ASK A SCIENTIST
Question: If heat rises how come the higher up you get the colder it gets?
Heat rises because air warmed by the sun expands in every direction, including up. Heat is molecular motion, with molecules moving faster the warmer anything is. The atmosphere of the earth gets denser closer to the ground because the earth's gravitation pulls its molecules slowly down toward the surface. The atmosphere near the earth's surface, which is the air we breathe, is quite dense. As its molecules are hit by sunlight's energy, they bounce against each other faster. This causes friction that heats up the air near the surface.
But, more importantly, much of the sunlight energy hitting the solid earth gets absorbed by its sunlit matter (rocks, pavement, and so on), after which some of it gets re-radiated as infra-red energy. This energy is similar to the microwaves emitted in a microwave oven. This re-radiation continues to happen after dark, as the earth's surface cools. All this radiation goes out into space and heats it up to some degree. But it doesn't heat up very much, because the further away we get from any planet, the planet's gravitational pull weakens, and the density of molecules in space becomes thinner. The re-radiated infra-red energy just keeps going until it hits a molecule, gets absorbed and is re-radiated again in a different direction, getting weaker each time. After a while it hits fewer and fewer molecules in the space in between stars.
Ultimately, it is the thinness of gaseous matter in space, or its low density, that prevents space from heating up. Energy radiated from stars just passes right though it. And space cannot get denser because, cosmologists tell us, space is expanding, and so it must be getting thinned out even more over time. This is also why all the light radiated from all the stars in the universe doesn't heat up space, which would cause the night sky to be bright instead of dark as it is.
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail firstname.lastname@example.org. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).