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September Weather in Longmont, United States

Last updated: June 24, 2025

The average temperature in Longmont, United States during September is a comfortable 19°C (66°F). However, temperatures can vary significantly, with a minimum of 0°C (32°F) and a maximum of 37°C (98°F). Expect 27 mm (1.1 in) of precipitation across 5 days, and enjoy an average humidity level of 63%.

September Temperature in Longmont

In September in Longmont, the average temperature is 19°C (66°F). Maximum temperatures can reach 37°C (98°F), while minimum temperatures can drop as low as 0°C (32°F). It’s advisable to pack accordingly to enjoy the diverse climate during your travels.

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

The chart illustrates September temperature patterns in Longmont, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Longmont, September typically has an average temperature of 19°C (66°F), with highs reaching up to 37°C (98°F) and lows dropping to around 0°C (32°F).
JanFebMarAprMayJunJulAugSepOctNovDec303040485668747266494033
Data source: Temperature data for Longmont 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 Longmont

In Longmont in September, the precipitation amount is 27 mm (1.1 in) over a period of 5 days.

September Precipitation Trends in Longmont

The chart displays monthly precipitation patterns in Longmont, 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 Longmont, September typically receives around 27 mm (1.1 inches) of precipitation spread over approximately 5 days.
JanFebMarAprMayJunJulAugSepOctNovDec1926565511942314327382315
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Longmont

In Longmont in September, humidity is 63%. With an average temperature of 19°C (66°F), this humidity feels like a warm embrace, making it pleasantly comfortable for exploring the area's sights while infusing the air with a refreshing vitality that's ideal for travel.

September Average Humidity in Longmont

This chart illustrates the average monthly humidity levels in Longmont, 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 Longmont is approximately 63%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec84%86%75%67%67%68%62%63%63%72%80%85%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Longmont

In Longmont in September, the maximum UV index is 9, which corresponds to a very high Exposure Category. The time to burn is just 15 minutes, so it's essential to take precautions. For detailed information, you can check the Hourly UV Index in Longmont.

Maximum UV Index Levels in Longmont for September

The chart shows the monthly maximum UV Index (UVI) for Longmont 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 Longmont typically reaches 9, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec46811111212119643
Data source: Longmont 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 Longmont Compares to Other Months

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

This table compares monthly weather conditions in Longmont, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather-1°C (30°F)19 mm (0.8 inches)84%moderate
February Weather-1°C (30°F)26 mm (1.0 inches)85%high
March Weather5°C (40°F)56 mm (2.2 inches)75%very high
April Weather9°C (48°F)55 mm (2.2 inches)67%extreme
May Weather13°C (56°F)119 mm (4.7 inches)67%extreme
June Weather20°C (69°F)42 mm (1.6 inches)68%extreme
July Weather23°C (75°F)31 mm (1.2 inches)62%extreme
August Weather22°C (73°F)43 mm (1.7 inches)63%extreme
September Weather19°C (66°F)27 mm (1.1 inches)63%very high
October Weather9°C (49°F)38 mm (1.5 inches)72%high
November Weather4°C (40°F)23 mm (0.9 inches)79%moderate
December Weather0°C (33°F)15 mm (0.6 inches)85%moderate

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