If you’re planning a trip to the Inland Empire or just moved into a new place near Euclid Avenue, you’ve probably heard the rumors. People love to say Southern California has "no seasons." Honestly, that’s just not true. Especially when it comes to the weather at Ontario California, things are a lot more nuanced than just "sunny and 75."
Ontario sits in a unique geographic pocket. To the north, you have the massive San Gabriel Mountains. To the west, the cooling influence of the Pacific Ocean is constantly trying to push through the Chino Hills. Because of this, Ontario deals with a Mediterranean climate that can feel like a desert one day and a coastal breeze-way the next.
The Reality of Summer in Ontario
Summer is no joke here. From late June through September, the thermometer regularly climbs into the 90s. It’s a dry heat, which locals will tell you is "better" than humidity, but 105°F is still 105°F.
In August, the average high hits about 94°F, but that’s just the average. You’ll definitely see stretches where it stays above 100°F for a week straight. It’s the kind of heat that makes the asphalt at Ontario International Airport shimmer and makes you regret wearing anything but linen. But here’s the thing: the humidity stays low, usually around 50% or less. This means as soon as the sun drops behind the mountains, the temperature actually plummets. You might be sweating at 4:00 PM and reaching for a light hoodie by 9:00 PM.
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Those Infamous Santa Ana Winds
You can't talk about weather at Ontario California without mentioning the Santa Anas. These aren't your typical breezes. They are hot, dry winds that blow in from the Great Basin and Mojave Desert, getting squeezed through mountain passes like the Cajon Pass.
When these hit, the air gets weirdly electric. The humidity can drop to single digits. Honestly, it’s a bit eerie. These winds are most common in the fall and early winter. While they bring "beautifully clear" skies because they blow all the smog out to sea, they also bring a massive fire risk. If you see a "Red Flag Warning" on the news, take it seriously. It means the brush is tinder-dry and the wind is ready to whip any spark into a nightmare.
Why the Wind Matters for Travelers
- Flight Delays: Strong gusts can occasionally mess with flight patterns at ONT.
- Air Quality: While the winds clear the air initially, they can also kick up dust or carry smoke from distant wildfires.
- Static Shock: Basically, expect to get zapped every time you touch a car door.
Winter is the Hidden Gem
If you hate the cold, you’ll love it here. "Winter" in Ontario is basically a mild spring for everyone else. January is usually the coldest month, but we’re talking highs of 67°F and lows of 45°F. You rarely see frost on your windshield.
Rain? We get some. February is typically the wettest month, averaging a little over 2 inches. But "wet" is relative. Ontario only gets about 13 inches of rain a year total. When it does rain, it usually comes in short, intense bursts rather than a week-long drizzle.
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One thing that surprises people is the "May Gray" and "June Gloom." Even though it’s heading into summer, the marine layer—that thick fog from the ocean—often rolls in during the morning. It makes the start of the day feel cool and overcast, but it usually burns off by noon. It’s nature’s air conditioning.
Planning Your Visit Based on the Sky
If you’re coming for a wedding or a convention, April and May are your best bets. The hills are actually green (briefly), the air is crisp, and the temperatures sit comfortably in the 70s.
October is also great, provided a Santa Ana event isn't happening. The "shoulder seasons" are where the weather at Ontario California really shines. You get the sun without the oppressive heat and the clear skies without the winter chill.
Actionable Weather Survival Tips
- The Layer Rule: Even in July, bring a light jacket. The desert-adjacent climate means big temperature swings between day and night.
- Hydration is Real: In the summer, you lose moisture faster than you realize because the sweat evaporates instantly. Drink way more water than you think you need.
- Check the AQI: Because Ontario is in a basin, air quality can sometimes be poor in the late afternoon during summer. If you have asthma, keep an eye on the Air Quality Index.
- Sunscreen Always: The SoCal sun is intense. Even on a "partly cloudy" day in March, you will get burned if you’re out at the Toyota Arena or walking around Ontario Mills.
Basically, Ontario weather is predictable until it isn't. You can count on the sun, but you have to respect the heat and the wind. If you keep an eye on the mountain peaks—which occasionally get a dusting of snow in the winter while you're wearing a t-shirt in the valley—you'll start to appreciate the weird, wonderful logic of the Inland Empire climate.