QUESTION:  What is the annual precipitation on Neptune and Pluto?

ANSWER from Wayne Baggett, Computer Sciences Corporation, Space Telescope
Science Institute, on April 29, 1996.
        
        The short answer is that we really don't know how much precipitation
falls on
Neptune and Pluto.  Especially in the case of Neptune, where we have never even 
observed a solid surface (we only see cloud tops), the definition of 
"precipitation" is not obvious, since the value would depend on where you
happen to be within the atmosphere.  For instance, on Earth if you determined
how much rain, snow, sleet, and hail fell at the height of cirrus clouds way 
up in the atmosphere, it could be very different from the value measured at the
ground.  

Because the measurements of Pluto's atmosphere are so hard to make, we don't
even have any good guesses for it's precipitation rates yet.  Pluto has a very
thin atmosphere only when it is closest to the sun, so precipitation on Pluto
is only possible during a small part of its orbit around the sun.  When Pluto
is farthest from the sun, its whole atmosphere is frozen onto its surface,
probably through a process similar to the formation of frost on cold winter
mornings here on Earth.  Maybe when you grow up, you can be the person to
discover how much snow Pluto gets! 

Now, let's look at your question in more detail.  You ask for "annual"
precipitation, which for the Earth would mean the amount of rain and stuff
which falls in one Earth year (the length of time it takes the Earth to orbit
the sun once).  Using "one Earth year" for the definition of "annual" for any
other planet could lead you to some erroneous results, because characteristics
such as rainfall, temperature, wind speed, and many others change with the
different seasons, which are determined by the planet's orbital period around
the sun. Since every planet has a different orbital period around the sun,
every planet's year is a different length, and "annual" would mean different
time periods for every planet.  To get a good measure of "annual"
precipitation, you would have to measure it over many orbital periods of the
planet in question.  Keep in mind that Pluto was only discovered in 1930 and it
has an orbital period of about 250 years, so we've only known about the planet
for about one-fourth of a Pluto orbit! 

Now, let's look at "precipitation."  On Earth, precipitation always consists of
water in either liquid (rain) or solid (snow, sleet, or hail) form, after it
condenses from its gaseous form (water vapor).  This combination is possible
only because the Earth's temperature is such that water can naturally exist in
all three forms.  On other planets, the temperature is different from Earth so
some other material will replace water as the substance involved in
precipitation.  On Mars, for instance, we know that there is carbon dioxide
"snow" (we call it "dry ice") at its north and south poles, so carbon dioxide
on Mars behaves similar to water on Earth.  Both Neptune and Pluto are much
farther from the sun than either Earth or Mars, and their temperatures are
correspondingly lower.  In both planets, precipitation probably consists of
methane (also called "natural gas"), with both solid and liquid on Neptune and
only solid on Pluto. 

Finally, remember that precipitation amounts can be very different from one
point on a planet to another; the Earth has deserts, which receive rain only on
occasion, and rain forests, where it rains almost every day.  Thus, to define
some sort of average precipitation for an entire planet would be only the very
first step in understanding how the atmosphere works with regard to the
"moisture" content and transport mechanisms. 

This closer examination of your question illustrates a problem which is
frequently encountered in astronomy: when applying the concepts we know and
understand from our everyday life on Earth to conditions in other places in the
universe, we have to be very careful or we can be easily mislead about what is
really going on.  One of the things which makes astronomy so difficult is that
the sizes, masses, distances, and time scales used to study the planets, stars,
and galaxies are completely outside our everyday experience, and really
appreciating these concepts can be a daunting task.