September weather forecast
Sunnyvale, CA

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The perfect blue sky, trees, buildings, and cute water fountain all reflected in the calm pool of water with birds in Sunnyvale, California
The perfect blue sky, trees, buildings, and cute water fountain all reflected in the calm pool of water with birds in Sunnyvale, California

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

The transition into September in Sunnyvale marks the average high-temperature at a still moderately hot 77.9°F (25.5°C), differing minimally from August's 79.3°F (26.3°C). In September, the temperature registers a significant dip to an average of 55.9°F (13.3°C) at night.

Humidity

September and October, with an average relative humidity of 63%, are the least humid months.

Rainfall

In Sunnyvale, California, 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 September, the average length of the day 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, in Sunnyvale, sunrise is at 7:03 am and sunset at 6:52 pm PDT.

Sunshine

In September, the average sunshine in Sunnyvale is 11.1h.

UV index

In September, the average daily maximum UV index in Sunnyvale is 5. A UV Index estimate of 3 to 5 represents a medium vulnerability from exposure to the Sun's UV radiation for the average person.
Note: An average maximum UV index of 5 in September leads to these recommendations:
Take necessary precautions - Defending against sun damage is advocated. The midday Sun can be dangerous due to its intense radiation; always find shade and reduce exposure. For a sun-safe wardrobe, go for clothes that are tightly woven and generously sized.
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Frequently asked questions

What are September temperatures in Sunnyvale?

The high-temperature in Sunnyvale during September is 77.9°F (25.5°C) and the low-temperature is 55.9°F (13.3°C).

What is the least humid month in Sunnyvale?

September and October, with an average relative humidity of 63%, are the least humid months in Sunnyvale, California.

What is the average humidity in September in Sunnyvale?

In Sunnyvale, the average relative humidity in September is 63%.

How many days does it rain in September in Sunnyvale?

In Sunnyvale, in September, it is raining for 2.2 days, with typically 0.08" (2mm) of accumulated precipitation.

How many hours of daylight does Sunnyvale have in September?

The average day length in September in Sunnyvale is 12h and 25min.

When are sunrise and sunset in September in Sunnyvale?

On the first day of September in Sunnyvale, California, sunrise is at 6:39 am and sunset at 7:36 pm. On the last day of the month, sunrise is at 7:03 am and sunset at 6:52 pm PDT.

How many hours of sunshine does Sunnyvale have in September?

In September, the average sunshine duration is 11.1h.

How high is the UV index in September in Sunnyvale?

In September, the average daily maximum UV index in Sunnyvale is 5. A UV Index value of 3 to 5 symbolizes a moderate health threat from exposure to Sun's UV rays.
Average humidity
Average rainfall days
Average daylight
Average sunshine
Average sunshine days
Average UV index
Average cloud cover
Average visibility

Average temperature in September
Sunnyvale, CA

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).

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Average pressure in September
Sunnyvale, CA

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).

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Average wind speed in September
Sunnyvale, CA

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).

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Average humidity in September
Sunnyvale, CA

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%).

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Average rainfall in September
Sunnyvale, CA

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").

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Average rainfall days in September
Sunnyvale, CA

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).

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Average daylight in September /
Average sunshine in September
Sunnyvale, CA

Average daylight in September / Average sunshine in September - Sunnyvale, CA
  • Average daylight in September: 12h and 2min

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 1min

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).

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Average sunshine days in September
Sunnyvale, CA

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).

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Average UV index in September
Sunnyvale, CA

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).

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Average cloud cover in September
Sunnyvale, CA

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).

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Average visibility in September
Sunnyvale, CA

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).

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Sunnyvale, California, USA

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