There are three main groups of factors that influence the spread of wildfires :-
a) Weather :
This includes the wind speed and direction, temperature and relative humidity. The wind speed has a major effect on the spread of the fire, and at times the fire can travel faster than a person can run. Trying to outrun a wildfire under such conditions is useless. Wind direction also plays a significant role as a changing wind causes the fire to burn in an erratic fashion, resulting in a very wide fire front. This factor can also result in firefighters becoming trapped by the fire. Temperature affects the ability of the fuel to burn, and the hotter and drier the air, the greater the radiated heat from the burning material, and the more likely adjacent fuels will burn as well. Relative humidity has an impact as this may influence the amount of moisture in between the fuel loads, and generally the drier the air, the hotter the fire will burn. To summarize, days most conducive to the spread of runaway fires are very hot and windy, with low humidity, and changing wind direction.
b) Terrain :
We know that fire prefers to burn uphill than downhill, and mountainous terrain creates its own local weather patters. Even on calm days, wind will be created by the updraft of the fire, and be channeled into the gullies and ravines.
c) Fuel :
Fuel types and loads have been discussed previously in detail. Heavy fuel loads that are dry and that allow sparks to be thrown ahead of the fire front by the strong winds cause a phenomenon known as “spotting”, whereby sometimes a new fire will be started as far as 3 km ahead of the existing fire.