Historical weather data, all-time temperature records, and monthly climate averages for Ontario, CA. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
Ontario sits in the Inland Empire, where the desert influence is real — this city has seen everything from 19° freezes to 109° scorchers since records began in 1986. Today's date has its own chapter in that story, shaped by the same Santa Ana winds and marine pushes that define Southern California's weather personality.
Ontario's all-time high of 109° hit on July 22, 2006, a reminder of just how punishing the Inland Empire's summer heat can get. On the other extreme, March 12, 1986 dropped to a stunning 19° — proof that this sun-soaked corner of SoCal isn't immune to a serious freeze.
Ontario is warmest from late July through August, with average highs peaking around 89-90° the first week of August. Winter is mild but not warm — December and January both average highs in the low 60s, with January also being the wettest stretch of the year. Spring and fall offer the sweet spot, with highs in the comfortable 68-77° range.