The main features of a wildfire include extreme heat, strong winds and low humidity. Additionally, wildfires require a sufficient amount of combustible fuel to sustain their initiation and advance.
Heat helps to initiate combustion when dry fuel has a sufficient source of oxygen. For this reason, wildfires tend to spread during the summer, in sunlight and especially on hot afternoons. Since high humidity hinders the ignition and spread of wildfires, they tend to occur during dry periods such as droughts and heat waves. Once wildfires have started and attained sufficient momentum, they create fronts that heat and dry the air and fuel they approach through convection and thermal radiation. Winds cause wildfires to spread faster and also change direction. They also cause phenomena such as spotting, when burning embers create additional fires, and crowning, when fire surges into treetops. Major wildfires sometimes create their own tornado-like winds known as fire whirls.
The ability of wildfires to ignite and spread is dependent on the type of available fuel and the topography of the land. Wildfires are sustained by ground fuels such as roots and peat, forest litter, surface vegetation like grass and shrubs, small and large trees, and material suspended from trees. When the landscape is hilly, wildfires burn more rapidly, as a fire travels faster uphill.