A severe type of wildfire where the heat and intensity build to the extent that they travel up the forest undergrowth and ignite the crowns of the trees. Most forest fires, especially controlled burns, clear only the scrub and undergrowth and are actually beneficial to the long-term health of the forest, giving older trees a larger share of rainfall. Crown fires are especially nasty because the fire can race from crown to crown, and without the protection of the forest canopy the wind feeds and spreads the fire extremely quickly.
According to The Economist, "Until the late 1970s, the government's policy was to extinguish all fires. Its motto became "Ten a.m. and ten acres"--the time by which it aimed to put out any fire, and the area to which it would be confined...Between 1931 and 1950 crown fires burned only 12,000 acres of Ponderosa pine trees in the south-west. In the past decade they have destroyed 350,000 acres. Ponderosa pines are tough, thick-barked trees. But they cannot survive crown fires."
Source: The Economist, "God, man and the fires", 12 August 2000.