"A layer of cold air sinks down close to the ground and gets overridden by warm air," Jeffrey said. That's kind of like a saucepan that you pour water in. You turn on the burner and it starts to boil. On a good day, that steam or pollution goes up and drifts away. But a temperature inversion is like putting a lid on the saucepan. The pollution builds up and the air just gets dirtier and dirtier. A lot of what you're seeing that you think is fog is really air pollution."