QUESTION:
How do the clouds form on Neptune?

Answer from Heidi Hammel on April 24, 1996:
Clouds form by the same process on Neptune as it is on Earth, there is
just a different gas involved (methane instead of water).  On Earth - when
it is cold in certain spots, water gases in our atmosphere condense to
form either water ice crystals or sometimes tiny water droplets.  That's
what our clouds are made of.  On Neptune, it is so cold that the methane
in the atmosphere condenses - the gas molecules stick together to form
ice crystals.  In the coldest parts, there are lots of ice crystals,
and that is a Neptune cloud!  (Neptune has a lot more methane gas than
water gas in its atmosphere; that's why it has methane clouds rather
than water clouds).