From AMS Glossary
A unit of energy per unit area once commonly employed in radiation theory; equal to one gram-calorie per square centimeter.
The langley is almost always used in conjunction with some time unit to express a flux density; but the time unit has been purposely separated in order that it may be chosen in a manner convenient to each particular problem. The unit was named in honor of the American scientist Samuel P. Langley, 1834–1906, who made many contributions to the knowledge of solar radiation. Modern meteorologists tend to use the mks unit W m-2.