QUESTION:
We know what rain in the form of water is like, but what would the helium rain on 
Neptune be like?

ANSWER from Heidi Hammel on April 24, 1996
That's a tough question.  I am not even sure Neptune has Helium rain -
it is Saturn that is famous for it, and Jupiter might have a little.
I think Neptune is much more likely to have a lot of methane snow
rather than Helium rain.  That's what we think the white clouds are on
Neptune - methane ice crystals.  If you have ever flown on an airplane
through clouds on Earth, that is probably what flying through Neptune's
upper atmosphere would be like - except much colder and VERY much darker,
like flying here at midnight on a moonless night.