Brushfires may be classified as forest fires or grass fires. Both of these are controlled and impacted hugely by a combination of three factors: fuel, topography and weather. Usually the source of fuel in a forest fire is the dead vegetation on the forest floor. The quantity of moisture present in the air determines how quickly this is likely to ignite. If the moisture content exceeds 30 per cent, then the vegetation is too wet and there is no danger of a fire. The more it falls below this threshold percentage, the more rapidly and erratically the fire can spread.
The movement of the fire is also dependent on the slope of the land—fire moves quicker uphill because there is more new fuel available. A ten-degree slope can double or halve the speed of a brushfire depending on its upward or downward trajectory. Also, forest fires move slower than grass fires because they feed on the dense undergrowth and hence have more fuel. Grass fires can move as fast as 20 km/h and are usually driven by the wind. Air temperature, humidity, wind and atmospheric stability impact the weather and, in turn, the fire itself. Wind is a key factor because the fire and the wind together create a self-sustaining loop—the energy of the wind is increased by the force of the fire, thus increasing the momentum of the fire itself.