Ontario shares the Inland Empire's hot-summer Mediterranean climate. Summers (June–September) are dry and very hot, regularly hitting 100°F (38°C) and sometimes higher; the haze from LA basin smog tends to settle here against the mountains. Winters are mild, with highs in the 60s F (15–20°C) and the occasional January storm clearing the air enough to see the snow on Mount San Antonio.
Spring (March–April) and autumn (October–November) are the best times to visit — warm, mostly clear, and with the San Gabriels visible up close. Avoid late summer if you can.