Any source of water vapor will do, including people. Waiting for the bus, mother and son turn to the sound of a nearby train. Though the train is more than five miles away, a temperature inversion makes it sounds as if it's coming down the street. The inversion, created when warm air rests on top of colder air, acts as a tunnel in which sound waves bounce for great distances. The boy sees a raven flying above the ice fog and points to it.
Ravens often roost close to town during a cold snap.
As the black bird flies through the air, its hyper metabolism keeps its body temperature at about 107 degrees. Through various adaptations, most animals are bothered very little by the cold, though chickadees adapted to life at bird feeders will probably die if people stop feeding them now. The mother walks back into the house, her toes tingling as her extremities go through a normal cycle of warming and cooling. Her toe temperature rises to 68 degrees after falling to 50 while waiting for the bus. She goes inside and moves toward her husband to give him a kiss.