September Weather in Grass Valley, United States

Last updated: June 24, 2025

In Grass Valley, United States, the average temperature in September hovers around 21°C (70°F). However, be prepared for a range as it can dip to a brisk 5°C (41°F) and soar to a sizzling 41°C (106°F). Expect an average precipitation of 23 mm (0.9 in) over just 2 days, complemented by an average humidity of 44%.

September Temperature in Grass Valley

In September in Grass Valley, the average temperature is 21°C (70°F). Maximums can reach 41°C (106°F), and minimums can drop to as low as 5°C (41°F). Be sure to pack a range of clothing to accommodate the varying temperatures during your travels.

September Weather in Grass Valley: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Grass Valley, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Grass Valley, September typically has an average temperature of 21°C (70°F), with highs reaching up to 41°C (106°F) and lows dropping to around 5°C (41°F).

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

September Precipitation in Grass Valley

In Grass Valley in September, the precipitation amount is 23 mm (0.9 in) over a span of 2 days.

September Precipitation Trends in Grass Valley

The chart displays monthly precipitation patterns in Grass Valley, 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 Grass Valley, September typically receives around 23 mm (0.9 inches) of precipitation spread over approximately 2 days.

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

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Millimeters (mm) Inches (in)

September Humidity in Grass Valley

In Grass Valley in September, humidity is 44%. With an average temperature of 21°C (70°F), this humidity feels like a refreshing embrace, making it a comfortable environment for exploring and enjoying outdoor activities without feeling overheated.

September Average Humidity in Grass Valley

This chart illustrates the average monthly humidity levels in Grass Valley, 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 Grass Valley is approximately 44%, 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 Grass Valley

In Grass Valley 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 Grass Valley.

Maximum UV Index Levels in Grass Valley for September

The chart shows the monthly maximum UV Index (UVI) for Grass Valley 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 Grass Valley typically reaches 9, which falls under the very high exposure category.

Data source: Grass Valley 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 Grass Valley Compares to Other Months

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

This table compares monthly weather conditions in Grass Valley, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
	Temperature	Precipitation	Humidity	UV
January Weather	7°C (45°F)	269 mm (10.6 inches)	84%	moderate
February Weather	8°C (46°F)	279 mm (11.0 inches)	84%	moderate
March Weather	9°C (48°F)	267 mm (10.5 inches)	83%	high
April Weather	13°C (55°F)	129 mm (5.1 inches)	81%	very high
May Weather	17°C (62°F)	76 mm (3.0 inches)	74%	very high
June Weather	22°C (72°F)	13 mm (0.5 inches)	64%	extreme
July Weather	26°C (79°F)	1 mm (0.0 inches)	44%	extreme
August Weather	25°C (77°F)	2 mm (0.1 inches)	36%	very high
September Weather	21°C (70°F)	23 mm (0.9 inches)	44%	very high
October Weather	15°C (60°F)	78 mm (3.1 inches)	65%	high
November Weather	10°C (51°F)	157 mm (6.2 inches)	84%	moderate
December Weather	7°C (44°F)	215 mm (8.4 inches)	83%	moderate