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September Weather in O'Fallon, United States

Last updated: June 24, 2025

In September, the average temperature in O'Fallon, United States is a pleasant 22°C (72°F). However, be prepared for a temperature range that can drop to a cool 7°C (45°F) at night or soar to a warm 35°C (95°F) during the day. Expect 56 mm (2.2 in) of precipitation over 6 days, contributing to an average humidity of 76%.

September Temperature in O'Fallon

In September in O'Fallon, the average temperature is 22°C (72°F). The maximum can reach 35°C (95°F), while the minimum can drop to as low as 7°C (45°F). This diverse temperature range makes it a perfect time for travelers to enjoy both warm days and cooler evenings while exploring the sights.

September Weather in O'Fallon: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in O'Fallon, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In O'Fallon, September typically has an average temperature of 22°C (72°F), with highs reaching up to 35°C (95°F) and lows dropping to around 7°C (45°F).
JanFebMarAprMayJunJulAugSepOctNovDec323546556776797672594439
Data source: Temperature data for O'Fallon 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 O'Fallon

In O'Fallon in September, the precipitation amount is 56 mm (2.2 in) spread over 6 days.

September Precipitation Trends in O'Fallon

The chart displays monthly precipitation patterns in O'Fallon, 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 O'Fallon, September typically receives around 56 mm (2.2 inches) of precipitation spread over approximately 6 days.
JanFebMarAprMayJunJulAugSepOctNovDec72781281211458497139561106852
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in O'Fallon

In O'Fallon in September humidity is 76%. With an average temperature of 22°C (72°F), this humidity feels like a warm embrace, making travel slightly uncomfortable as you navigate through the thick, saturated air.

September Average Humidity in O'Fallon

This chart illustrates the average monthly humidity levels in O'Fallon, 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 O'Fallon is approximately 76%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec82%80%80%80%82%83%81%80%76%76%85%86%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in O'Fallon

In O'Fallon 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 O'Fallon.

Maximum UV Index Levels in O'Fallon for September

The chart shows the monthly maximum UV Index (UVI) for O'Fallon 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 O'Fallon typically reaches 9, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec3569101111109653
Data source: O'Fallon 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 O'Fallon Compares to Other Months

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

This table compares monthly weather conditions in O'Fallon, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather0°C (32°F)72 mm (2.9 inches)82%moderate
February Weather2°C (35°F)78 mm (3.1 inches)80%moderate
March Weather8°C (47°F)128 mm (5.0 inches)80%high
April Weather13°C (55°F)121 mm (4.8 inches)80%very high
May Weather19°C (67°F)145 mm (5.7 inches)82%very high
June Weather25°C (76°F)84 mm (3.3 inches)83%extreme
July Weather26°C (80°F)97 mm (3.8 inches)81%extreme
August Weather24°C (76°F)139 mm (5.5 inches)80%very high
September Weather22°C (72°F)56 mm (2.2 inches)76%very high
October Weather15°C (59°F)110 mm (4.3 inches)76%high
November Weather7°C (44°F)68 mm (2.7 inches)85%moderate
December Weather4°C (39°F)52 mm (2.1 inches)86%moderate

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