American meteorologist who apparently gave the first essentially correct explanation of the thermodynamics of cloud formation and growth. He was also one of the first to use the telegraph for collecting meteorological observations.

Espy served as a meteorologist with the U.S. War Department and the U.S. Navy until 1852, when he continued his work at the Smithsonian Institution, Washington, D.C. He presented his theory of clouds and his more general (though incorrect) theory of storms before scientific audiences in the United States and Europe and in his Philosophy of Storms (1841).