Climate averages 1991–2020
Sunnyvale weather
in September.
A warm, very dry month: the 3rd warmest and the 3rd driest of the year. Meets the pleasant-weather rule
Long-term climate patterns, not a forecast for a particular trip.
September is a warm month in Sunnyvale: daytime highs average 26.7°C and nights cool to about 15.2°C. By daytime temperature it is the 3rd warmest of the 12 months.
Expect about 2.5 mm of rain on fewer than one day, which makes September a very dry month (the 3rd driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 28 min on an average September day.
The nearest other city in this atlas is Mountain View (5 km away). In September its highs average 26.1°C, 0.6°C cooler than Sunnyvale, with 1.9 mm of rain.
Among the 498 cities listed in the United States, Sunnyvale ranks joint 250th of 498 for the warmest September highs and joint 36th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; Sunnyvale is 0.2°C cooler than that mean.
From September to October, average highs fall by 3.5°C to 23.3°C, and monthly rainfall rises from 2.5 mm to 30 mm.
The annual picture
September in context
Monthly rainfall
Average total · mmAverages from hourly ERA5 reanalysis data. Actual weather varies from year to year. How these numbers are calculated
Every month, side by side
Sunnyvale climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 14.3°C6.2°C | 6.2°C | 129 | 9.1 | 9.910 h |
| February | 15.3°C7.2°C | 7.2°C | 135 | 9.5 | 10.811 h |
| March | 16.9°C8.4°C | 8.4°C | 100 | 9.3 | 11.912 h |
| April | 18.6°C9.2°C | 9.2°C | 46 | 5.6 | 13.113 h |
| May | 21.3°C10.8°C | 10.8°C | 22 | 3.4 | 14.214 h |
| June | 24.6°C12.7°C | 12.7°C | 3.9 | 1.0 | 14.715 h |
| July | 26.9°C14.5°C | 14.5°C | 0.8 | 0.1 | 14.514 h |
| August | 27.1°C15.0°C | 15.0°C | 1.0 | 0.2 | 13.614 h |
| September, selected | 26.7°C15.2°C | 15.2°C | 2.5 | 0.5 | 12.512 h |
| October | 23.3°C13.3°C | 13.3°C | 30 | 3.0 | 11.311 h |
| November | 17.6°C9.6°C | 9.6°C | 62 | 6.0 | 10.210 h |
| December | 14.0°C6.6°C | 6.6°C | 119 | 9.2 | 9.610 h |
Rainfall is the average of the 30 separate monthly totals. A rain day is a local calendar day with at least 1 mm of precipitation. Daylight is the calculated sunrise-to-sunset length, not measured sunshine.
Compare places
September in nearby cities
- Mountain View in SeptemberUnited States · 5 kmHigh 26.1°C · Low 15.0°C · 1.9 mm rain
- San Jose in SeptemberUnited States · 13 kmHigh 27.8°C · Low 15.6°C · 2.5 mm rain
- Milpitas in SeptemberUnited States · 13 kmHigh 28.0°C · Low 16.0°C · 1.9 mm rain
- Fremont in SeptemberUnited States · 20 kmHigh 27.2°C · Low 15.4°C · 1.9 mm rain
- Redwood City in SeptemberUnited States · 22 kmHigh 24.6°C · Low 14.5°C · 1.8 mm rain
- Union City in SeptemberUnited States · 25 kmHigh 27.4°C · Low 15.1°C · 1.9 mm rain
- San Mateo in SeptemberUnited States · 33 kmHigh 24.0°C · Low 14.2°C · 1.8 mm rain
- Hayward in SeptemberUnited States · 34 kmHigh 26.5°C · Low 14.3°C · 1.7 mm rain
Read the numbers correctly
Climate, not a promise.
These patterns help compare months. They cannot tell you the weather on a specific day of your trip.
Sources, method and limitsHow these numbers are made (what the highs, lows and rain days mean, the sources and the model cell used for Sunnyvale) is explained once on the Sunnyvale year overview and on the Sources, method and limits page.
San Jose lies in the same rainfall cell, so its rainfall figures are identical to these; temperatures differ because they come from smaller 0.1° cells.