Elementary CA: Industrial chimneys

When the gases in a steel chimney are heated, then the gases expand. The hot gases occupy larger volume than before. The weight of gases per cubic metre becomes less. As a result of this, the unit pressure at the bottom of chimney due to weight of hot gases also becomes less than the unit pressure due to weight of cold air outside of chimney. The difference between two pressure results in the flow of burnt gases up the chimney.