September weather forecastSan Mateo, CA
September
Weather in September
Come September, San Mateo begins transitioning into autumn, yet maintaining the warmth characteristic of the summer months. September stands out as having one of the highest maximum monthly temperatures, equal to August, resting at 71.2°F (21.8°C). With summer winding down, changes in weather conditions are not drastic, but a subtle decrease signals an impending arrival of autumn. The combination of pleasant temperatures and ample daylight makes this month thoroughly enjoyable.Temperature
Temperatures reach their maximum during August and September, recording an average high-temperature of 71.2°F (21.8°C).Humidity
In September, the average relative humidity is 72%.Rainfall
Months with rainfall are January through May, September through December. September is the first month it rains. Throughout September, in the average 0.9 rainfall days, it receives 0.04" (1mm) of precipitation. In San Mateo, during the entire year, the rain falls for 32.6 days and collects up to 2.13" (54mm) of precipitation.Daylight
In San Mateo, the average length of the day in September is 12h and 25min.On the first day of September in San Mateo, 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 is 11h.UV index
The months with the highest UV index are June through September, with an average maximum UV index of 5. A UV Index value of 3 to 5 symbolizes a medium health hazard from unsafe exposure to UV radiation for the average person.Note: During September, 5 as the daily maximum UV index converts into the following advice:
Put precautions into practice - Protective measures against sun-induced damage are advised. Direct Sun exposure should be minimized especially during midday when radiation is most intense. To best shield against UV radiation, choose sun-safe garments and dependable sunglasses.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
Average temperature in September
San Mateo, CA
- Average high temperature in September: 71.2°F
The warmest months (with the highest average high temperature) are August and September (71.2°F).
The month with the lowest average high temperature is December (55.8°F).
- Average low temperature in September: 56.7°F
The month with the highest average low temperature is September (56.7°F).
The coldest month (with the lowest average low temperature) is January (46.9°F).
Average pressure in September
San Mateo, 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
San Mateo, CA
- Average wind speed in September: 5.9mph
The windiest month (with the highest average wind speed) is April (8mph).
The calmest months (with the lowest average wind speed) are August, September and October (5.9mph).
Average humidity in September
San Mateo, CA
- Average humidity in September: 72%
The months with the highest relative humidity are March and August (76%).
The months with the lowest relative humidity are October and November (70%).
Average rainfall in September
San Mateo, CA
- Average rainfall in September: 0.04"
The wettest month (with the highest rainfall) is February (0.55").
The driest months (with the least rainfall) are June, July and August (0").
Average rainfall days in September
San Mateo, CA
- Average rainfall days in September: 0.9 days
The month with the highest number of rainy days is March (6.8 days).
The month with the least rainy days is July (0.3 days).
Average daylight in September /
Average sunshine in September
San Mateo, 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
San Mateo, CA
- Average sunshine days in September: 28.7 days
The month with the most sunshine days is July (30.4 days).
The month with the least sunshine days is February (23.3 days).
Average UV index in September
San Mateo, 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 and December (UV index 3).
Average cloud cover in September
San Mateo, CA
- Average cloud cover in September: 13
The month with the most cloud cover is March (Cloud cover 35).
The month with the least cloud cover is July (Cloud cover 11).
Average visibility in September
San Mateo, 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).
