NOVA: Either really heavy rain,

[shortylickens]

or light hail.

Cant tell which.

 

[ElFenix]

look outside the window.

if there are little ice balls on the ground, it's hail. if there's just pooling water, it's heavy rain.

 

[shortylickens]

It seems like the iceballs disintegrate instantly, but they do impact.

 

[BurnItDwn]

Maybe he works inside a building with no windows?

Datacenter I was in years ago was a converted KMart. Literally no windows at all, only fake skylights it was "always sunny"