September Weather in Aspen Colorado, United States
Daily high temperatures decrease by 9°F, from 68°F to 59°F, rarely falling below 48°F or exceeding 74°F.
Daily low temperatures decrease by 8°F, from 40°F to 32°F, rarely falling below 25°F or exceeding 47°F.
For reference, on July 8, the hottest day of the year, temperatures in Aspen typically range from 43°F to 73°F, while on January 28, the coldest day of the year, they range from 7°F to 29°F.
Average High and Low Temperature in September in Aspen
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 in Aspen
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 month of September in Aspen experiences gradually decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 32% to 25%. The lowest chance of overcast or mostly cloudy conditions is 25% on September 19.
The clearest day of the month is September 19, with clear, mostly clear, or partly cloudy conditions 75% of the time.
For reference, on February 12, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 51%, while on September 19, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 75%.
Cloud Cover Categories in September in Aspen
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. In Aspen, the chance of a wet day over the course of September is gradually decreasing, starting the month at 23% and ending it at 21%.
For reference, the year's highest daily chance of a wet day is 29% on August 1, and its lowest chance is 13% on June 17.
Over the course of September in Aspen, the chance of a day with only rain decreases from 23% to 17%, the chance of a day with mixed snow and rain increases from 0% to 3%, and the chance of a day with only snow remains an essentially constant 1% throughout.
Probability of Precipitation in September in Aspen
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 Aspen is essentially constant, remaining about 1.1 inches throughout, and rarely exceeding 2.3 inches or falling below 0.3 inches.
The highest average 31-day accumulation is 1.2 inches on September 19.
Average Monthly Rainfall in September in Aspen
As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day snowfall during September in Aspen is increasing, starting the month at 0.1 inches, when it rarely exceeds 0.2 inches or falls below -0.0 inches, and ending the month at 1.4 inches, when it rarely exceeds 4.0 inches or falls below -0.0 inches.
Average Monthly Snowfall in September in Aspen
Over the course of September in Aspen, 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, 36 seconds.
The shortest day of the month is September 30, with 11 hours, 49 minutes of daylight and the longest day is September 1, with 13 hours, 2 minutes of daylight.
Hours of Daylight and Twilight in September in Aspen
The earliest sunrise of the month in Aspen is 6:35 AM on September 1 and the latest sunrise is 26 minutes later at 7:02 AM on September 30.
The latest sunset is 7:38 PM on September 1 and the earliest sunset is 46 minutes earlier at 6:51 PM on September 30.
Daylight saving time is observed in Aspen during 2023, 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 5:41 AM and sets 14 hours, 56 minutes later, at 8:36 PM, while on December 22, the shortest day of the year, it rises at 7:23 AM and sets 9 hours, 25 minutes later, at 4:48 PM.
Sunrise & Sunset with Twilight in September in Aspen
The figure below presents a compact representation of the sun's elevation (the angle of the sun above the horizon) and azimuth (its compass bearing) for every hour of every day in the reporting period. The horizontal axis is the day of the year and the vertical axis is the hour of the day. For a given day and hour of that day, the background color indicates the azimuth of the sun at that moment. The black isolines are contours of constant solar elevation.
Solar Elevation and Azimuth in September in Aspen
The figure below presents a compact representation of key lunar data for September 2023. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases. The label associated with each bar indicates the date and time that the phase is obtained, and the companion time labels indicate the rise and set times of the Moon for the nearest time interval in which the moon is above the horizon.
Moon Rise, Set & Phases in September in Aspen
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 Aspen is essentially constant during September, remaining around 0% throughout.
Humidity Comfort Levels in September in Aspen
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 Aspen is gradually increasing during September, increasing from 4.3 miles per hour to 5.0 miles per hour over the course of the month.
For reference, on April 11, the windiest day of the year, the daily average wind speed is 6.2 miles per hour, while on August 1, the calmest day of the year, the daily average wind speed is 4.0 miles per hour.
Average Wind Speed in September in Aspen
The hourly average wind direction in Aspen throughout September is predominantly from the west, with a peak proportion of 55% on September 30.
Wind Direction in September in Aspen
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
The growing season in Aspen typically lasts for 2.8 months (86 days), from around June 17 to around September 11, rarely starting before May 28 or after July 9, and rarely ending before August 24 or after September 28.
During September in Aspen, the chance that a given day is within the growing season is very rapidly decreasing falling from 76% to 7% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in September in Aspen
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
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days in Aspen are gradually increasing during September, increasing by 126°F, from 886°F to 1,012°F, over the course of the month.
Growing Degree Days in September in Aspen
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 Aspen is gradually decreasing during September, falling by 0.9 kWh, from 6.2 kWh to 5.3 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in September in Aspen
For the purposes of this report, the geographical coordinates of Aspen are 39.191 deg latitude, -106.818 deg longitude, and 8,904 ft elevation.
The topography within 2 miles of Aspen contains large variations in elevation, with a maximum elevation change of 2,808 feet and an average elevation above sea level of 8,629 feet. Within 10 miles contains large variations in elevation (6,437 feet). Within 50 miles also contains extreme variations in elevation (8,950 feet).
The area within 2 miles of Aspen is covered by trees (68%) and shrubs (30%), within 10 miles by trees (73%) and shrubs (13%), and within 50 miles by trees (58%) and shrubs (22%).
This report illustrates the typical weather in Aspen, 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 Aspen.
For each station, the records are corrected for the elevation difference between that station and Aspen 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 Aspen is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Aspen and a given station.
The stations contributing to this reconstruction are:
To get a sense of how much these sources agree with each other, you can view a comparison of Aspen and the stations that contribute to our estimates of its temperature history and climate. Please note that each source's contribution is adjusted for elevation and the relative change present in the MERRA-2 data.
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Algorithms 2nd Edition , 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 airports and weather stations are provided by AskGeo.com .
Maps are © OpenStreetMap contributors.
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.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.
Please review our full terms contained on our Terms of Service page.