September Weather in Keystone, United States

Last updated: June 24, 2025

In September, the average temperature in Keystone, United States is a pleasant 20°C (67°F). However, be prepared for some fluctuations, as temperatures can dip to a minimum of 3°C (37°F) and soar up to a maximum of 33°C (92°F). With an average precipitation of 121 mm (4.8 in) spread over 9 days, it’s wise to carry an umbrella. Additionally, expect a humidity level of 78%, creating a warm, somewhat muggy atmosphere. Stay comfortable and enjoy your time!

September Temperature in Keystone

In September in Keystone, the average temperature is 20°C (67°F). Maximums can reach 33°C (92°F), while minimums can drop as low as 3°C (37°F). This pleasant climate makes Keystone an ideal destination for travelers seeking a mix of warmth and cool relief during their journey.

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

The chart illustrates September temperature patterns in Keystone, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Keystone, September typically has an average temperature of 20°C (67°F), with highs reaching up to 33°C (92°F) and lows dropping to around 3°C (37°F).

Data source: Temperature data for Keystone generated using information from the Copernicus Climate Change Service.

September Precipitation in Keystone

In Keystone in September, the precipitation amount is 121 mm (4.8 in) over the course of 9 days.

September Precipitation Trends in Keystone

The chart displays monthly precipitation patterns in Keystone, 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 Keystone, September typically receives around 121 mm (4.8 inches) of precipitation spread over approximately 9 days.

Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.

Choose your preferred precipitation unit

Millimeters (mm) Inches (in)

September Humidity in Keystone

In Keystone in September humidity is 78%. With an average temperature of 20°C (67°F), this humidity feels like walking through a warm, thick blanket, making travel a bit sticky and requiring frequent breaks to find shade and cool refreshments.

September Average Humidity in Keystone

This chart illustrates the average monthly humidity levels in Keystone, 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 Keystone is approximately 78%, offering insight into how humid the weather typically is during this month.

Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Keystone

In Keystone 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 Keystone.

Maximum UV Index Levels in Keystone for September

The chart shows the monthly maximum UV Index (UVI) for Keystone 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 Keystone typically reaches 8, which falls under the very high exposure category.

Data source: Keystone 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 Keystone Compares to Other Months

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

This table compares monthly weather conditions in Keystone, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
	Temperature	Precipitation	Humidity	UV
January Weather	-6°C (21°F)	45 mm (1.8 inches)	81%	moderate
February Weather	-5°C (23°F)	52 mm (2.1 inches)	81%	moderate
March Weather	3°C (38°F)	73 mm (2.9 inches)	81%	high
April Weather	9°C (48°F)	85 mm (3.3 inches)	82%	very high
May Weather	16°C (62°F)	162 mm (6.4 inches)	78%	very high
June Weather	23°C (73°F)	143 mm (5.6 inches)	81%	extreme
July Weather	24°C (76°F)	91 mm (3.6 inches)	79%	very high
August Weather	22°C (72°F)	64 mm (2.5 inches)	77%	very high
September Weather	20°C (67°F)	121 mm (4.8 inches)	78%	very high
October Weather	11°C (52°F)	143 mm (5.6 inches)	78%	high
November Weather	3°C (38°F)	45 mm (1.8 inches)	87%	moderate
December Weather	-1°C (30°F)	40 mm (1.6 inches)	84%	moderate