Radioactive snow gauge
From Glossary of Meteorology
radioactive snow gauge
A device that automatically and continuously records the water equivalent of snow on a given surface as a function of time.
A small sample of a radioactive salt is placed in the ground in a lead-shielded collimator that directs a beam of radioactive particles vertically upward. A Geiger–Müller counting system (located above the snow level) measures the amount of depletion of radiation caused by the presence of the snow.