[AS380 Mark Richards]

Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the explosive conflagration consuming the rugged landscape on the edge of Yosemite National Park, forestry experts say.

The fire had ravaged about 455 square kilometres by Tuesday, the biggest in the Sierra's recorded history and one of the largest on record in California.

Containment increased to 20 per cent but the number of destroyed structures rose to 101 and some 4,500 structures remained threatened. The types of lost buildings were not specified. Firefighters were making stands at Tuolumne City and other mountain communities.

The blaze was just 15 hectares when it was discovered near a road in Stanislaus National Forest on Aug. 17, but firefighters had no chance of stopping it in the early days.

AirSource have been called on to provide water bombing duties using a Sikorsky Erickson Sky Crane and Bell 212 helicopters in support to the fire crews working on the ground. Nearly 3,700 firefighters are on the scene to fight the blaze.

AirSource are also assisting the Air National Guard by proviing pilots for the Hercules C-130J which is flying low level passes dropping fire retardent on the fire lines.

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[Buckeye Roy]

Incredible videos you put on the detail sheet. Great job!