You’re driving down I-880, and suddenly the sky changes. One minute it’s that classic East Bay gold, and the next, you’re hitting a wall of gray mist near the Alvarado district. That is the weather in Union City California in a nutshell. It’s a bit of a chameleon. While people often lump the whole Bay Area into one "foggy" bucket, Union City plays by its own set of rules because of where it sits—wedged between the bay and the hills.
Honestly, it’s not just "sunny" or "rainy." It’s specific.
The Microclimate Game: Why It’s Never Just One Temp
Union City is basically a weather crossroads. You've got the cool, salty air pushing in from the San Francisco Bay to the west and the dry, inland heat trying to sneak over the hills from the Tri-Valley area to the east.
This creates a weird, localized tug-of-war.
On a typical July afternoon, you might see 78°F near the Union Landing shopping center, while just a few miles up toward the hills near Dry Creek Pioneer Regional Park, it feels five degrees warmer. The hills act like a giant radiator. They soak up the sun and block those brisk bay breezes from cooling down the eastern neighborhoods as quickly as the flats.
Summer Isn't What You Think
If you’re coming from the East Coast or the Midwest, you’re probably expecting a sweltering July.
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Nope.
In Union City, the hottest months are actually August and September. It’s that "Indian Summer" vibe everyone talks about in Northern California. Average highs hover around 79°F, but it's an arid, bone-dry heat. You won't be dripping in sweat the second you walk outside because the humidity usually stays low—around 60% or less.
But here’s the kicker: the wind.
June is actually the windiest month here. You'll get these steady gusts around 13-14 mph coming off the water. It’s enough to make a 75-degree day feel like you need a light hoodie if you're standing in the shade. Locals know the "Bay Area Layer" isn't a suggestion; it's a survival strategy.
The Wet Season and the 2026 Outlook
Winter in Union City is short, wet, and kinda moody.
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January 2026 has already shown us some teeth. Earlier this month, we saw temperatures dipping into the low 40s at night, with a few days struggling to get past 56°F. Most of the city's 18 to 20 inches of annual rain falls between December and March.
- January & February: These are the "soakers." You get these atmospheric rivers that dump inches of rain in 48 hours.
- The Flood Factor: If you live near the Alameda Creek, you know the drill. The city actually sets up a 24/7 sandbag station at the Corporation Yard (34650 7th Street) during these months.
- Frost? Rare, but it happens. We usually see a handful of nights where the mercury hits 35°F, but it’s never going to snow. If you see white on the ground, it’s either hail or you’ve driven too far up toward Mount Hamilton.
Surviving the "Grey-vember" Transitions
November is actually the calmest month in terms of wind. It’s that eerie, still time of year where the hills turn from toasted brown to a vibrant, electric green. It happens almost overnight once the first real rain hits.
But don't let the calm fool you.
This is when the Tule fog starts. It’s thick. It’s "can't-see-your-own-hood-ornament" thick. It rolls in from the Central Valley and settles in the low-lying parts of Union City, making the morning commute on the Decoto Road exit a total gamble.
Practical Steps for Dealing with Union City Weather
Forget the "Ultimate Guide" fluff. If you live here or you're just passing through, here is the ground-level reality of how to handle the weather in Union City California without losing your mind.
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1. Watch the AQI in the Fall
Because of the way Union City sits in a bowl against the hills, smoke from North Bay or Central Valley wildfires can get trapped here. Even if the fire is 100 miles away, the air quality can tank in hours. Keep an eye on the Spare the Air alerts during September and October.
2. The 4:00 PM Rule
In the summer, the temperature can drop 15 degrees in two hours. At 3:00 PM, you’re in shorts. By 5:30 PM, the bay breeze kicks in and you’re shivering. Always keep a denim jacket or a windbreaker in the trunk of your car.
3. Home Maintenance is Real
Because we get those heavy, concentrated bursts of rain in the winter, clogged gutters are the #1 cause of property damage here. Clear them in October. If you wait until the first big storm in December, you’ll be competing with every other person at the Home Depot on Whipple Road for a ladder and a pair of gloves.
4. Best Time to Visit?
If you want the absolute best "Chamber of Commerce" weather, aim for May or June. The hills are still green, the sky is a deep blue, and the temperature sits in that sweet spot of 68°F to 72°F. It’s perfect for hitting the trails at Masonic Home or just walking around the Union Landing area without melting or freezing.
Basically, Union City weather is predictable once you understand the geography. It's a mix of bay influence and hill protection. Stay layered, watch the wind, and enjoy those September sunsets—they're easily some of the best in the East Bay.
To stay ahead of the next big shift, make sure your home's drainage is clear before the November transition hits, and keep a backup power bank ready for those occasional wind-related outages in the hills.