Original Author: Horst Dunkhorst

A steam locomotive emits plums of smoke as it operates. The exhaust from the smokestack is initially pushed into the atmosphere due to the internal pressure of the exhaust system. It continues to rise due to buoyancy, as the exhaust is hotter than the surrounding air and therefore less dense.

It is interesting to see how the smoke stack changes in definition and color as it mixes with the atmosphere. Darker smoke indicates a high concentration of fuel particles (coal, wood, etc.) in the smoke. An efficient engine will have a lighter colored smoke, as most of the fuel will be burned and converted to heat. As the smoke rises, it dissipates into the atmosphere, diluting the smoke and giving it that hazy appearance and lighter color.