Wildfire is the term used to describe any uncontrolled fire in a forest, grassland or land sown to crops. The fire can be caused by lightning or by humans, and it can spread quickly under certain conditions.
Wildfires require three key elements to ignite: fuel, heat and oxygen. This is sometimes referred to as the “fire triangle.”
Without people’s interference, lightning strikes cause most natural wildfires in the United States. But people start the majority of human-caused wildfires. They can ignite a wildfire when they leave a campfire unattended, or they may throw a lit cigarette out of their window while driving through a forest. The heat from the sun and the wind can also cause a wildfire to spread by itself.
The topography of a landscape can help determine how fast a wildfire moves, Roise says. The elevation and aspect (direction of the slope) can play a role in wildfire progression, as well as the temperature and moisture levels of the air. For example, the side of a mountain facing the sun will dry out faster than the other sides because it receives more sunlight. The air will be warmer at higher elevations, as well, which can preheat fuels that are further uphill to burn more easily once the fire reaches them.
Climate change is a big contributor to the wildfires that occur in California, because it means hotter temperatures and drier vegetation. The number of people living in the wildland-urban interface (the area where homes and forests intersect) has also increased, putting more lives in danger.