California Climate Zone 15
Climate zone 15 covers the low desert — the Coachella and Imperial valleys and the Colorado River corridor, the hottest assignment in the state. The Energy Commission assigns 52 of California's ZIP codes to it, making it the 15th largest of the sixteen.
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Where zone 15 is
The Energy Commission assigns 52 ZIP codes to zone 15 — about 1.9 per cent of every ZIP code it lists, and the 15th largest of the sixteen. Places in it include Palm Springs, Indio, Palm Desert, El Centro, Brawley, Blythe and Borrego Springs.
The zone falls in two separate areas rather than one block, which is why a project in one part can feel unrelated to a project in the other. The ZIP codes fall in the prefix ranges 920xx, 922xx, 923xx, 925xx.
What drives compliance in zone 15
The most cooling-dominated assignment in California. Summer conditions are extreme enough that cooling capacity, duct performance and solar heat gain control the outcome almost entirely, and heating is close to irrelevant. Prescriptive glazing requirements are among the strictest in the state.
Where zone 15 ends
Zone 15 covers the Coachella and Imperial valleys and the Colorado River corridor, and it is the second smallest assignment by ZIP count despite covering a large area — a reflection of how sparsely the low desert is settled compared with the coast.
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.