September weather forecastSunnyvale, CA
September
Weather in September
September welcomes a steady decline in temperature, with the average high temperature falling to 77.9°F (25.5°C). Despite the drop in temperature, the average daylight hours remain appreciable at 12.4, and rainfall remains minimal. The UV index and visibility stay consistent with that of August, indicating no significant change in sunshine impact. The transition towards fall is evident in the golden landscape and pleasant breeze in September. A seasonal balance provides Sunnyvale residents with the quintessential Californian autumn experience.Temperature
Sunnyvale experiences a transition to a still warm 77.9°F (25.5°C) in average high-temperatures with the arrival of September, not greatly differing from August's 79.3°F (26.3°C). Sunnyvale, amid September nights, sees a noteworthy disparity from daily highs with an average temperature of 55.9°F (13.3°C).Humidity
September and October, with an average relative humidity of 63%, are the least humid months.Rainfall
In Sunnyvale, in September, it is raining for 2.2 days, with typically 0.08" (2mm) of accumulated precipitation. Throughout the year, there are 50 rainfall days, and 3.9" (99mm) of precipitation is accumulated.Daylight
In Sunnyvale, the average length of the day in September is 12h and 25min.On the first day of the month, sunrise is at 6:39 am and sunset at 7:36 pm. On the last day of September, sunrise is at 7:03 am and sunset at 6:52 pm PDT.
Sunshine
The average sunshine in September is 11.1h.UV index
The average daily maximum UV index in September is 5. A UV Index reading of 3 to 5 represents a moderate health risk from unsafe exposure to UV radiation for average individuals.Note: In September, the UV index of 5 transforms into the following advice:
Guard yourself - Precautionary measures against sun damage are advocated. The mid-day Sun emits the most powerful UV radiation, so seek shade and minimize exposure. Wearing a wide-brim hat can block approximately 50% of UV radiation, protecting the eyes.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
Average temperature in September
Sunnyvale, CA
- Average high temperature in September: 77.9°F
The warmest month (with the highest average high temperature) is July (79.9°F).
The month with the lowest average high temperature is December (56.8°F).
- Average low temperature in September: 55.9°F
The month with the highest average low temperature is August (56.8°F).
The coldest months (with the lowest average low temperature) are February and December (44.2°F).
Average pressure in September
Sunnyvale, CA
- Average pressure in September: 29.92"Hg
The month with the highest atmospheric pressure is January (30.13"Hg).
The months with the lowest atmospheric pressure are June and September (29.92"Hg).
Average wind speed in September
Sunnyvale, CA
- Average wind speed in September: 4.6mph
The windiest month (with the highest average wind speed) is April (6.2mph).
The calmest month (with the lowest average wind speed) is September (4.6mph).
Average humidity in September
Sunnyvale, CA
- Average humidity in September: 63%
The month with the highest relative humidity is March (73%).
The months with the lowest relative humidity are September and October (63%).
Average rainfall in September
Sunnyvale, CA
- Average rainfall in September: 0.08"
The wettest months (with the highest rainfall) are February and March (0.75").
The driest month (with the least rainfall) is July (0").
Average rainfall days in September
Sunnyvale, CA
- Average rainfall days in September: 2.2 days
The month with the highest number of rainy days is March (9.6 days).
The month with the least rainy days is August (0.4 days).
Average daylight in September /
Average sunshine in September
Sunnyvale, CA
- Average daylight in September: 12h and 24min
The month with the longest days is June (Average daylight: 14h and 42min).
The month with the shortest days is December (Average daylight: 9h and 36min).
- Average sunshine in September: 11h and 6min
The month with the most sunshine is July (Average sunshine: 12h and 6min).
The month with the least sunshine is December (Average sunshine: 5h and 54min).
Average sunshine days in September
Sunnyvale, CA
- Average sunshine days in September: 27.5 days
The month with the most sunshine days is August (30.1 days).
The month with the least sunshine days is March (19.9 days).
Average UV index in September
Sunnyvale, CA
- Average UV index in September: 5
The month with the highest UV index is June (UV index 6).
The months with the lowest UV index are February, November and December (UV index 3).
Average cloud cover in September
Sunnyvale, CA
- Average cloud cover in September: 10
The month with the most cloud cover is March (Cloud cover 35).
The month with the least cloud cover is July (Cloud cover 7).
Average visibility in September
Sunnyvale, CA
- Average visibility in September: 6mi
The months with the highest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
The months with the lowest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
