California Climate Zone 10
Climate zone 10 covers the Inland Empire and inland San Diego County — the western inland valleys of Southern California. The Energy Commission assigns 171 of California's ZIP codes to it, making it the 8th largest of the sixteen.
Title 24 Reports from $199 100% Online
Where zone 10 is
The Energy Commission assigns 171 ZIP codes to zone 10 — about 6.3 per cent of every ZIP code it lists, and the 8th largest of the sixteen. Places in it include Riverside, San Bernardino, Ontario, Corona, Escondido, Temecula, Fontana and Hemet.
The zone is geographically contiguous — it occupies one continuous block of the state rather than being scattered. The ZIP codes fall in the prefix ranges 917xx, 919xx, 920xx, 921xx, 922xx, 923xx, 924xx, 925xx, 928xx.
What drives compliance in zone 10
Summers are hot and dry enough that mechanical performance, not the envelope, usually settles the outcome. Duct leakage, equipment sizing and refrigerant charge do more work in this zone than almost anywhere else, which is also why field verification turns up so often on Inland Empire alterations — an older duct run in an unconditioned attic is the typical weak point.
Where zone 10 ends
Zone 10's eastern boundary is the sharpest in Southern California. It runs through the San Gorgonio Pass, where Beaumont sits in zone 10 and Banning, immediately adjacent, sits in the desert zone — the point at which coastal air stops reaching inland.
Boundaries follow conditions rather than administrative lines, so the assignment is resolved from the project ZIP code rather than inferred from the city on the drawings.
Cities we cover in zone 10
7 of our city guides cover places in this zone, and some of them cross a zone boundary inside the city.