From AMS Glossary
The mechanical, chemical, or biological action of the atmosphere, hydrometeors, and suspended impurities on the form, color, or constitution of exposed material; to be distinguished from erosion.
Mechanical weathering results from the disintegrating action of high or low temperature, large changes of temperature, frost, or the impact of wind-borne sand or water (e.g., frost action, scouring, etc.). Chemical weathering is due to the chemical action of atmospheric constituents, especially acid impurities, in a moist atmosphere or in rainwater (e.g. corrosion, oxidation, etc.). Biological agents are mainly fungi that attack organic material (e.g., rotting, mildew, etc.).
Brooks, C. E. P. 1950. Climate in Everyday Life. 179–187.