Five Cloud Terms

It's a rainy day here in central Maine, and the sky is somewhere between white and gray.

In Mrs. White's fifth-grade science class we learned about four types of clouds, but my perfunctory Internet research this morning suggests that cloud classification is more complicated than that. Nevertheless, here are the four types of clouds I learned in elementary school, and another word for something white you see in the sky.

My nephew Henry, who is not quite seven, takes a special interest in weather, so maybe he'll have something to add.

1. Cumulus. The puffy white clouds you see on sunny days. They form at a lower level than other clouds and can turn into storm-condition clouds, which are called cumulonimbus.

2. Nimbus. We learned this as a separate cloud type in fifth grade, but it appears to be an attribute of clouds, not a separate type. It means that the cloud brings precipitation, as in cumulonimbus or nimbostratus. (A related cool term is virga, precipitation that falls from a cloud but evaporates before it hits the ground.)

3. Stratus. "Stratus" means "layer," and these are the clouds that make a gray day. They can start at lower altitudes and go quite high, where they become altostratus and cirrostratus. Cirrostratus clouds carry ice crystals. They are relatively transparent, and give the sun and moon a halo effect. Today's clouds are nimbostratus, since they've brought rain.

4. Cirrus. Cirrus clouds form at high altitude and appear as wispy strands — the "mare's tail" or "mackerel sky" (which is actually formed by cirrocumulus clouds). Cirrus clouds look the way they do because of winds, and people who know about these things can tell a lot about the wind by the way cirrus clouds look and move. The sailor's rhyme goes, "Mare's tails and mackerel scales/Make tall ships carry low sails."

5. Contrail. Not clouds at all, but the condensed water vapor left by airplane exhaust; "contrail" is a portmanteau word made from "condensation trail." Still, they're pretty in a clear blue sky, and I always thought it would be fun to ride along with a skywriter sometime.