From Glossary of Meteorology
A letter, number, diagrammatic sign, or character used in weather records or on weather maps to indicate meteorological phenomena, both past and present, in a concise and accurate form.
Symbols were first suggested and used by J. A. Lambert in 1771; his symbols included only clouds, rain, snow, fog, and thunder. The symbology pertaining to surface observations in the United States is contained in the "Manual of Surface Observations," 7th ed., 1955; that pertaining to the preparation of weather maps and weather analysis is contained in "Preparation of Weather Maps," U.S. Weather Bureau, 1942, and "Weather Analysis Symbols," U.S. Weather Bureau, 1950.