Average Weather in September in San Dimas California, United States
Daily high temperatures decrease by 6°F, from 91°F to 85°F, rarely falling below 74°F or exceeding 100°F.
Daily low temperatures decrease by 4°F, from 65°F to 61°F, rarely falling below 55°F or exceeding 72°F.
For reference, on August 23, the hottest day of the year, temperatures in San Dimas typically range from 66°F to 92°F, while on December 26, the coldest day of the year, they range from 44°F to 65°F.
Average High and Low Temperature in September
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
The month of September in San Dimas experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 11% throughout the month. The lowest chance of overcast or mostly cloudy conditions is 9% on September 7.
The clearest day of the month is September 7, with clear, mostly clear, or partly cloudy conditions 91% of the time.
For reference, on February 20, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 44%, while on September 7, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 91%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In San Dimas, the chance of a wet day over the course of September is gradually increasing, starting the month at 1% and ending it at 3%.
For reference, the year's highest daily chance of a wet day is 20% on February 19, and its lowest chance is 0% on June 23.
Probability of Precipitation in September
The average sliding 31-day rainfall during September in San Dimas is essentially constant, remaining about 0.2 inches throughout, and rarely exceeding 0.9 inches or falling below -0.0 inches.
Average Monthly Rainfall in September
Over the course of September in San Dimas, 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, 1 minute, implying an average daily decrease of 2 minutes, 5 seconds, and weekly decrease of 14 minutes, 37 seconds.
The shortest day of the month is September 30, with 11 hours, 52 minutes of daylight and the longest day is September 1, with 12 hours, 52 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in San Dimas is 6:24 AM on September 1 and the latest sunrise is 20 minutes later at 6:45 AM on September 30.
The latest sunset is 7:16 PM on September 1 and the earliest sunset is 40 minutes earlier at 6:36 PM on September 30.
Daylight saving time is observed in San Dimas during 2018, 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 5:40 AM and sets 14 hours, 26 minutes later, at 8:06 PM, while on December 21, the shortest day of the year, it rises at 6:53 AM and sets 9 hours, 53 minutes later, at 4:45 PM.
Sunrise & Sunset with Twilight in September
The chance that a given day will be muggy in San Dimas is essentially constant during September, remaining within 1% of 2% throughout.
For reference, on August 10, the muggiest day of the year, there are muggy conditions 4% of the time, while on November 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in September
The average hourly wind speed in San Dimas is essentially constant during September, remaining within 0.1 miles per hour of 5.3 miles per hour throughout.
For reference, on December 31, the windiest day of the year, the daily average wind speed is 7.9 miles per hour, while on September 8, the calmest day of the year, the daily average wind speed is 5.2 miles per hour.
The lowest daily average wind speed during September is 5.2 miles per hour on September 9.
Average Wind Speed in September
Wind Direction in September
San Dimas 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 surface water temperature in San Dimas is essentially constant during September, remaining within 1°F of 68°F throughout.
Average Water Temperature in September
Temperatures in San Dimas 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 in September
The average accumulated growing degree days in San Dimas are rapidly increasing during September, increasing by 666°F, from 3,650°F to 4,316°F, over the course of the month.
Growing Degree Days in September
The average daily incident shortwave solar energy in San Dimas is decreasing during September, falling by 1.2 kWh, from 7.0 kWh to 5.8 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in September
For the purposes of this report, the geographical coordinates of San Dimas are 34.107 deg latitude, -117.807 deg longitude, and 997 ft elevation.
The topography within 2 miles of San Dimas contains very significant variations in elevation, with a maximum elevation change of 958 feet and an average elevation above sea level of 980 feet. Within 10 miles contains very significant variations in elevation (5,315 feet). Within 50 miles also contains extreme variations in elevation (10,062 feet).
The area within 2 miles of San Dimas is covered by artificial surfaces (83%) and shrubs (17%), within 10 miles by artificial surfaces (53%) and shrubs (42%), and within 50 miles by shrubs (53%) and artificial surfaces (24%).
This report illustrates the typical weather in San Dimas 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 San Dimas.
For each station, the records are corrected for the elevation difference between that station and San Dimas 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 San Dimas is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between San Dimas and a given station.
The stations contributing to this reconstruction are: Ontario International Airport (80%, 15 kilometers, east); Mount Wilson (2.6%, 28 kilometers, northwest); and Fullerton Municipal Airport (18%, 31 kilometers, southwest).
Time zones for aiports and weather stations are provided by AskGeo.com .