Photo: Twitter - Santa Cruz SO

Some evacuations were lifted as cooler weather gave firefighters a boost in their struggle with a wildfire burning through dry brush that is threatening hundreds of structures in a remote area of California's Santa Cruz Mountains.

Mandatory evacuations were lifted Wednesday for Santa Cruz County but evacuation orders remained in effect for neighbouring Santa Clara County, where most of the 300 threatened structures are located, the California Department of Forestry and Fire Protection said.

The wildfire had charred more than 4 square miles and was 22 per cent contained by Wednesday evening, said Cal Fire Battalion Chief Scott McLean.

A 10-degree drop in temperatures and increased humidity helped fire crews. The cooling trend was expected to last through the week.

"Tomorrow is expected to be much cooler and each day will get progressively better but we still have very active fire behaviour," McLean said.

The blaze won't be fully contained before next week, McLean estimated.

It was among three blazes burning in Northern California during a time of year when the drought-stricken state sees its largest and most damaging wildfires, state forestry officials said.

The blaze broke out Monday during a statewide heat wave that brought witheringly low humidity and temperatures in the upper 90s.

It gutted at least one home and threatened 300 buildings, though it was not clear how many were homes or smaller structures.