September weather forecast
Sonoma, CA

Come September, Sonoma experiences the onset of the transition from summer to fall. While the mercury continues to read high temperatures, there is a noticeable decrease from August. Outdoor experiences remain enjoyable owing to the significant duration of sunlight and relatively high daytime temperatures. Even though the month heralds the end of dry summer days with the return of minimal rainfall, the weather remains uniformly sunny and dry. September introduces a slightly increased level of humidity, signaling the change of seasons.

Temperature

September's advent in Sonoma, California, denotes a minor fluctuation in the average high-temperature to a still warm 81.1°F (27.3°C) from August's 82.9°F (28.3°C). Despite a significant deviation from daily highs, September sees an average nighttime temperature of 54.7°F (12.6°C) in Sonoma.

Heat index

For September, the heat index is calculated to be 82.4°F (28°C). If one sustains activity during exposure, it could lead to a sense of fatigue, with a chance of heat cramps.
Thinking about the heat index, one should account for its values in shaded areas with gentle breezes. The heat index has the potential to rise by 15 Fahrenheit (8 Celsius) degrees with direct sunshine exposure.
Note: The heat index, also known as 'feels like' or 'apparent temperature', determines the feeling of heat when you account for the relative humidity. An individual's temperature experience can be influenced by extra factors, including metabolic differences, activity levels, and clothing. With the sun shining directly, it's crucial to understand that it can increase the heat index by up to 15 Fahrenheit (8 Celsius) degrees. Heat index values are particularly relevant to babies and toddlers. Young children typically face greater hazards than adults because they do not sweat as much. Their larger skin surface in comparison to their little bodies and the high heat production from their active nature compounds their vulnerability.

Humidity

The least humid month is September, with an average relative humidity of 59%.

Rainfall

In Sonoma, in September, it is raining for 2.4 days, with typically 0.08" (2mm) of accumulated precipitation. Throughout the year, in Sonoma, there are 73.3 rainfall days, and 15.47" (393mm) of precipitation is accumulated.

Daylight

In September, the average length of the day in Sonoma is 12h and 25min.
On the first day of the month, sunrise is at 6:40 am and sunset at 7:39 pm. On the last day of September, in Sonoma, sunrise is at 7:05 am and sunset at 6:53 pm PDT.

Sunshine

In Sonoma, California, the average sunshine in September is 11h.

UV index

In September, the average daily maximum UV index in Sonoma is 5. A UV Index of 3 to 5 symbolizes a moderate threat to health from unsafe exposure to UV radiation for ordinary individuals.
Note: In September, a daily UV index of 5 turns into the following recommendations:
Take precautions. Light skin individuals may get burned in less than 30 minutes. The midday Sun can be harmful due to its potent UV radiation; seek shade and reduce exposure. Limit sun-induced ocular harm with sunglasses that feature UVA and UVB filtration.