The Santa Ana winds fanning wildfires that have killed at least 25 people in Southern California and destroyed more than 10,000 houses, businesses and other structures in Greater Los Angeles are flaring up again.

In recent days, however, the region’s powerful Santa Ana winds—which have been fanning the flames—have begun to slow down. This lull has offered firefighters a reprieve and a key opportunity to make progress against the blazes, but forecasts suggest the Santa Ana will return next week. What are these gusts, and how have they become so strong?