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September Weather in Boulder, United States

Last updated: June 24, 2025

In Boulder, United States, the average temperature in September is a mild 12°C (54°F). However, be prepared for the extremes: temperatures can dip as low as -7°C (19°F) or soar to 32°C (91°F). Expect to experience around 43 mm (1.7 in) of precipitation over 7 days, with an average humidity of 68% that adds to the atmospheric charm of this location.

September Temperature in Boulder

In September in Boulder, the average temperature is 12°C (54°F). Maximums can reach 32°C (91°F), and minimums can drop as low as -7°C (19°F). Be prepared for a wide range of temperatures when traveling during this month.

September Weather in Boulder: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Boulder, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Boulder, September typically has an average temperature of 12°C (54°F), with highs reaching up to 32°C (91°F) and lows dropping to around -7°C (19°F).
JanFebMarAprMayJunJulAugSepOctNovDec181528344657656354382718
Data source: Temperature data for Boulder generated using information from the Copernicus Climate Change Service.
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The average of daily temperatures (24 hours) calculated across all days in the month.

September Precipitation in Boulder

In Boulder in September, the total precipitation amount is 43 mm (1.7 in) spread over a period of 7 days.

September Precipitation Trends in Boulder

The chart displays monthly precipitation patterns in Boulder, with values for each month alongside September’s data, which is highlighted for easy comparison. It shows the amount of precipitation, providing insights into how September’s conditions compare to those of other months. In Boulder, September typically receives around 43 mm (1.7 inches) of precipitation spread over approximately 7 days.
JanFebMarAprMayJunJulAugSepOctNovDec2356406892112443943443432
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Boulder

In Boulder in September, humidity is 68%. With an average temperature of 12°C (54°F), this humidity feels like a damp chill that can make exploring the area a bit uncomfortable, often requiring warm layers to stay cozy while sightseeing.

September Average Humidity in Boulder

This chart illustrates the average monthly humidity levels in Boulder, with September’s data highlighted for easy comparison. It shows the fluctuation in humidity throughout the year, helping you understand how conditions vary by season. In September, average humidity in Boulder is approximately 68%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec87%88%83%78%76%78%77%71%68%72%84%91%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Boulder

In Boulder in September, the maximum UV index is 8, which corresponds to a very high Exposure Category. This means that the time to burn is only 15 minutes. For more detailed information, you can refer to the Hourly UV Index in Boulder.

Maximum UV Index Levels in Boulder for September

The chart shows the monthly maximum UV Index (UVI) for Boulder in September, representing the highest UV radiation level. The UVI measures the intensity of ultraviolet radiation from the sun, indicating the potential risk of sunburn and UV-related health issues. In September, the maximum UV index in Boulder typically reaches 8, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec34689111098532
Data source: Boulder September UVI data generated using information from the Copernicus Climate Change Service.

UV Risk Categories

  •  Extreme (11+): Avoid the sun, stay in shade.
  •  Very High (8-10): Limit sun exposure.
  •  High (6-7): Use SPF 30+ and protective clothing.
  •  Moderate (3-5): Midday shade recommended.
  •  Low (0-2): No protection needed.

How September Weather in Boulder Compares to Other Months

Weather in Boulder varies notably across the year, with each month offering distinct climate conditions. This page compares September’s weather to other months in Boulder, focusing on differences in temperature, rainfall, humidity, and UV levels.

This table compares monthly weather conditions in Boulder, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather-8°C (18°F)23 mm (0.9 inches)87%moderate
February Weather-9°C (16°F)55 mm (2.2 inches)88%moderate
March Weather-2°C (28°F)40 mm (1.6 inches)83%high
April Weather1°C (34°F)68 mm (2.7 inches)78%very high
May Weather8°C (46°F)92 mm (3.6 inches)76%very high
June Weather14°C (57°F)112 mm (4.4 inches)78%extreme
July Weather18°C (65°F)44 mm (1.7 inches)77%very high
August Weather17°C (64°F)39 mm (1.5 inches)71%very high
September Weather12°C (54°F)43 mm (1.7 inches)68%very high
October Weather3°C (38°F)44 mm (1.7 inches)72%moderate
November Weather-3°C (28°F)34 mm (1.3 inches)84%moderate
December Weather-8°C (19°F)32 mm (1.3 inches)90%low

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