This has been suggested prior, but it's been at least a month, so I'll give this a bump.
Prior suggestions (some made by me) have included:
a) Night/Day cycle. Not noticed underground, necessarily.
b) Rain.
c) Rainbows following rain.
d) Drifting clouds.
e) Storms
f) Lightning
You're the first to suggest blowing debris; I like it!
I would add:
g) moving tumbleweeds.
h) dust storms
i) The occasional passing tornado that throws everything about but moves on.