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September Weather in Grinnell, United States

Last updated: June 24, 2025

In September, the average temperature in Grinnell, United States is a pleasant 20°C (68°F). Expect temperatures to range from a cool 4°C (39°F) to a warm 33°C (92°F). With an average precipitation of 118 mm (4.7 in) over 9 days, you might want to keep an umbrella handy. Additionally, the average humidity hovers around a comfortable 73%, ensuring a refreshing atmosphere throughout the month.

September Temperature in Grinnell

In September in Grinnell, the average temperature is 20°C (68°F). Maximum temperatures can reach up to 33°C (92°F), while minimums can drop as low as 4°C (39°F). It's a great time for travelers to enjoy mild weather and plan outdoor activities.

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

The chart illustrates September temperature patterns in Grinnell, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Grinnell, September typically has an average temperature of 20°C (68°F), with highs reaching up to 33°C (92°F) and lows dropping to around 4°C (39°F).
JanFebMarAprMayJunJulAugSepOctNovDec222339496273767368523830
Data source: Temperature data for Grinnell 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 Grinnell

In Grinnell in September, the precipitation amount is 118 mm (4.7 in), occurring over a span of 9 days.

September Precipitation Trends in Grinnell

The chart displays monthly precipitation patterns in Grinnell, 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 Grinnell, September typically receives around 118 mm (4.7 inches) of precipitation spread over approximately 9 days.
JanFebMarAprMayJunJulAugSepOctNovDec4348798116613381721181454638
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Grinnell

In Grinnell in September, humidity is 73%. With an average temperature of 20°C (68°F), this humidity feels like a warm embrace, making outdoor activities somewhat sticky, yet the atmosphere remains pleasant for leisurely strolls and sightseeing.

September Average Humidity in Grinnell

This chart illustrates the average monthly humidity levels in Grinnell, 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 Grinnell is approximately 73%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec78%80%77%78%77%81%81%77%73%75%85%83%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Grinnell

In Grinnell 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 Grinnell.

Maximum UV Index Levels in Grinnell for September

The chart shows the monthly maximum UV Index (UVI) for Grinnell 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 Grinnell typically reaches 8, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec35689111098643
Data source: Grinnell 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 Grinnell Compares to Other Months

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

This table compares monthly weather conditions in Grinnell, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather-6°C (22°F)43 mm (1.7 inches)78%moderate
February Weather-5°C (23°F)48 mm (1.9 inches)80%moderate
March Weather4°C (39°F)79 mm (3.1 inches)77%high
April Weather9°C (49°F)81 mm (3.2 inches)77%very high
May Weather17°C (62°F)166 mm (6.5 inches)77%very high
June Weather23°C (74°F)133 mm (5.2 inches)81%extreme
July Weather24°C (76°F)81 mm (3.2 inches)81%very high
August Weather23°C (73°F)71 mm (2.8 inches)77%very high
September Weather20°C (68°F)118 mm (4.7 inches)73%very high
October Weather11°C (53°F)145 mm (5.7 inches)75%high
November Weather3°C (38°F)46 mm (1.8 inches)85%moderate
December Weather-1°C (31°F)38 mm (1.5 inches)83%moderate

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