Snow melts when the light level is high enough, however, it does not melt based on the light level of the block below it.
simple test:

1. /fill ~-16 ~-1 ~-16 ~16 ~-3 ~16 stone
   /fill ~-15 ~-1 ~-15 ~15 ~-1 ~15 glass
   /fill ~-15 ~-2 ~-15 ~15 ~-2 ~15 glowstone

2. run this twice with ~ 1 second in between /fill ~-15 ~ ~-15 ~15 ~ ~15 snow
   → Observe no snow melts at areas which remained covered after the first few ticks
3. Break a single snow layer
   → Observe snow now melts around the broken block