This is what controllers see (this is a portion of the entire screen at JFK tower) on ASDE-X even if operating in zero/zero visibility. This is the newest technology for ATC. Every airplane on the field's transponder is being picked up by radar and reflected on the screen. If any airport vehicles breach a runway, they will be visible on this screen. Note that areas in red reflect closed taxiways (construction/repair). At the time this photo was taken JFK was operating on the 4's and there was light snow falling (RVR4000).
