[rconti]

Stupid question: Wouldn't the snow cover the vault lights? (I know, they could clear them by clearing the sidewalks...)

[sampo]

Snow also scatters light a lot, if it's not too dirty. One example I found, it takes about 8 cm (3 inches) of snow to reduce the light irradiance to half.

[jrace]

It would. But not 100%. Same with skylights, depending on the depth and density some sunlight still makes it

[jrochkind1]

They weren't clearing the sidewalks to let light through, they were (and we are) clearing the sidewalks so that people could walk on them! Anyway, with or without prism tiles.