September Weather in Silverthorne Colorado, United States

Daily high temperatures decrease by 9°F, from 67°F to 58°F, rarely falling below 47°F or exceeding 74°F.

Daily low temperatures decrease by 8°F, from 43°F to 34°F, rarely falling below 27°F or exceeding 48°F.

For reference, on July 15, the hottest day of the year, temperatures in Silverthorne typically range from 47°F to 72°F, while on January 28, the coldest day of the year, they range from 10°F to 28°F.

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.

Sauze d'Oulx, Italy (5,268 miles away) is the far-away foreign place with temperatures most similar to Silverthorne (view comparison).

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The month of September in Silverthorne experiences gradually decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 33% to 27%. The lowest chance of overcast or mostly cloudy conditions is 26% on September 18.

The clearest day of the month is September 18, with clear, mostly clear, or partly cloudy conditions 74% of the time.

For reference, on March 4, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 49%, while on September 18, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 74%.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Silverthorne, the chance of a wet day over the course of September is rapidly decreasing, starting the month at 25% and ending it at 18%.

For reference, the year's highest daily chance of a wet day is 33% on July 23, and its lowest chance is 12% on January 28.

Over the course of September in Silverthorne, the chance of a day with only rain decreases from 25% to 15%, 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 0% throughout.

Rainfall

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 Silverthorne is gradually decreasing, starting the month at 1.1 inches, when it rarely exceeds 1.8 inches or falls below 0.4 inches, and ending the month at 0.8 inches, when it rarely exceeds 1.6 inches or falls below 0.2 inches.

Snowfall

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 Silverthorne 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 1.2 inches, when it rarely exceeds 3.8 inches or falls below -0.0 inches.

Over the course of September in Silverthorne, 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, 14 minutes, implying an average daily decrease of 2 minutes, 33 seconds, and weekly decrease of 17 minutes, 53 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, 2 minutes of daylight.

The earliest sunrise of the month in Silverthorne is 6:32 AM on September 1 and the latest sunrise is 27 minutes later at 6:59 AM on September 30.

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

Daylight saving time is observed in Silverthorne during 2025, 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 5:36 AM and sets 14 hours, 59 minutes later, at 8:35 PM, while on December 21, the shortest day of the year, it rises at 7:21 AM and sets 9 hours, 22 minutes later, at 4:43 PM.

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.

The figure below presents a compact representation of key lunar data for September 2025. 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.

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

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 Silverthorne is gradually increasing during September, increasing from 4.1 miles per hour to 5.0 miles per hour over the course of the month.

For reference, on January 13, the windiest day of the year, the daily average wind speed is 7.1 miles per hour, while on August 1, the calmest day of the year, the daily average wind speed is 3.7 miles per hour.

The hourly average wind direction in Silverthorne throughout September is predominantly from the west, with a peak proportion of 66% on September 30.

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 Silverthorne typically lasts for 3.4 months (105 days), from around June 5 to around September 19, rarely starting before May 18 or after June 24, and rarely ending before September 3 or after October 5.

During September in Silverthorne, the chance that a given day is within the growing season is very rapidly decreasing falling from 93% to 19% over the course of the month.

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 Silverthorne are gradually increasing during September, increasing by 118°F, from 821°F to 939°F, over the course of the month.

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 Silverthorne is gradually decreasing during September, falling by 0.8 kWh, from 6.1 kWh to 5.3 kWh, over the course of the month.

For the purposes of this report, the geographical coordinates of Silverthorne are 39.632 deg latitude, -106.074 deg longitude, and 8,753 ft elevation.

The topography within 2 miles of Silverthorne contains very significant variations in elevation, with a maximum elevation change of 1,332 feet and an average elevation above sea level of 9,109 feet. Within 10 miles contains very significant variations in elevation (5,043 feet). Within 50 miles also contains extreme variations in elevation (9,035 feet).

The area within 2 miles of Silverthorne is covered by shrubs (50%) and trees (41%), within 10 miles by trees (69%) and snow and glaciers (18%), and within 50 miles by trees (58%) and shrubs (20%).