QUESTION:
How it could rain Helium because it's the lightest element on Earth?

ANSWER from Trisha Borgman on April25, 1996:
Well, y'all have raised an excellent point!  I assume that, since
we don't have to worry about raining helium on Earth, you are
talking about raining helium on another planet.  However, just
because helium is a very light element doesn't mean that it isn't
affected by gravity.  On Earth, it would be difficult for helium
to fall because helium is lighter than our air (oxygen and nitrogen).
However, what if our atmosphere was composed of hydrogen? Helium
is heavier than hydrogen; therefore, the helium would fall straight
through the hydrogen atmosphere.  Another point which is important
to consider is the phase of the helium as it "rains".  On Earth,
our normal temperatures and atmospheric pressures cause helium to
be in its gaseous phase. However, with a little work, we can force
helium to be in a liquid phase.  On other planets, it is possible
for helium to be in the liquid phase naturally.  That, combined
with the fact that other planets have very different atmospheres,
make it possible for helium to "rain" on other planets.

ANSWER from Bryan Miller on May 2, 1996:
I am interested to know where you heard about helium rain.  I've never
heard of it and if I knew the source of the question I could make more
detailed comments.  One correction is that helium is not the lightest
element, that title belongs to hydrogen, the most abundant element in
the universe.  However, helium is still much lighter than most of the
other gases that make up the Earth's atmosphere.  This makes it
difficult for the Earth to retail helium in its atmosphere; most of it
drifts away into space.  Most of the helium used by humans is found in
natural gas.  Another important property of helium is that it is
chemically neutral; it does not react strongly with other elements.
This makes it useful in certain industrial processes but it also makes
it unlikely that one would ever see helium rain.  The formation of rain
is partially dependent on the condensation of water molecules around
dust particles in the atmosphere.  This is aided by the fact that water
molecules are not symmetrical and so have a small electric field that
allows them to bond more easily with other molecules.  Being inert,
helium does not form droplets at normal temperatures.  The only way to
produce a helium liquid is to cool it to extremely low temperatures.