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September Weather in Fullerton, United States

Last updated: June 24, 2025

In Fullerton, United States, the average temperature in September hovers around 20°C (69°F), with a minimum reaching 4°C (40°F) and a maximum soaring to 35°C (96°F). Expect about 57 mm (2.2 in) of precipitation spread over 7 days, along with an average humidity of 77%. Perfect weather for enjoying both the warmth and occasional showers!

September Temperature in Fullerton

In September in Fullerton, the average temperature is 20°C (69°F). Maximum temperatures can reach 35°C (96°F), while minimum temperatures can drop as low as 4°C (40°F). Be prepared for a range of weather conditions when traveling during this month in Fullerton.

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

The chart illustrates September temperature patterns in Fullerton, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Fullerton, September typically has an average temperature of 20°C (69°F), with highs reaching up to 35°C (96°F) and lows dropping to around 4°C (40°F).
JanFebMarAprMayJunJulAugSepOctNovDec252540496175777469514030
Data source: Temperature data for Fullerton 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 Fullerton

In Fullerton in September, the precipitation amount is 57 mm (2.2 in) over the course of 7 days.

September Precipitation Trends in Fullerton

The chart displays monthly precipitation patterns in Fullerton, 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 Fullerton, September typically receives around 57 mm (2.2 inches) of precipitation spread over approximately 7 days.
JanFebMarAprMayJunJulAugSepOctNovDec2920895714297658457832242
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Fullerton

In Fullerton in September, humidity is 77%. With an average temperature of 20°C (69°F), this humidity feels like a sultry embrace, making travel comfortable yet slightly sticky, as the moist air clings to your skin with every step you take.

September Average Humidity in Fullerton

This chart illustrates the average monthly humidity levels in Fullerton, 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 Fullerton is approximately 77%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec73%81%69%71%80%83%84%84%77%72%80%79%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Fullerton

In Fullerton 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 Fullerton.

Maximum UV Index Levels in Fullerton for September

The chart shows the monthly maximum UV Index (UVI) for Fullerton 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 Fullerton typically reaches 8, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec3569101110108643
Data source: Fullerton 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 Fullerton Compares to Other Months

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

This table compares monthly weather conditions in Fullerton, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather-4°C (26°F)28 mm (1.1 inches)73%moderate
February Weather-4°C (25°F)20 mm (0.8 inches)81%moderate
March Weather4°C (40°F)89 mm (3.5 inches)69%high
April Weather9°C (49°F)57 mm (2.2 inches)71%very high
May Weather16°C (61°F)142 mm (5.6 inches)80%very high
June Weather24°C (75°F)97 mm (3.8 inches)83%extreme
July Weather25°C (78°F)65 mm (2.5 inches)84%very high
August Weather23°C (74°F)84 mm (3.3 inches)84%very high
September Weather20°C (69°F)57 mm (2.2 inches)77%very high
October Weather11°C (52°F)83 mm (3.3 inches)72%high
November Weather4°C (40°F)22 mm (0.9 inches)80%moderate
December Weather-1°C (31°F)42 mm (1.6 inches)79%moderate

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