Don Poitras writes:

If it's dark enough that you can't see much outside, then it _is_ IMC.

Nighttime isn't a meteorological condition. That's why you don't see
nighttime indicators in METARs. The key point is that you don't see much,
which allows you to fly IFR. I suppose that if you look out the window you
might see something, but you have to look, whereas during the day, the scenery
outside is hard to ignore.

--
Transpose mxsmanic and gmail to reach me by e-mail.