What you need to know about how wildfires spread

Trees burn in the Bootleg Fire in southern Oregon on July 25. (Bootleg Fire Incident Command/AP)

High temperatures. Low humidity. Little rainfall. Dry vegetation. Fast winds.

Wildfires depend on a combination of environmental conditions to start and spread. As global temperatures rise, research shows these conditions are appearing more intensely and frequently — escalating the risk of wildfires.

This summer, the United States has suffered through record-breaking heat, destructive fires and stifling smoke. Wildfires have burned more than a million acres in at least 12 states. Anomalously high temperatures and extremely dry conditions, particularly in the western United States, are expected to exacerbate fire conditions into the fall.

Here are a few key facts about how wildfires spread and how to protect yourself.