nomadseason

September Weather in Foster City, United States

Last updated: June 24, 2025

In September, the average temperature in Foster City, United States is a pleasant 18°C (66°F), making it an ideal time to explore the area. Expect temperatures to range from a cool 10°C (50°F) to a warm 39°C (103°F). With only 2 mm (0.1 in) of precipitation over 0 days, you'll likely enjoy clear skies. The average humidity sits at a comfortable 55%, ensuring a delightful atmosphere for any outdoor activities.

September Temperature in Foster City

In September in Foster City, the average temperature is 18°C (66°F), with maximums reaching up to 39°C (103°F) and minimums dropping as low as 10°C (50°F). Be prepared for warm days and cooler nights when planning your travel to Foster City.

September Weather in Foster City: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Foster City, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Foster City, September typically has an average temperature of 18°C (66°F), with highs reaching up to 39°C (103°F) and lows dropping to around 10°C (50°F).
JanFebMarAprMayJunJulAugSepOctNovDec525354565863636565625652
Data source: Temperature data for Foster City generated using information from the Copernicus Climate Change Service.
Choose your preferred temperature unit
The average of daily temperatures (24 hours) calculated across all days in the month.

September Precipitation in Foster City

In Foster City in September, the precipitation amount is 2 mm (0.1 in) during a span of 0 days.

September Precipitation Trends in Foster City

The chart displays monthly precipitation patterns in Foster City, 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 Foster City, September typically receives around 2 mm (0.1 inches) of precipitation spread over approximately 0 days.
JanFebMarAprMayJunJulAugSepOctNovDec1051008633242012244788
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
Choose your preferred precipitation unit

September Humidity in Foster City

In Foster City in September humidity is 55%. With an average temperature of 18°C (66°F), this humidity feels like a refreshing embrace, making for comfortable travels as long as you stay hydrated and enjoy the mild weather.

September Average Humidity in Foster City

This chart illustrates the average monthly humidity levels in Foster City, 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 Foster City is approximately 55%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec82%84%82%78%71%64%59%54%55%62%78%80%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Foster City

In Foster City 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 Foster City.

Maximum UV Index Levels in Foster City for September

The chart shows the monthly maximum UV Index (UVI) for Foster City 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 Foster City typically reaches 9, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec3589101111109743
Data source: Foster City 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 Foster City Compares to Other Months

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

This table compares monthly weather conditions in Foster City, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather11°C (52°F)105 mm (4.1 inches)82%moderate
February Weather11°C (53°F)100 mm (3.9 inches)84%moderate
March Weather12°C (54°F)86 mm (3.4 inches)82%very high
April Weather13°C (57°F)33 mm (1.3 inches)78%very high
May Weather15°C (59°F)24 mm (0.9 inches)71%very high
June Weather17°C (63°F)2 mm (0.1 inches)64%extreme
July Weather17°C (63°F)0 mm (0.0 inches)58%extreme
August Weather18°C (65°F)0 mm (0.0 inches)54%very high
September Weather18°C (66°F)2 mm (0.1 inches)55%very high
October Weather17°C (63°F)24 mm (0.9 inches)62%high
November Weather13°C (56°F)47 mm (1.9 inches)78%moderate
December Weather11°C (52°F)88 mm (3.5 inches)80%moderate

Compare Foster City's Climate with Cities Worldwide