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Average Weather in Portola California, United States

In Portola, the summers are warm, arid, and mostly clear and the winters are long, chilly, wet, and partly cloudy. Over the course of the year, the temperature typically varies from 25°F to 84°F and is rarely below 14°F or above 91°F.

The warm season lasts for 3.0 months, from June 17 to September 18, with an average daily high temperature above 75°F. The hottest day of the year is August 3, with an average high of 84°F and low of 54°F.

The cold season lasts for 3.9 months, from November 17 to March 13, with an average daily high temperature below 50°F. The coldest day of the year is December 31, with an average low of 25°F and high of 41°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 Portola12 AM4 AM8 AM12 PM4 PM8 PM12 AMJanFebMarAprMayJunJulAugSepOctNovDecfreezingchillycoldcoolcomfortablewarm
The average hourly temperature, color coded into bands: frigid < 15°F < freezing < 32°F < chilly < 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 Portola, 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 Portola begins around May 30 and lasts for 4.6 months, ending around October 19. On July 29, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 89% of the time, and overcast or mostly cloudy 11% of the time.

The cloudier part of the year begins around October 19 and lasts for 7.4 months, ending around May 30. On March 2, the cloudiest day of the year, the sky is overcast or mostly cloudy 58% of the time, and clear, mostly clear, or partly cloudy 41% of the time.

Cloud Cover

Cloud Cover in Portolaclearercloudiercloudier0%10%20%30%40%50%60%70%80%90%100%JanFebMarAprMayJunJulAugSepOctNovDecMar 258%Mar 258%Jul 2911%Jul 2911%May 3035%May 3035%Oct 1935%Oct 1935%overcastmostly cloudypartly cloudymostly clearclear
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 Portola varies significantly throughout the year.

The wetter season lasts 5.7 months, from October 29 to April 21, with a greater than 19% chance of a given day being a wet day. The chance of a wet day peaks at 35% on March 1.

The drier season lasts 6.3 months, from April 21 to October 29. The smallest chance of a wet day is 2% on August 11.

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 in Portola changes throughout the year.

Rain alone is the most common for 12 months, from January 2 to December 27. The highest chance of a day with rain alone is 20% on March 13.

Mixed snow and rain is the most common for 6.0 days, from December 27 to January 2. The highest chance of a day with mixed snow and rain is 13% on February 18.

Daily Chance of Precipitation

Daily Chance of Precipitation in Portolarain0%10%20%30%40%50%60%70%80%90%100%JanFebMarAprMayJunJulAugSepOctNovDecMar 135%Mar 135%Aug 112%Aug 112%Jan 229%Jan 229%Dec 2729%Dec 2729%Oct 2919%Oct 2919%Apr 2119%Apr 2119%snowmixedrain
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. Portola experiences extreme seasonal variation in monthly rainfall.

The rainy period of the year lasts for 8.9 months, from September 16 to June 14, with a sliding 31-day rainfall of at least 0.5 inches. The most rain falls during the 31 days centered around February 18, with an average total accumulation of 4.1 inches.

The rainless period of the year lasts for 3.1 months, from June 14 to September 16. The least rain falls around July 31, with an average total accumulation of 0.1 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.

Snowfall

We report snowfall in liquid-equivalent terms. The actual depth of new snowfall is typically between 5 and 10 times the liquid-equivalent amount, assuming the ground is frozen. Colder, drier snow tends to be on the higher end of that range and warmer, wetter snow on the lower end.

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Portola experiences significant seasonal variation in monthly liquid-equivalent snowfall.

The snowy period of the year lasts for 6.0 months, from October 27 to April 26, with a sliding 31-day liquid-equivalent snowfall of at least 0.1 inches. The most snow falls during the 31 days centered around January 3, with an average total liquid-equivalent accumulation of 1.1 inches.

The snowless period of the year lasts for 6.0 months, from April 26 to October 27. The least snow falls around August 1, with an average total liquid-equivalent accumulation of 0.0 inches.

Average Liquid-Equivalent Monthly Snowfall

The average liquid-equivalent snowfall (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 rainfall.

Sun

The length of the day in Portola varies significantly over the course of the year. In 2017, the shortest day is December 21, with 9 hours, 21 minutes of daylight; the longest day is June 21, with 14 hours, 60 minutes of daylight.

Hours of Daylight and Twilight

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 5:33 AM on June 14, and the latest sunrise is 2 hours, 1 minute later at 7:34 AM on November 4. The earliest sunset is at 4:37 PM on December 7, and the latest sunset is 3 hours, 57 minutes later at 8:34 PM on June 27.

Daylight saving time (DST) is observed in Portola 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 Portola12 AM2 AM4 AM6 AM8 AM10 AM12 PM2 PM4 PM6 PM8 PM10 PM12 AMJanFebMarAprMayJunJulAugSepOctNovDecJun 145:33 AMJun 145:33 AM8:34 PMJun 278:34 PMJun 27Dec 74:37 PMDec 74:37 PM7:34 AMNov 47:34 AMNov 4Mar 12DSTMar 12DSTDSTNov 5DSTNov 5daynightnightnightnight
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 Portola, 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 Portola0%10%20%30%40%50%60%70%80%90%100%JanFebMarAprMayJunJulAugSepOctNovDecJul 30%Jul 30%drydry
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 Portola does not vary significantly over the course of the year, remaining within 0.3 miles per hour of 2.7 miles per hour throughout.

Average Wind Speed

Average Wind Speed in Portola0.0 mph0.5 mph1.0 mph1.5 mph2.0 mph2.5 mph3.0 mph3.5 mph4.0 mph4.5 mph5.0 mphJanFebMarAprMayJunJulAugSepOctNovDecMar 213.1 mphMar 213.1 mphAug 62.4 mphAug 62.4 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 Portola varies throughout the year.

The wind is most often from the south for 1.7 months, from January 31 to March 22 and for 1.1 months, from November 4 to December 7, with a peak percentage of 42% on February 23. The wind is most often from the west for 6.2 months, from March 22 to September 28, with a peak percentage of 61% on July 23. The wind is most often from the east for 1.2 months, from September 28 to November 4 and for 1.8 months, from December 7 to January 31, with a peak percentage of 36% on October 20.

Wind Direction

Wind Direction in PortolaESWESE0%20%40%60%80%100%JanFebMarAprMayJunJulAugSepOctNovDecwestsoutheastnorth
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 experiences extreme seasonal variation over the course of the year.

The brighter period of the year lasts for 3.5 months, from May 10 to August 24, with an average daily incident shortwave energy per square meter above 7.3 kWh. The brightest day of the year is July 1, with an average of 8.6 kWh.

The darker period of the year lasts for 3.5 months, from November 1 to February 16, with an average daily incident shortwave energy per square meter below 3.5 kWh. The darkest day of the year is December 19, with an average of 2.1 kWh.

Average Daily Incident Shortwave Solar Energy

Average Daily Incident Shortwave Solar Energy in Portolabrightdarkdark0 kWh1 kWh2 kWh3 kWh4 kWh5 kWh6 kWh7 kWh8 kWh9 kWh10 kWhJanFebMarAprMayJunJulAugSepOctNovDecJul 18.6 kWhJul 18.6 kWhDec 192.1 kWhDec 192.1 kWhMay 107.3 kWhMay 107.3 kWhAug 247.3 kWhAug 247.3 kWhNov 13.4 kWhNov 13.4 kWhFeb 163.5 kWhFeb 163.5 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 Portola are 39.810 deg latitude, -120.469 deg longitude, and 5,105 ft elevation.

The topography within 2 miles of Portola contains very significant variations in elevation, with a maximum elevation change of 928 feet and an average elevation above sea level of 5,067 feet. Within 10 miles contains very significant variations in elevation (3,376 feet). Within 50 miles also contains extreme variations in elevation (10,062 feet).

The area within 2 miles of Portola is covered by shrubs (68%) and trees (31%), within 10 miles by trees (52%) and shrubs (33%), and within 50 miles by trees (51%) and shrubs (42%).

Data Sources

This report illustrates the typical weather in Portola, 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 Portola.

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

The stations contributing to this reconstruction are: Truckee Tahoe Airport (34%, 62 kilometers, southeast); Blue Canyon-Nyack Airport (37%, 63 kilometers, south); and Reno/Tahoe International Airport (29%, 70 kilometers, southeast).

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.