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).


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 September in Alta Sierra is increasing, starting the month at 0.2 inches, when it rarely exceeds 0.7 inches, and ending the month at 0.8 inches, when it rarely exceeds 2.4 inches.

Average Monthly Rainfall in September

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.


Over the course of September in Alta Sierra, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1 hour, 13 minutes, implying an average daily decrease of 2 minutes, 31 seconds, and weekly decrease of 17 minutes, 35 seconds.

The shortest day of the month is September 30, with 11 hours, 48 minutes of daylight and the longest day is September 1, with 13 hours, 1 minute of daylight.

Hours of Daylight and Twilight in September

Hours of Daylight and Twilight in September in Alta Sierra181522291122334455667788991010111112121313141415151616171718181919202021212222232324242525262627272828292930300 hr24 hr4 hr20 hr8 hr16 hr12 hr12 hr16 hr8 hr20 hr4 hr24 hr0 hrAugOctSep 2212 hr, 8 minSep 2212 hr, 8 minnightnightdaydaySep 113 hr, 1 minSep 113 hr, 1 min
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 of the month in Alta Sierra is 6:33 AM on September 1 and the latest sunrise is 26 minutes later at 6:59 AM on September 30.

The latest sunset is 7:34 PM on September 1 and the earliest sunset is 46 minutes earlier at 6:47 PM on September 30.

Daylight saving time is observed in Alta Sierra during 2017, but it neither starts nor ends during September, so the entire month is in daylight saving time.

For reference, on June 21, the longest day of the year, the Sun rises at 5:38 AM and sets 14 hours, 55 minutes later, at 8:33 PM, while on December 21, the shortest day of the year, it rises at 7:20 AM and sets 9 hours, 25 minutes later, at 4:44 PM.

Sunrise & Sunset with Twilight in September

The solar day over the course of September. 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.


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 Alta Sierra is essentially constant during September, remaining around 0% throughout.

For reference, on May 16, 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 September

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.


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 Alta Sierra is essentially constant during September, remaining within 0.1 miles per hour of 5.4 miles per hour throughout.

For reference, on December 18, the windiest day of the year, the daily average wind speed is 6.2 miles per hour, while on September 24, the calmest day of the year, the daily average wind speed is 5.3 miles per hour.

The lowest daily average wind speed during September is 5.3 miles per hour on September 24.

Average Wind Speed in September

The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.
The wind direction in Alta Sierra during September is predominantly out of the south from September 1 to September 27 and the east from September 27 to September 30.

Wind Direction in September

Wind Direction in September in Alta SierraSE181522291122334455667788991010111112121313141415151616171718181919202021212222232324242525262627272828292930300%100%10%90%20%80%30%70%40%60%50%50%60%40%70%30%80%20%90%10%100%0%AugOcteastwestsouthnorth
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).

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 in Alta Sierra is rapidly decreasing during September, falling by 1.6 kWh, from 6.8 kWh to 5.2 kWh, over the course of the month.

Average Daily Incident Shortwave Solar Energy in September

The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.


For the purposes of this report, the geographical coordinates of Alta Sierra are 39.142 deg latitude, -121.054 deg longitude, and 2,119 ft elevation.

The topography within 2 miles of Alta Sierra contains very significant variations in elevation, with a maximum elevation change of 971 feet and an average elevation above sea level of 2,092 feet. Within 10 miles contains very significant variations in elevation (3,205 feet). Within 50 miles contains large variations in elevation (9,331 feet).

The area within 2 miles of Alta Sierra is covered by shrubs (93%), within 10 miles by shrubs (56%) and trees (24%), and within 50 miles by trees (42%) and cropland (19%).

Data Sources

This report illustrates the typical weather in Alta Sierra 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 Alta Sierra.

For each station, the records are corrected for the elevation difference between that station and Alta Sierra 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 Alta Sierra is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Alta Sierra and a given station.

The stations contributing to this reconstruction are: Auburn Municipal Airport (72%, 21 kilometers, south); Beale Air Force Base (18%, 33 kilometers, west); and Blue Canyon-Nyack Airport (10%, 33 kilometers, northeast).

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.


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.