QUESTION:
Does Pluto have four seasons?

ANSWER from Megan Donahue on May 28, 1996
If Pluto was like Earth, with great big weather systems, fronts, 
hurricanes, and such, Hubble would almost certainly be able to see them. 
However, for years we've known that Pluto's atmosphere is fairly clear, 
and has very few if any real clouds. We've known this because when 
astronomers measure the change in Pluto's brightness as it rotates, 
the data always looks the same, week after week, year after year. 
If Pluto had variable cloudiness, the data we'd get would vary.

Although Pluto doesn't have weather like on Earth, it does have seasons, and
many scientists believe that over the next few years Pluto will approach a
point in its seasonal cycle where its atmosphere may start snowing onto the
surface. If it does, the Hubble Space Telescope should see the change as
bright snows cover more of the planet. That would really be something! Stay
tuned, we hope Hubble will take the time to watch for this, because we think
the snows come only once every orbit, which lasts 250 years.
- I didn't know this myself, so I learned from your question, too! I
also asked Alan a question of my own, so I can add a little bit to
his answer...
 
First, a question for you: WHY DOES THE EARTH HAVE FOUR SEASONS? 
 
The path the earth takes around the Sun is very nearly round, so the seasons
aren't affected by how close the Earth is to the Sun. As a matter of
fact, when we in the northern hemisphere are experiencing summer, the
Sun is the farthest away. People often think that the seasons change as 
the distance between the Earth and the Sun changes, and that isn't so!
 
The Earth's seasons occur because the Earth's rotation axis is tilted. 
During our summer the northern part of the Earth is tilted towards the 
Sun, during the winter the northern part is tilted away. One way to help 
see this just sitting there in your chair is to imagine that your head is the
Sun and that your fist is a planet. Stick your thumb up straight then tilt
it towards you a little bit, pointed at the wall high behind you. 
Move your fist around your head, keeping the thumb pointed in the
same direction (don't worry about looking silly, Alan reassured 
me that planetary scientists do this all the time.) 
If you keep your thumb in the same orientation while
moving it around, you'll observe that sometimes your thumb is pointing
towards you, when it's in front of you (summer), sometimes away, behind 
your head (winter), and sometimes it's pointing neither
direction, when it's on either side of you (fall and spring). 

Pluto's seasons are complicated because it has seasons for two
reasons. Pluto's axis is tilted by a lot - it is lying on its side.
So one of the reasons Pluto has seasons is the same reason the Earth
has seasons - a tilted axis. If you tilt your thumb so that it is
nearly parallel to the ground you can see that the northern half of
Pluto has an extremely long day during much of its orbit, and extremely
long nights during the other part!
 
But unlike the Earth's orbit around the Sun, Pluto's orbit is not
very circular. Its distance from the Sun changes a significant amount 
during its 248-year orbit. The distance between the Sun and Pluto varies
between 30 and 50 times the distance from the Earth to the Sun. This variation
dramatically changes the intensity of the Sun's radiation on Pluto, no
matter what its orientation, unlike the Earth, whose orbital distance
does not change very much at all. 

When Earth has its "equinox" (equal times day and night, which happens
twice a year), it is during the spring and fall, as we would expect. 
(Remember your explanation about why Earth has seasons.) Pluto's equinoxes
occur twice an orbit, once when it is almost at its farthest distance 
from the Sun and once when it is almost at its closest. If Pluto's 
distance from the Sun did not vary at all, its "spring" and "fall"
would occur near its equinoxes. But its distance DOES vary quite a bit, 
so the seasons on Pluto are more complex and more difficult to
predict (so far) than Earth seasons.
   
Briefly, any planet whose rotation axis is tipped relative to its 
orbit will have four seasons. Mercury, Venus, Jupiter don't really have 
seasons. Earth, Mars, Saturn, Uranus, Neptune and Pluto all will have 
four seasons.