Fire needs three things — fuel, oxygen and heat, the “fire triangle.” Once burning, how fast and how fiercely a wildfire spreads is governed mostly by fuel (how dry and how plentiful), weather and terrain.
Weather is the biggest day-to-day driver. Low humidity and high temperatures dry the fuel; strong, gusty wind pushes the flame front and throws embers ahead of it. Many of the most destructive fires occur during dry downslope wind events such as the Santa Ana and Diablo winds.
Terrain matters because fire moves faster uphill — flames preheat the slope above them. Narrow canyons can channel wind and funnel fire, and steep ground gives crews little time to respond.
Forecasters issue a Red Flag Warning when warm temperatures, very low humidity and strong winds combine to make any new ignition likely to grow rapidly. Those are the conditions when a single spark can become a major fire within hours.