Union City (CA) Temperature by Month
Union City, California, United States of America has an average annual maximum temperature of 20°C (68°F), with moderate seasonal shifts ranging from 15°C (59°F) in December to 25°C (77°F) in September. Below you'll find a full monthly breakdown and a comparison with cities worldwide.
Union City Monthly Temperatures
The weather in Union City changes moderately throughout the year, offering enough variation to appreciate each season. Nights are cooler, with lows ranging from 14°C (57°F) to 6°C (43°F).
The chart below illustrates the average maximum day and minimum night temperatures in Union City by month:
Low temperatures are most often recorded between 4 AM and 6 AM, while highs typically occur around 3 PM. September, the city's warmest month, sees 270 hours of sunshine.
The chart below shows the average temperature throughout the year:
Temperature: Union City vs the United States of America
The map below shows the annual temperature across the United States of America. You can also select individual months if you want to compare a specific time of year.
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Union City vs World: Temperature Compared
Union City's average annual maximum temperature is 20°C (68°F). To put that in context, here's how it compares to a few well-known destinations:
Rome, Italy averages 20°C (68°F) annually, with reliably warm summers and comfortable winters.
Zermatt, Switzerland averages just 4°C (39°F) annually due to its altitude, with very cold winters and cool summers even at its warmest.
San Francisco, USA averages 19°C (66°F) annually, but with little seasonal variation — summers are often cool and foggy, winters mild.
Brisbane, Australia averages 26°C (79°F) a year, with warm winters and hot, humid summers.
What Does the Temperature Feel Like in Union City?
Temperature alone doesn't tell the whole story — humidity plays a big role in how warm or cold it actually feels. High humidity in summer makes the heat feel more intense, particularly once temperatures climb above 25°C. In winter, the same humidity can make cold air feel sharper than the thermometer suggests.
In Union City, December is the coolest month, with average highs of 15°C (59°F) and humidity around 78% — considered high. In September, the warmest month, temperatures average 25°C (77°F) with 73% humidity — conditions that feel high. For a full picture, see our humidity page.
Climate temperature data is typically calculated as a 30-year average. This smooths out year-to-year variability and gives a more reliable picture of what a place is actually like, rather than what happened in any single unusual year.
The readings come from a range of sources — land-based weather stations, ocean buoys, ships, and satellites. That data is collected by weather services around the world, then pooled, quality-checked, and averaged to produce the climate records you see here.
For cities and regions with significant elevation, altitude is one of the biggest factors shaping local temperatures. As a rule of thumb, temperatures fall by around 6°C for every 1,000 metres gained — so a city at 2,000 metres will typically be around 12°C cooler than a city at sea level in the same region. Higher ground also tends to see more dramatic day-to-night temperature swings, since thinner air loses heat faster after sunset.
For more on Union City's weather — including monthly rainfall, sunshine hours, and humidity — visit our Union City climate page.
