The climate at Riverside is mediterranean with mild winters and occasional rains, Santa Ana winds from the desert, and rare early morning frosts.
Summers are hot with occasional air inversion layers.
Rainfall increases from 250 mm at Riverside to 1000 mm along the coastal slopes of the mountains, then decreases to 50 to 150 mm in the Mojave and Colorado deserts. Snow is common in the mountains above 2000 m, where ski resorts operate for several months. Strong environmental gradients produce remarkable vegetation diversity in the region. Chaparral and other brushland cover in the coastal valleys shift into majestic conifer forests on mountain summits, beyond which lay vast expanses of creosote bush scrub and alkali sink vegetation in the deserts. Mountain and desert wildlands are mostly still in their pre-European pristine setting, and contrast with rapidly changing land use in the interior coastal valleys surrounding the campus.