[CF-metadata] snow temperature

[Jonathan Gregory]

Dear Burkhardt and Karl

(1) Yes, I agree that surface_temperature_where_snow should be the name for the
quantity you describe. I will add it to the list.

(2) "Sea surface" temperature is a special name, which we retained because it
is the customary name for this observational quantity, even though it is not
the temperature at the surface, but that of some rather poorly defined level
where ships measure it (usually through cooling water intake). The skin
temperature at the surface of the sea is surface_temperature (same as snow
and any other surface).

(3) "Mean sea level" means a particular geodetic surface, regardless of
sea ice.

Best wishes

Jonathan