You’re standing in downtown San Jose, it's 75 degrees, and you're thinking about driving up to the Lick Observatory. You figure a light hoodie is enough. Honestly? You’ve probably already made your first mistake.
Mt hamilton ca weather is a weird, fickle beast that doesn't care about the forecast in the valley. While the Santa Clara Valley stays predictable, the peak of Mt Hamilton—sitting at roughly 4,209 feet—is basically playing by its own set of rules. It’s one of the few places in the Bay Area where you can start your drive in shorts and end it in a legitimate snowstorm.
The Thermal Inversion Nobody Tells You About
Most people assume it gets colder as you go up. Usually, they're right. But Mt Hamilton is famous for "thermal inversions." This is where the cool air gets trapped in the valley, and the mountaintop actually ends up warmer than San Jose.
I’ve seen days in July where it’s a foggy 65 degrees at the base, but once you break through the cloud layer near the summit, it’s a crisp, sunny 80 degrees. It feels like you’ve climbed into a different state. That said, August remains the heavy hitter for heat, with record highs hitting 103°F in the past. If you’re visiting in late summer, the sun at that altitude will cook you faster than you think. Use the high-SPF stuff.
Snow in California? It Happens More Than You'd Think
If you’re looking for the "Winter Wonderland" version of the Bay Area, this is it. While the valley gets rain, the peak gets hit with some serious powder.
On average, Mt Hamilton gets about 18 inches of snow a year. Sometimes it's just a dusting that melts by noon. Other times? It’s a total lockout. Back in February 2001, the mountain got slammed with 30 inches. Even as recently as March 2006, a single night dumped over a foot of snow on the telescopes.
When the snow hits, Caltrans often shuts down State Route 130. Don't be that person who tries to "brave it" in a Prius without chains. The road is narrow, the drops are steep, and the ice doesn't play nice.
A Quick Look at the Current Setup
Right now, as of January 18, 2026, the mountain is being surprisingly chill. The current temperature is 57°F with a light 2 mph breeze coming from the east. It's mostly sunny, which is perfect for a drive, but don't get too comfortable.
The forecast for today shows a high of 66°F, but the low is dropping to 50°F tonight. If you’re staying for the stars, that 16-degree drop feels like a lot more when you’re standing on a concrete observation deck.
Planning for the Lick Observatory Visit
If you’re heading up for one of the public evening tours, the weather is your biggest hurdle. The astronomers need "photometric" skies—basically, zero clouds and low humidity—to do their best work.
- Check the Wind: If the wind speed is high (we’re talking 25+ mph), they might not even open the dome shutters. Current winds are a measly 4 mph, which is ideal.
- Humidity Matters: High humidity can cause condensation on the massive mirrors. Tonight’s humidity is around 41%, which is pretty safe for the equipment.
- The "Hour" Rule: It’s only 15 miles from San Jose, but the drive takes a full hour. The road is a relentless series of hairpins. If there’s even a hint of "difficult driving conditions" like fog or light sleet, double that time.
Why the Shoulders are the Best Time to Go
October and May are the "secret" months for mt hamilton ca weather. You avoid the summer heat inversions and the winter road closures. The air is often the clearest during these months, meaning you can actually see Yosemite to the east and the Pacific Ocean to the west on a single turn of the head.
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The National Weather Service has had a station up there since the Lick Observatory opened, and the data shows that May is when the mountain starts to dry out, though they've seen measurable snow as late as May in the past.
Basically, if you’re planning a trip, bring a heavy jacket even if you don't think you'll need it. The mountain doesn't care about your plans. It has its own.
Actionable Next Steps for Your Trip
- Check the Caltrans QuickMap: Specifically look for SR 130. If it says "closed," it means the snow or a landslide has taken over.
- Layer Like a Pro: Base layer for the sun, windbreaker for the peak. The wind at the summit is almost always 5–10 degrees colder than the thermometer says due to the wind chill.
- Gas Up in San Jose: There is zero fuel on the mountain. If you run low while idling in a snow delay, you’re in for a very long walk.
- Watch the Dew Point: If the temperature and dew point are within 3 degrees of each other, expect thick fog that will turn your one-hour drive into a terrifying two-hour crawl.