If you’re packing for a trip to the heart of Silicon Valley, you might glance at a weather app and think you’ve got it all figured out. But the weather San Jose in Celsius tells a different story than the generic "sunny California" trope you see in movies. It’s a city of microclimates, where moving just 10 kilometers can change the air temperature by five degrees.
San Jose sits in a unique topographical bowl. It’s shielded by the Santa Cruz Mountains to the west and the Diablo Range to the east. This creates a "rain shadow" effect, making San Jose significantly drier and warmer than its famous neighbor, San Francisco. While people in SF are shivering in the summer fog, San Jose is often basking in actual heat.
The Real Numbers: A Month-by-Month Breakdown
Most of the world thinks in Celsius, so let’s drop the Fahrenheit confusion. On average, the annual mean temperature in San Jose is about 16°C. But averages are deceptive.
In the dead of winter—usually January—you’re looking at daily highs around 15°C to 16°C. It’s crisp. It’s sweater weather. At night, it can dip down to 6°C or 7°C. You won't see snow here; the last time it really "snowed" in the valley was 1976, though you'll occasionally see a dusting on the peaks of Mount Hamilton.
By the time April rolls around, the city starts to glow. Highs climb to 20°C or 21°C. This is arguably the best time to visit. The hills are still green from the winter rains before the summer sun turns them into "California Gold" (which is just a fancy term for dried-up grass).
Summer is where things get interesting. July and August see average highs of 27°C to 29°C. However, heatwaves are becoming more frequent. It's not uncommon to hit 35°C or even 38°C during a September spike. September is actually often the hottest month here because the marine layer—that thick Pacific fog—finally gives up and retreats.
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Why the Rain Shadow Matters
San Jose is technically a "Mediterranean climate," but it almost qualifies as semi-arid. Why? Because the Santa Cruz Mountains act like a giant wall. As wet air comes off the Pacific, it hits those mountains, rises, cools, and dumps all its rain on the redwoods in places like Boulder Creek.
By the time that air reaches downtown San Jose, it’s dry.
This is why San Jose only gets about 380mm to 400mm of rain per year. For context, that’s less than half of what some coastal towns get just 30 kilometers away. Most of this rain falls between November and March. If you’re visiting in July, the chance of rain is basically zero. You have a better chance of seeing a self-driving car than a rain cloud in mid-August.
Understanding the San Jose Microclimates
Don't assume the temperature at Mineta San Jose International Airport (SJC) is what you'll feel everywhere.
- North San Jose / Alviso: Since it’s right on the edge of the San Francisco Bay, it stays cooler. The bay breeze acts like natural air conditioning. If it's 28°C downtown, it might only be 24°C near the water.
- South San Jose / Almaden Valley: This area is further from the bay and closer to the foothills. It traps heat. On a summer afternoon, expect it to be 2-3 degrees warmer here than in the city center.
- The Hills: If you drive up to the Lick Observatory on Mount Hamilton, the temperature drops fast. It’s about 1,280 meters up. You can easily see a 10-degree difference from the valley floor.
What to Actually Wear
Packing for the weather San Jose in Celsius requires a strategy locals call "The Onion." Layers are everything.
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Even on a day that hits 30°C, the temperature will plummet the moment the sun goes down. Because the humidity is low—usually hovering around 40% to 50% in the afternoon—the air doesn't hold onto heat. You can go from wearing a T-shirt at 5:00 PM to needing a denim jacket or a light puffer by 8:00 PM.
Honestly, the "dry heat" thing is real. 28°C in San Jose feels a lot more comfortable than 28°C in New York or London because you aren't swimming in your own sweat. But the sun is intense. Since San Jose has about 300 sunny days a year, the UV index is frequently high.
Surprising Weather Facts
Most people don't realize that San Jose is one of the windiest cities in the Bay Area during the spring. In April and May, "onshore flow" kicks in. As the Central Valley (to the east) heats up, it sucks in cool air from the ocean. This air tunnels through the Golden Gate and down the Santa Clara Valley.
You’ll get these afternoon gusts that reach 20 to 30 km/h. It’s great for clearing out smog, but it’s annoying if you’re trying to play tennis or wear a hat.
Also, the "Diablo Winds" are a thing in the autumn. These are hot, dry winds that blow from the inland deserts toward the coast. They usually happen in October. This is when the fire risk is highest. The sky can turn a weird, hazy orange, and temperatures can suddenly jump to 32°C even though it's nearly November.
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How to Plan Your Trip Around the Climate
If you want the "perfect" San Jose experience, aim for late May or early October.
In May, the days are long, the wind hasn't gotten too crazy yet, and the temperatures are a steady 23°C to 25°C. In October, you get "Indian Summer." The crowds at Santana Row or the SAP Center are manageable, and the evenings are incredibly mild.
Avoid February if you hate gray skies. While it's not freezing, it's the "wettest" month, and the dampness makes the 14°C high feel a lot colder than it actually is.
Quick Temperature Reference
To keep it simple, here is a rough guide to what you'll encounter:
- Winter (Dec-Feb): Highs of 14°C - 16°C, Lows of 6°C - 8°C. Bring a coat.
- Spring (Mar-May): Highs of 18°C - 23°C, Lows of 9°C - 12°C. Perfect for hiking.
- Summer (Jun-Aug): Highs of 26°C - 29°C, Lows of 13°C - 15°C. Very dry.
- Fall (Sep-Nov): Highs of 20°C - 27°C, Lows of 10°C - 14°C. Watch for heat spikes in Sept.
Actionable Next Steps
If you are headed to San Jose soon, don't just check the 10-day forecast once. Check the "RealFeel" or "Apparent Temperature" which accounts for the wind and sun. Always carry a light layer, even in the summer, because the temperature drop after sunset is sudden and significant. If you plan on visiting the Santa Cruz Mountains for wine tasting, expect it to be significantly cooler and potentially foggier than downtown. Stick to the shade during the peak sun hours of 1:00 PM to 4:00 PM to avoid the strongest UV exposure.