Why are industrial smokestacks generally so tall?
Noble thought Jac, but remember they started building tall smokestacks before anyone cared about pollution. Smokestacks are built for the purpose of enclosing a column of hot gases, thereby producing a draft. Due to their high temperature the gaseous products of combustion have a lower density than the surrounding air. Hence the weight of the hot gases in the smokestack is less than the weight of an equally high column of air, producing pressure (i.e., weight per unit) differences that drive the hot gases up the smokestack. The difference in pressure is equal to the height of the smokestack multiplied by the difference in densities of hot gas and air, and this is how we calculate the draft.