Weather is a key factor in starting and spreading wildfires -- particularly drought, which dries out vegetation. Trees, underbrush, dry grassy fields, pine needles, dry leaves and twigs can all cause and spread forest fires because they burn faster, like kindling, than large logs or stumps. The more fuel that is present, the more intensely the fire will burn and the faster it will spread. When the fuel is very dry, such as after a long drought, it is consumed much faster, and the fire is much more difficult to contain. And as the fire spreads, it generates heat that evaporates the moisture in potential fuel materials just beyond it, making it easier for those to ignite.
Wind can also help spread a forest fire, and is the most unpredictable factor. Winds supply the fire with extra oxygen and push it across the land at a faster rate. Because the wind generally flows uphill, fires also travel faster up a slope than downhill. Wildfires can even generate their own winds, called fire whirls, which resemble tornados. They arise from the vortices created by the fire's heat, and can be so strong they have been known to hurl flaming logs and burning debris over long distances.
Sometimes it is better to let forests burn. Small wildfires are necessary to prevent buildup of dead brush and saplings, which can act as dry kindling if it is allowed to accumulate too much. So to prevent massive, widespread wildfires, scientists often set controlled burns -- fires set deliberately to imitate Nature's own control mechanism. The U.S. Forest Service uses controlled burns on windless humid days to clear away excess wood. They plan to control burn a million acres a year (about 2,000 square miles) by 2015, which will help put out future wildfires faster by taking away a major fuel source.