Anew set of stamps from the U.S. Postal Service is sure to put weather lovers on cloud nine. Each of the 15 stamps in the series, which goes on sale nationwide today to launch National Stamp Collecting Month, shows a different view of clouds.

Clouds can be fluffy and fun, or dark and scary. Some clouds keep to themselves, surrounded only by blue sky. Others travel in packs that blanket Earth and block out the sun.

Why are we so fascinated by clouds?

Maybe because we like to guess what animal or object a cloud looks like. Or maybe because clouds are one of the few elements of weather that we can actually see: Wind, temperature and humidity are invisible, and you're never supposed to look directly at the sun.

Clouds are collections of water droplets or ice particles. Most clouds form when rising air cools, which lets the water molecules slow down and attach to one another. The result is a cloud.

Oddly enough, the sun helps create many clouds. When it heats some areas more than others, any warm air surrounded by cooler air will rise (because it's less dense), cool off and make a cloud.

"Pressure" describes the force of the weight of the air above a certain area. When atmospheric pressure is low, air rises more easily. Low pressure often means clouds and maybe storms; high pressure usually means sunny, dry conditions.

Clouds are grouped by their shape, using Latin words. Most fall into one of three groupings: cirrus (pronounced SEAR-us, the Latin word for a curl of hair), stratus (STRAIGHT-us or STRAT-us, meaning spread out) and cumulus (KYOO-myoo-lus, or heap).

Dan Stillman explains what the new stamps show us.

The Lowdown

Cumulus, the lowest clouds we see in the sky, form when patches of warm air rise from the ground. These clouds have flat bottoms and puffy tops, and come in various sizes.

The shortest members of the family are cumulus humilis. These are fair-weather clouds that form in a stable atmosphere and often morph into shapes resembling animals or objects.

When afternoon heating makes the atmosphere less stable (causing air to rise even higher), cumulus humilis can sprout up and become cumulus congestus. These clouds have tops that look like cauliflower, and can produce lots of rain.

A cumulus congestus that keeps growing might turn into the biggest, baddest kind of cloud -- the cumulonimbus, or thunderstorm cloud, which can produce heavy rain, thunder and lightning, hail and tornadoes.

Stuck in the Middle

Stratus refers to a type of cloud that spreads out like a blanket.

In cloud talk, putting "alto" before a word describes clouds that form in the middle levels of the atmosphere (just as "alto" describes a mid-range singing voice). Altostratus refers to layered clouds that sometimes darken the sky as a storm approaches. Altostratus translucidus allow the sun or moon to shine through.

Altocumulus castellanus clouds have towers that resemble castles. Altocumulus lenticularis clouds form when moist air is forced over and around a mountain; they look like spaceships and are sometimes mistaken for unidentified flying objects!

Sky High

The highest clouds are cirrus. Indeed, these clouds do look like thin strands of hair. They form waaaay up in the sky where it's very cold, so they are made up mostly of ice crystals.

Cirrus radiatus are a special type in which the cloud's strands appear to spread out from a single point, like spokes on a bike wheel. Cirrostratus fibratus are thin, sheet-like clouds that bend the sun or moon's light, producing a halo ring.

Mammatus clouds look like pouches hanging from the sky. Unlike other clouds, they form in sinking, rather than rising, air. Mammatus can be found under other clouds, especially below cumulonimbus.