temperature inversion
"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."
"Meteorologist Joe DeNardo said that temperature inversion is like putting a lit cigarette in a bowl and then placing a blanket on top of that; the smoke continually fills up the bowl and has no way of escaping. Temperature inversions were not unusual for Donora, but this one was different because it lasted for five days."
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