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September Weather in Redwood City, United States

Last updated: June 24, 2025

In September, the average temperature in Redwood 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 Redwood City

In September in Redwood 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 Redwood City.

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

The chart illustrates September temperature patterns in Redwood City, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Redwood 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 Redwood City 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 Redwood City

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

September Precipitation Trends in Redwood City

The chart displays monthly precipitation patterns in Redwood 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 Redwood 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.
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September Humidity in Redwood City

In Redwood 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 Redwood City

This chart illustrates the average monthly humidity levels in Redwood 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 Redwood 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 Redwood City

In Redwood 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 Redwood City.

Maximum UV Index Levels in Redwood City for September

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

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

This table compares monthly weather conditions in Redwood 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

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