Astronomy Fun Fact #71
For most of its year, Pluto doesn't have an atmosphere. But when it reaches it's closest approach to the Sun, the heat thaws the frozen nitrogen, methane, and carbon monoxide on Pluto's surface. The ices evaporate and the gases provide Pluto with a temporary atmosphere. When Pluto moves farther away from the Sun, those same gases freeze and fall back onto Pluto's surface.
Update: after a conversation with a real life Pluto expert, I learned that there's actually some debate about whether or not the entire atmosphere freezes and falls back to the ground. But people are pretty sure that the atmosphere gets thinner as Pluto moves away from the Sun.