Union City Weather

May is Union City's peak month, and the numbers make a strong case for it. May 2025 posted a comfort score of 90 out of 100, average highs of 70.2°F, average lows of 54°F, 13.3 hours of sunshine per day, and zero rainfall for the entire month. That last part is worth emphasizing: zero inches. The city was dry, warm, and bright through all of May. July and August run close behind, consistently scoring 88-89 across both years of data, with highs in the mid-70s and enough sunshine to make afternoons feel genuinely summer-like. What sets May apart is that it combines the warmth of summer with slightly longer days and no marine fog to speak of. April is also excellent, scoring 88 in 2025 with highs near 70°F and a mere 0.26 inches of rain. If you have flexibility, the late April through early July window is when Union City performs most reliably, offering the kind of weather that makes East Bay living feel like a permanent gift.

Union City runs comfortably warm in summer without tipping into the oppressive heat that hits the further inland East Bay cities. Average highs from June through August cluster in the low-to-mid 70s: June at 70.5°F, July at 74.7°F, August at 76.3°F. On the hottest heat wave days, Union City can push into the mid-to-upper 80s, but that's the exception. The city's position near the southern bay allows marine air to moderate temperatures, so even when Livermore and Brentwood are baking at 95°F-plus, Union City tends to stay 10 to 15 degrees cooler. Mornings are reliably mild, with lows sitting in the upper 50s to low 60s even at summer's peak. The comfort scores bear this out: July and August both averaged 88-89 out of 100 across the two-year dataset. Afternoon breezes off the bay contribute to this, making the heat feel manageable without crossing into uncomfortable wind territory. Summer here is warm enough to actually feel like summer, which is more than most of the coastal Bay Area can say.

Union City winters are mild by most standards but feel genuinely cool when you're used to the warm months. Average lows from December through February sit in the low-to-mid 40s, with December 2024 bottoming out around 40°F on colder nights. Daytime highs in the same window range from 57 to 60°F. Frost is rare and snow is essentially unheard of at 75 feet of elevation. The bigger discomfort in winter is rain, not cold. January 2025 brought 4.99 inches of rainfall, the wettest single month in the dataset, and paired with a comfort score of 75. November and December can each deliver 3 to 4.5 inches. That said, winter storms in the Bay Area tend to arrive in discrete systems and leave behind beautiful clear days. December 2024 actually managed a comfort score of 82 despite nearly an inch of rain, because the storms were brief and sunny spells dominated. A proper jacket and rain layer from November through February is the practical preparation. Heavy insulation is not necessary.

Union City follows the Bay Area's Mediterranean pattern almost perfectly: essentially no rain from late spring through early fall, then a wet season that runs from October through March. The peak wet months in the data are January 2025 (4.99 inches), February 2025 (4.68 inches), and November 2024 (4.44 inches). October is variable: 2024 brought 3.37 inches while 2025 came in at 4.37 inches, so by October you should start treating rain as a real possibility. The summer dry season is remarkably consistent: June through September averaged under 0.25 inches per month across both years, and May 2025 had none at all. The city logs roughly 77 rainy days per year, which leaves about 275 days that qualify as dry and comfortable. For practical planning, arriving between May and September means you're essentially guaranteed dry weather. The Bay Area's rainiest months follow the same pattern across the region, so Union City is predictable in this regard rather than quirky.

Less than you might expect for a Bay Area city. Union City averages about 1.8 hours of morning fog per day across the dataset, which is meaningfully lower than what you'd encounter in San Francisco, Daly City, or even parts of the Oakland hills. The marine layer that pours through the Golden Gate and blankets the western bay tends to thin considerably by the time it reaches Union City's position at the southern end. Morning fog that does form here typically burns off by 9 or 10 a.m. The foggiest months are November 2024 and January 2025, each around 2.1 hours per day, while summer months stay in the 1.5-2 hour range. June 2025 was the foggiest summer month at 2.6 hours, but still modest. The result is that Union City gets genuine sunshine: 9.8 hours per day on average, with May 2025 hitting a remarkable 13.3 hours. If you're escaping coastal fog, Union City is a reliable improvement. Why morning fog forms in the Bay Area helps explain why some East Bay cities see more of it than others.

Yes, substantially so during the months that matter most to visitors. Union City's summer average high of roughly 74°F compares favorably to San Francisco's typical July highs of 62-65°F. The difference is most stark in July and August, when Union City sits in clear sunshine at 74-76°F while much of San Francisco is wrapped in the marine layer. The reason is distance from the Pacific. San Francisco is positioned directly at the bay's entrance, fully exposed to cold ocean air. Union City sits at the bay's southern end, giving incoming marine air enough overland travel to warm up. The Bay Area's microclimate system creates these 10 to 15 degree differences across surprisingly short distances. In winter the gap narrows, since both cities see similar cool, wet conditions, though Union City still tends to run a degree or two warmer on average. For anyone visiting the Bay Area specifically to feel actual warmth, Union City consistently delivers what the coast cannot.

Union City sits between Hayward to the north and Fremont to the south, and weather-wise it occupies a similar range. All three cities share the same basic advantage over the coast: enough inland distance to shed the worst fog while still benefiting from bay breezes that keep summer from getting brutal. Fremont tends to run slightly warmer, especially further inland near the foothills, and its average comfort score of 85 edges Union City's 84. Hayward is comparable, though its lower elevation neighborhoods near the bay shoreline can feel cooler and breezier on windy afternoons. Coyote Hills Regional Park to the northwest of Union City sits right on the bay margin and catches more wind than any of the surrounding towns. Garin Regional Park in the Hayward hills to the northeast tends to be clearer but slightly cooler due to elevation. Union City's position makes it a reasonable middle ground: warmer than the coast, not as warm as inland valleys, and usually calm enough for comfortable outdoor activity.

For a large stretch of the year, Union City evenings are genuinely comfortable for outdoor dining. From May through October, temperatures after sunset settle into the low-to-mid 60s, and the wind off the bay is rarely aggressive enough to spoil a patio dinner. July and August evenings typically hold in the upper 50s to low 60s by 8 p.m., which is cooler than, say, Sacramento but warm enough with a light jacket. September is particularly good: 2025's September averaged a comfort score of 87 with highs near 70°F and only 1.57 inches of rain over the entire month. April and May evenings cool quickly but stay manageable if you bring a layer. November through February is where outdoor dining becomes a gamble: evenings drop into the mid-to-upper 40s and rain is possible any night. A fleece or light jacket handles May through October evenings reliably. Heavier layers become necessary once November arrives. The practical window for relaxed outdoor dining in Union City is roughly late April through early October.

Union City is one of the more forgiving Bay Area destinations for dressing. The marine fog is modest, the winds are manageable, and the temperature swings are real but not dramatic. In summer, a short-sleeved shirt handles afternoons comfortably, but mornings still start in the upper 50s to low 60s, so a light fleece or long-sleeve layer is worth carrying. The daily temperature range from low to high is typically 15 to 18 degrees, which is narrower than what you'd manage on the coast. Spring and fall require a bit more thought: April mornings can start near 52°F and climb to nearly 70°F by mid-afternoon. A zip jacket you can remove and stow is the right call. For winter visits from November through February, a real jacket and rain gear are the main additions. Heavy coats are overkill, but a waterproof outer layer makes a meaningful difference when a storm rolls through. Sunscreen matters year-round because Union City sees close to 10 hours of sunshine per day on average, and UV exposure is real even when temperatures feel mild. Comfortable walking shoes are all you need for terrain at this elevation.

Union City's seasons are distinct but none of them are extreme. Spring (March through May) is the most dynamic, moving from cool and occasionally rainy in March to genuinely warm and dry by May. March 2025 averaged 10.6 hours of sunshine and only 0.92 inches of rain, which is a good month by any measure. By May, it's as good as it gets. Summer (June through September) is the dry season, reliably warm and sunny with comfort scores in the 86-89 range. Fog is present but brief, burning off most mornings before 10 a.m. Fall (October through November) is when the wet season begins to reassert itself. October can go either way: sometimes beautiful, sometimes wet. November reliably brings rain and comfort scores drop to the low 70s. Winter (December through February) is the rainy season, but not dramatically so. December 2024 managed a comfort score of 82 because storms were short and sunny spells were long. The Bay Area's four seasons don't match the standard calendar, and Union City's data shows exactly that: the best weather runs May through September, and winter is mild with intermittent rain rather than relentlessly gray.