Sofia: Good morning. My presentation is about tule fog, the thick fog that fills California's Central Valley in winter.

Sofia: Tule fog is named after the tule, a tall reed that grows in the valley's wetlands. It forms on calm, clear winter nights, especially after rain has soaked the ground. As the night goes on, the ground loses heat, and the air just above it cools until its moisture turns into tiny droplets of water.

Sofia: Because the mountains around the valley hold the cold air in place, the fog can sit there for days. In the thickest tule fog, drivers may be able to see only a few car lengths ahead, which is why it has caused serious highway crashes.

Sofia: So tule fog is not just bad weather. It is the valley's shape, its wet soil and its long winter nights working together. And once you understand how it forms, you can predict when it will be dangerous.