As the first major atmospheric river of the winter arrives in Los Angeles, it brings with it the hope that the fire risk has finally receded, the danger that severe landslides could occur in the fire-scarred hills around the city, and the possibility that Southern California’s rainy season is, at long last, going to begin in earnest. Many accounts of the Palisades and Eaton fires have attributed their intensity in part to a delay in the winter rains, and that framing is not wrong. However, it doesn’t answer a basic question: why does Los Angeles receive virtually all its rain during the winter?

Other weather and infrastructure explainers:

• The Associated Press: What’s an atmospheric river? A pineapple express? AP explains the weather phenomenon
• The Associated Press: What causes mudslides, and what can be done to lessen the danger?
• UC San Diego Today: Atmospheric rivers explain atypical El Niño and La Niña Years
• KVPR (Fresno, Calif.): Podcast: Examining the state of California’s levees, a crucial part of water infrastructure