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Rain on Snow
When rain falls on ripe snow, the water itself doesn't cause snow to disappear; however, the accompanying warm air temperature increases the melt rate. What impact does rain actually have on the snowpack?According to the Corps of Engineers' Snow Hydrology manual,
(click on the image to see detail)
Mr = 0.00695 (Tr - 32) Pr
where Mr is the melt, in inches of water, Tr is
the mean rain temperature (F), and Pr is the inches of rainfall.
Therefore, it would take about 10 inches of rain at 48F to melt one inch
of snow water content.
Much of work to develop and verify this equation was done at the Central Sierra Snow Laboratory located at Soda Springs near Donner Summit. Considerable work remains to refine the physical mechanisms of how the rain water migrates through the pack.
Only after prolonged, warm rainfall will snowmelt be a major contributor to runoff. This did occur in February, 1986 and during the first week of 1997.
Dense snow makes shovelling difficult and leaves abundant "Sierra cement" on the ski slopes.
California Cooperative Snow Surveys