September weather forecastPortola Valley, CA
September
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.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
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: 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: 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: 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: 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: 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: 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 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: 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: 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: 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: 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).
