September weather forecast
Portola Valley, CA

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Weather in September

September, the first month of the autumn in Portola Valley, is also an agreeable month, with an average temperature ranging between min 56.7°F (13.7°C) and max 72.5°F (22.5°C).

Temperature

In September, the average high-temperature registers at a still pleasant 72.5°F (22.5°C), nearly identical to August's 73°F (22.8°C). An average nighttime temperature of 56.7°F (13.7°C) is noted in Portola Valley, California, during the month of September.

Humidity

In September, the average relative humidity is 71%.

Rainfall

In September, in Portola Valley, the rain falls for 2.4 days. Throughout September, 0.08" (2mm) of precipitation is accumulated. In Portola Valley, during the entire year, the rain falls for 57.5 days and collects up to 4.76" (121mm) of precipitation.

Daylight

In Portola Valley, the average length of the day in September is 12h and 25min.
On the first day of September in Portola Valley, sunrise is at 6:40 am and sunset at 7:37 pm. On the last day of the month, sunrise is at 7:04 am and sunset at 6:53 pm PDT.

Sunshine

The average sunshine in September in Portola Valley is 11h.

UV index

June through September, with an average maximum UV index of 5, are months with the highest UV index. A UV Index value of 3 to 5 symbolizes a moderate health risk from exposure to the Sun's UV rays for the ordinary person.
Note: The UV index of 5 during September translates into the following guidance:
Take measures. Individuals with light skin might experience burns in under 30 minutes. Direct exposure to the Sun during its peak hours around mid-day should be minimized for safety. A hat with a broad brim is indispensable, filtering out up to half of UV rays.

Frequently asked questions

What are the temperatures in Portola Valley in September?

The high-temperature in Portola Valley during September is 72.5°F (22.5°C) and the low-temperature is 56.7°F (13.7°C).

What is the average humidity in September in Portola Valley?

In September, the average relative humidity in Portola Valley, California, is 71%.

How many days does it rain in September in Portola Valley?

In Portola Valley, during September, the rain falls for 2.4 days and regularly aggregates up to 0.08" (2mm) of precipitation.

How many hours of daylight does Portola Valley have in September?

In Portola Valley, California, the average day length in September is 12h and 25min.

When are sunrise and sunset in September in Portola Valley?

On the first day of September, sunrise is at 6:40 am and sunset at 7:37 pm. On the last day of the month, sunrise is at 7:04 am and sunset at 6:53 pm PDT.

How long on average sun shines in September in Portola Valley?

The average sunshine duration in September in Portola Valley, California, is 11h.

What are the months with the highest UV index in Portola Valley?

The months with the highest UV index are June through September, with an average maximum UV index of 5.

How high is the UV index in September in Portola Valley?

The average daily maximum UV index in September is 5. A UV Index of 3 to 5 symbolizes a medium health risk from unsafe 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
Portola Valley, CA

Average temperature in September - Portola Valley, CA
  • Average high temperature in September: 72.5°F

The warmest month (with the highest average high temperature) is August (73°F).
The month with the lowest average high temperature is December (55.8°F).

  • Average low temperature in September: 56.7°F

The months with the highest average low temperature are August and September (56.7°F).
The coldest months (with the lowest average low temperature) are January and February (46.4°F).

Average pressure in September
Portola Valley, CA

Average pressure in September - Portola Valley, CA
  • Average pressure in September: 29.92"Hg

The month with the highest atmospheric pressure is January (30.13"Hg).
The month with the lowest atmospheric pressure is September (29.92"Hg).

Average wind speed in September
Portola Valley, CA

Average wind speed in September - Portola Valley, CA
  • Average wind speed in September: 5.7mph

The windiest month (with the highest average wind speed) is April (7.6mph).
The calmest month (with the lowest average wind speed) is August (5.5mph).

Average humidity in September
Portola Valley, CA

Average humidity in September - Portola Valley, CA
  • Average humidity in September: 71%

The month with the highest relative humidity is March (76%).
The month with the lowest relative humidity is October (68%).

Average rainfall in September
Portola Valley, CA

Average rainfall in September - Portola Valley, CA
  • Average rainfall in September: 0.08"

The wettest month (with the highest rainfall) is February (0.98").
The driest month (with the least rainfall) is June (0.04").

Average rainfall days in September
Portola Valley, CA

Average rainfall days in September - Portola Valley, CA
  • Average rainfall days in September: 2.4 days

The month with the highest number of rainy days is March (10.6 days).
The month with the least rainy days is June (0.9 days).

Average daylight in September /
Average sunshine in September
Portola Valley, CA

Average daylight in September / Average sunshine in September - Portola Valley, 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 0min

The month with the most sunshine is July (Average sunshine: 12h and 0min).
The month with the least sunshine is December (Average sunshine: 5h and 54min).

Average sunshine days in September
Portola Valley, CA

Average sunshine days in September - Portola Valley, CA
  • Average sunshine days in September: 25.8 days

The months with the most sunshine days are June and July (26.3 days).
The month with the least sunshine days is March (18.3 days).

Average UV index in September
Portola Valley, CA

Average UV index in September - Portola Valley, CA
  • Average UV index in September: 5

The months with the highest UV index are June, July, August and September (UV index 5).
The months with the lowest UV index are February, November and December (UV index 3).

Average cloud cover in September
Portola Valley, CA

Average cloud cover in September - Portola Valley, CA
  • Average cloud cover in September: 12

The month with the most cloud cover is March (Cloud cover 35).
The month with the least cloud cover is July (Cloud cover 9).

Average visibility in September
Portola Valley, CA

Average visibility in September - Portola Valley, 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).

Portola Valley, California, USA

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