Firefighters held the line against two massive wildfires that have ravaged parts of Los Angeles for the past week, even as desert winds and a parched landscape presented extremely hazardous conditions.

Santa Ana winds occur when air flows west from a region of high pressure over the dry Great Basin to lower pressure off the California coast, According to Accuweather. As that cool interior air flows over and through mountain passes, it accelerates, sinks and compresses.