What does "Nil and Nil" mean?

Well, 'nil' is short for nonexistant, or nothing, and in this case
each nil refers to a weather component, surface definition and horizon
definition.  The airplanes need both in order to fly, so they can see
to land- today we had none of either- it was a 'nil and nil' kind of
day...

but we made the most of it, making more measurements in the borehole.
Still, we'd rather be flying!

 [Camp in 'nil and nil' conditions- it's impossible to see the horizon or identify surface features, this is what we often call 'flat' light.  Very different from a windy storm, but the effects on aviation are the same!]