Climate averages 1991–2020
Union City weather
in December.
A cool, moderately wet month: the coolest and the 10th driest of the year.
Long-term climate patterns, not a forecast for a particular trip.
December is a cool month in Union City: daytime highs average 14.0°C and nights cool to about 6.3°C. By daytime temperature it is the coolest month of the year.
Expect about 83 mm of rain on roughly 9 days, which makes December a moderately wet month (the 10th driest of the 12 months). Daylight, from sunrise to sunset, lasts about 9 h 37 min on an average December day.
The nearest other city in this atlas is Fremont (6 km away). In December its highs average 14.0°C, about as warm as Union City, with 83 mm of rain.
Among the 498 cities listed in the United States, Union City ranks joint 163rd of 498 for the warmest December highs and joint 287th of 498 for the driest December (rainfall, lowest first). The mean December high of those 498 cities is 10.5°C; Union City is 3.5°C warmer than that mean.
From December to January, average highs rise by 0.3°C to 14.3°C, and monthly rainfall rises from 83 mm to 91 mm.
The annual picture
December 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
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, selected | 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
December in nearby cities
- Fremont in DecemberUnited States · 6 kmHigh 14.0°C · Low 6.5°C · 83 mm rain
- Hayward in DecemberUnited States · 10 kmHigh 13.8°C · Low 6.2°C · 81 mm rain
- Pleasanton in DecemberUnited States · 15 kmHigh 13.9°C · Low 5.8°C · 79 mm rain
- San Ramon in DecemberUnited States · 21 kmHigh 13.6°C · Low 5.6°C · 81 mm rain
- Milpitas in DecemberUnited States · 21 kmHigh 14.5°C · Low 6.8°C · 83 mm rain
- Redwood City in DecemberUnited States · 23 kmHigh 13.8°C · Low 7.5°C · 94 mm rain
- Mountain View in DecemberUnited States · 24 kmHigh 14.2°C · Low 7.1°C · 83 mm rain
- Livermore in DecemberUnited States · 24 kmHigh 13.6°C · Low 5.5°C · 79 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 Union City) is explained once on the Union City year overview and on the Sources, method and limits page.
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.