Climate averages 1991–2020
Union City weather
by month.
See how the year changes. August has the highest average daily maximum; July has the lowest average monthly rainfall.
Pleasant-weather months by our simple rule: AprilMayJuneJulyAugustSeptemberOctober
Long-term climate patterns, not a forecast for a particular trip.
Across the year Union City averages a daytime high of 21.1°C and a nighttime low of 10.7°C, with about 478 mm of rain on roughly 55 days. August is the warmest month (average high 28.0°C) and December the coolest (14.0°C). February is the wettest month with 96 mm; July is the driest with under 1 mm.
Months with pleasant weather by a simple rule (average high between 18°C and 29°C, average low of at least 5°C and at most 8 rain days): April, May, June, July, August, September and October. This looks at temperature and rain only, not humidity, wind, sea temperature or crowds.
The annual picture
A year in Union City
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
Union City climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 14.3°C5.9°C | 5.9°C | 91 | 8.9 | 9.910 h |
| February | 15.5°C6.9°C | 6.9°C | 96 | 9.1 | 10.811 h |
| March | 17.2°C8.2°C | 8.2°C | 75 | 9.1 | 11.912 h |
| April | 19.1°C9.2°C | 9.2°C | 40 | 5.7 | 13.113 h |
| May | 22.1°C11.1°C | 11.1°C | 19 | 3.4 | 14.214 h |
| June | 25.6°C13.0°C | 13.0°C | 4.0 | 0.9 | 14.715 h |
| July | 27.9°C14.8°C | 14.8°C | 0.9 | 0.2 | 14.514 h |
| August | 28.0°C15.1°C | 15.1°C | 1.0 | 0.2 | 13.614 h |
| September | 27.4°C15.1°C | 15.1°C | 1.9 | 0.4 | 12.512 h |
| October | 23.7°C13.1°C | 13.1°C | 23 | 2.9 | 11.311 h |
| November | 17.8°C9.3°C | 9.3°C | 43 | 5.7 | 10.210 h |
| December | 14.0°C6.3°C | 6.3°C | 83 | 8.9 | 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
Nearby cities
- FremontUnited States · 6 kmHigh 20.9°C · Low 10.8°C · 478 mm rain a year
- HaywardUnited States · 10 kmHigh 20.5°C · Low 10.3°C · 455 mm rain a year
- PleasantonUnited States · 15 kmHigh 21.4°C · Low 10.2°C · 436 mm rain a year
- San RamonUnited States · 21 kmHigh 21.0°C · Low 10.0°C · 455 mm rain a year
- MilpitasUnited States · 21 kmHigh 21.5°C · Low 11.2°C · 478 mm rain a year
- Redwood CityUnited States · 23 kmHigh 19.3°C · Low 10.8°C · 523 mm rain a year
- Mountain ViewUnited States · 24 kmHigh 20.2°C · Low 10.8°C · 478 mm rain a year
- LivermoreUnited States · 24 kmHigh 21.6°C · Low 10.0°C · 436 mm rain a year
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 limitsWhat do the high and low temperatures mean?
We average the daily maximum and daily minimum for each calendar month over 1991–2020. They are not record extremes, a forecast range or temperatures at a fixed time of day.
How is a rain day counted?
A local calendar day (standard time, UTC-8, no daylight saving) with at least 1 mm of total precipitation counts as one rain day. We count these for each year and month, then average across 30 years. The average does not predict which days will be wet.
Where do the numbers come from?
ERA5-Land (temperature) and ERA5 (precipitation) reanalysis from the Copernicus Climate Change Service, read from the Open-Meteo open-data archive. Temperature comes from the 0.1° model cell at 37.60°, -122.00° (2 km from the city centre), adjusted up by 0.4°C for the difference between the city's elevation (19 m) and the cell's (86 m). Rain comes from the 0.25° cell at 37.50°, -122.00°. Fremont, Mountain View and Milpitas lie in the same rainfall cell, so their rainfall figures are identical to these; temperatures differ because they come from smaller 0.1° cells. These are model estimates, not weather-station readings; coast, hills and city streets can differ.
Does this page show sunshine, humidity or sea temperature?
No. The data used here contain air temperature and precipitation only. Daylight hours are calculated from the city's latitude (sunrise to sunset), not measured sunshine.