September Weather in Fairfax, United States

Last updated: June 24, 2025

In September, the average temperature in Fairfax, United States hovers around 18°C (65°F), but brace yourself for a range between a chilly 1°C (35°F) and a warm 33°C (92°F). Expect 118 mm (4.6 in) of precipitation spread over 9 days, contributing to an average humidity of 81%, creating a refreshingly damp atmosphere.

September Temperature in Fairfax

In September in Fairfax, the average temperature is 18°C (65°F), with maximums that can reach 33°C (92°F) and minimums that can dip as low as 1°C (35°F). This makes it a great time to explore, so be sure to pack for both warm days and cooler evenings!

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

The chart illustrates September temperature patterns in Fairfax, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Fairfax, September typically has an average temperature of 18°C (65°F), with highs reaching up to 33°C (92°F) and lows dropping to around 1°C (35°F).

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

September Precipitation in Fairfax

In Fairfax in September, the precipitation amount is 118 mm (4.6 in) over the course of 9 days.

September Precipitation Trends in Fairfax

The chart displays monthly precipitation patterns in Fairfax, 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 Fairfax, September typically receives around 118 mm (4.6 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 Fairfax

In Fairfax in September, humidity is 81%. With an average temperature of 18°C (65°F), this humidity feels like a thick blanket wrapping around you, making every step feel heavier and the air thick and sultry—a challenging environment for traveling.

September Average Humidity in Fairfax

This chart illustrates the average monthly humidity levels in Fairfax, 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 Fairfax is approximately 81%, 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 Fairfax

In Fairfax 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 Fairfax.

Maximum UV Index Levels in Fairfax for September

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

Data source: Fairfax 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 Fairfax Compares to Other Months

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

This table compares monthly weather conditions in Fairfax, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
	Temperature	Precipitation	Humidity	UV
January Weather	-9°C (16°F)	26 mm (1.0 inches)	87%	moderate
February Weather	-9°C (16°F)	27 mm (1.1 inches)	90%	moderate
March Weather	0°C (33°F)	62 mm (2.5 inches)	85%	moderate
April Weather	6°C (43°F)	80 mm (3.1 inches)	84%	very high
May Weather	15°C (60°F)	124 mm (4.9 inches)	78%	very high
June Weather	22°C (72°F)	106 mm (4.2 inches)	80%	very high
July Weather	23°C (74°F)	123 mm (4.8 inches)	81%	very high
August Weather	21°C (71°F)	124 mm (4.9 inches)	81%	very high
September Weather	18°C (65°F)	118 mm (4.6 inches)	81%	very high
October Weather	9°C (48°F)	97 mm (3.8 inches)	84%	moderate
November Weather	1°C (34°F)	30 mm (1.2 inches)	88%	moderate
December Weather	-4°C (24°F)	40 mm (1.6 inches)	84%	low