Ask the Experts; August 2009; Scientific American Magazine; by Herman Merte; 1 Page(s)

Herman Merte, a professor emeritusof mechanical engineering at the University of Michigan at Ann Arbor, explains:

What one sees drifting above a hot bath—often called “steam”—is in fact tiny drops of liquid water that have coalesced out of the gaseous mixture of air and water vapor above the fluid’s surface. The vapor, itself an invisible gas, arises from evaporation, whereby water molecules escape from a liquid. Evaporation is a slower process than boiling but is accelerated when water heats up (gaining increased energy).