Weather

Late fall through early spring is the best time to begin learning how to understand some of the things that clouds can tell you about weather. These months bring a wider variety of clouds to much of North America.

Cumulonimbus clouds—thunderstorms—pose the biggest weather danger to pilots during warmer months when hot, humid air feeds cumulus clouds that can grow into thunderstorms. As the days grow shorter and the sun drops lower in the sky, northern regions cool more than the South. The resulting temperature contrasts supply the energy that powers the large extratropical cyclones that move generally west to east later in the fall, through winter, and into spring.

Contrasting temperatures create cold fronts where cool or cold air is replacing warmer air at the surface, and warm fronts where warmer air is advancing. These fronts, particularly the warm fronts, create a wider variety of clouds than those you’ll see during a warm-weather flight.

A brief cloud guide. We still use the system for naming clouds—with a few additions and changes—that London businessman and amateur scientist Luke Howard developed and first presented in 1802. It categorizes clouds by their heights above the ground and whether they are generally flat (stratus) or lumpy and piled up (cumulus).

In general, stratus clouds form when a large mass of air rises at more or less the same rate over a large part of the sky—maybe all of the sky you can see—making them generally but not perfectly flat on the top and bottom.

Cumulus clouds form when bubbles of air are rising as air is sinking nearby. While the bottoms typically are flat, the tops can rise like castles in some places and hardly at all in others. You’ll usually see breaks between the clouds.

Clouds higher than 20,000 feet are called cirrus clouds or have names beginning with “cirro.” Since they’re so high, they are made mostly of ice crystals, although scientists have found supercooled water drops (water that is still liquid although colder than 32 degrees Fahrenheit). Cirrus clouds have wisps of snow falling from them (often called “mares’ tales”) that quickly evaporate. More or less solid sheets of these high clouds are called cirrostratus clouds, while those with lumps are cirrocumulus clouds.

Clouds between roughly 6,500 feet above the ground and 20,000 feet have names beginning with “alto,” such as altostratus and altocumulus.

Clouds below 6,000 feet don’t have a prefix indicating their height. If they are lumpy, they are cumulus clouds of various kinds. If they are flat, they are stratus clouds or have “strato” or “stratus” in their name, such as nimbostratus. The “nimbo” tells you that rain or snow is falling from the clouds. Stratocumulus clouds are sheets of lumpy clouds.

To tell stratocumulus, altocumulus, and cirrocumulus clouds apart, extend your arm toward the lumps in the clouds. If your little finger covers a lump, the clouds are cirrocumulus. If your thumb roughly covers one of the lumps, the clouds are altocumulus. If your fist is needed to cover a lump, you’re looking at stratocumulus clouds.

Rising air holds clouds up. Clouds are made of tiny drops of water or small ice crystals that are always falling. For a cloud to stay together and in the sky, air must be rising into it at least as fast as the water drops or ice crystals are falling at what’s known as their terminal velocity, which depends on their size and weight.

Air cools as it rises; when it cools to the dew point, the water vapor in the air begins condensing into tiny drops of water or sublimating into tiny ice crystals. These create the clouds we see. In a strong thunderstorm, that air can be rising at 100 mph. In an ordinary stratus cloud the air will be rising as slowly as a few inches an hour. Air does not have to be rising straight up to form clouds; it can be ascending at an angle, such as when warm air ahead of a surface warm front is sliding up and over colder air next to the ground.

The typical terminal velocities in this chart show how fast the air has to be rising for clouds, rain drops of certain sizes, and even hail to form.

Some typical terminal velocities

Typical cloud drop: 0.02 mph

Small raindrop: 4.5 mph

Large raindrop: 20 mph

Large hailstone: 100 mph

Growing cumulus clouds have generally flat bottoms showing where the rising air has cooled enough for water vapor in the air to begin condensing into cloud drops or­—if the air becomes cold enough—depositing directly into ice crystals.