Average Weather in Altadena California, United States
In Altadena, the summers are short, hot, arid, and clear and the winters are long, cool, wet, and partly cloudy. Over the course of the year, the temperature typically varies from 43°F to 88°F and is rarely below 36°F or above 97°F.
Based on the tourism score, the best time of year to visit Altadena for warm-weather activities is from early June to early October.
The hot season lasts for 2.9 months, from June 26 to September 24, with an average daily high temperature above 83°F. The hottest day of the year is August 23, with an average high of 88°F and low of 65°F.
The cool season lasts for 3.8 months, from November 23 to March 18, with an average daily high temperature below 69°F. The coldest day of the year is December 25, with an average low of 43°F and high of 64°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
In Altadena, 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 Altadena begins around April 24 and lasts for 6.3 months, ending around November 4. On September 7, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 91% of the time, and overcast or mostly cloudy 9% of the time.
The cloudier part of the year begins around November 4 and lasts for 5.7 months, ending around April 24. On February 22, the cloudiest day of the year, the sky is overcast or mostly cloudy 44% of the time, and clear, mostly clear, or partly cloudy 56% 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 Altadena varies throughout the year.
The wetter season lasts 4.3 months, from November 22 to March 30, with a greater than 10% chance of a given day being a wet day. The chance of a wet day peaks at 20% on February 19.
The drier season lasts 7.7 months, from March 30 to November 22. The smallest chance of a wet day is 0% on July 2.
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 20% on February 19.
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. Altadena experiences significant seasonal variation in monthly rainfall.
The rainy period of the year lasts for 6.2 months, from October 18 to April 24, 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 3.1 inches.
The rainless period of the year lasts for 5.8 months, from April 24 to October 18. The least rain falls around July 6, with an average total accumulation of 0.0 inches.
Average Monthly Rainfall
The length of the day in Altadena varies significantly over the course of the year. In 2018, the shortest day is December 21, with 9 hours, 52 minutes of daylight; the longest day is June 21, with 14 hours, 26 minutes of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:40 AM on June 12, and the latest sunrise is 1 hour, 34 minutes later at 7:14 AM on November 3. The earliest sunset is at 4:42 PM on December 4, and the latest sunset is 3 hours, 26 minutes later at 8:08 PM on June 29.
Daylight saving time (DST) is observed in Altadena during 2018, starting in the spring on March 11, lasting 7.8 months, and ending in the fall on November 4.
Sunrise & Sunset with Twilight and Daylight Saving Time
The perceived humidity level in Altadena, 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, staying within 1% of 1% throughout.
Humidity Comfort Levels
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
The average hourly wind speed in Altadena experiences significant seasonal variation over the course of the year.
The windier part of the year lasts for 5.9 months, from November 8 to May 4, with average wind speeds of more than 6.9 miles per hour. The windiest day of the year is December 31, with an average hourly wind speed of 8.6 miles per hour.
The calmer time of year lasts for 6.1 months, from May 4 to November 8. The calmest day of the year is September 6, with an average hourly wind speed of 5.3 miles per hour.
Average Wind Speed
The predominant average hourly wind direction in Altadena varies throughout the year.
The wind is most often from the west for 3.9 months, from March 5 to July 1 and for 1.5 months, from September 4 to October 18, with a peak percentage of 49% on May 22. The wind is most often from the south for 2.1 months, from July 1 to September 4, with a peak percentage of 47% on July 30. The wind is most often from the north for 4.6 months, from October 18 to March 5, with a peak percentage of 37% on January 1.
Altadena 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 3.3 months, from July 5 to October 13, with an average temperature above 66°F. The day of the year with the warmest water is August 22, with an average temperature of 68°F.
The time of year with cooler water lasts for 4.7 months, from December 10 to May 1, with an average temperature below 60°F. The day of the year with the coolest water is February 11, with an average temperature of 58°F.
Average Water Temperature
Best Time of Year to Visit
To characterize how pleasant the weather is in Altadena 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 Altadena for general outdoor tourist activities is from early June to early October, with a peak score in the first 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 Altadena for hot-weather activities is from early July to early September, with a peak score in the third week of August.
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.
Temperatures in Altadena are sufficiently warm year round that it is not entirely meaningful to discuss the growing season in these terms. We nevertheless include the chart below as an illustration of the distribution of temperatures experienced throughout the year.
Time Spent in Various Temperature Bands and the Growing Season
Growing Degree Days
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 3.9 months, from April 25 to August 20, with an average daily incident shortwave energy per square meter above 7.5 kWh. The brightest day of the year is June 18, with an average of 8.6 kWh.
The darker period of the year lasts for 3.2 months, from November 6 to February 12, with an average daily incident shortwave energy per square meter below 4.1 kWh. The darkest day of the year is December 25, with an average of 3.0 kWh.
Average Daily Incident Shortwave Solar Energy
For the purposes of this report, the geographical coordinates of Altadena are 34.190 deg latitude, -118.131 deg longitude, and 1,253 ft elevation.
The topography within 2 miles of Altadena contains large variations in elevation, with a maximum elevation change of 2,756 feet and an average elevation above sea level of 1,456 feet. Within 10 miles contains large variations in elevation (5,892 feet). Within 50 miles also contains extreme variations in elevation (10,062 feet).
The area within 2 miles of Altadena is covered by artificial surfaces (72%) and shrubs (27%), within 10 miles by artificial surfaces (49%) and shrubs (41%), and within 50 miles by shrubs (47%) and artificial surfaces (24%).
This report illustrates the typical weather in Altadena, 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 Altadena.
For each station, the records are corrected for the elevation difference between that station and Altadena 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 Altadena is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Altadena and a given station.
The stations contributing to this reconstruction are: Mount Wilson (4.2%, 8 kilometers, northeast); Burbank Bob Hope Airport (52%, 21 kilometers, west); Los Angeles / USC Campus Downtown (26%, 24 kilometers, southwest); and Fullerton Municipal Airport (18%, 38 kilometers, south).
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.
Time zones for aiports and weather stations are provided by AskGeo.com .