From AMS Glossary
Specially constructed board used to identify the surface of snow or ice that has been covered by more recent snowfall.
Snow boards are used as an aid in obtaining representative samples of solid precipitation at times when the catch in the gauge is considered erroneous, for example, in windy weather and wet snow. They provide the reference level for the measurement of new snowfall and ice. They are constructed of thin metal, wood, or other light material so as to not sink in the snow. They should be at least 40 cm by 40 cm and covered with white cloth or plastic.
NASA/National Weather Service 1972. National Weather Service Observing Handbook No. 2, Substation Observations. NOAA, US GPO.