Physical Properties: Wildfires occur when all of the necessary elements of a fire triangle come together in a susceptible area: an ignition source is brought into contact with a combustible material such as vegetation, that is subjected to sufficient heat and has an adequate supply of oxygen from the ambient air. A high moisture content usually prevents ignition and slows propagation, because higher temperatures are required to evaporate any water within the material and heat the material to its fire point. Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity, and are therefore less susceptible to wildfires. Less dense material such as grasses and leaves are easier to ignite because they contain less water than denser material such as branches and trunks. Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain. When this balance is not maintained, plants dry out and are therefore more flammable, often a consequence of droughts. A wildfire front is the portion sustaining continuous flaming combustion, where unburned material meets active flames, or thesmoldering transition between unburned and burned material. As the front approaches, the fire heats both the surrounding air and woody material through convection and thermal radiation. First, wood is dried as water is vaporized at a temperature of100°C (212°F). Next, the pyrolysis of wood at 230°C (450°F) releases flammable gases. Finally, wood can smoulder at 380°C (720°F) or, when heated sufficiently, ignite at 590°C (1,000°F). Even before the flames of a wildfire arrive at a particular location, heat transfer from the wildfire front warms the air to 800°C (1,470°F), which pre-heats and dries flammable materials, causing materials to ignite faster and allowing the fire to spread faster