Ten Major Cloud Types
In biblical times, Job asked, "Can any understand the spreadings of the clouds?" We're certainly still trying. In modern times, we understand that clouds represent the basic building blocks to our weather. The foundation consists of 10 major cloud types. In addition to cirrus, stratus, cumulus, and nimbus clouds, there are cirrostratus, cirrocumulus, altostratus, altocumulus, stratocumulus, nimbostratus, and cumulonimbus clouds. The following table places these cloud types into the four major cloud groups.
Major Cloud Groups
The earliest sky watchers said, "Mackerel sky and mare's tails make lofty ships carry low sails." The mare's tails are the cirrus clouds, which, combined with cirrocumulus, often indicate the approach of wind and rain, thus it's time for the low sails. Other variations include, "Mackerel scales, furl your sails," and, "A mackerel sky, not 24 hours dry."
These clouds are found at elevations of 20,000 feet (6,000 meters) and higher. The cold air at these elevations causes the small cloud droplets to freeze into ice crystals. Actually, because the droplets are so small, they can exist in liquid form below 32 degrees, and are called supercooled. The ice crystals act as prisms and cause light to separate into its many colors. High clouds can deliver some spectacular optical effects, including a red sky in the morning and at night. Cirrus clouds are in this grouping and are joined by cirrostratus clouds, which appear as thin sheets across the sky. The thin, feathery cirrus clouds thicken to form cirrostratus. The sun or the moon will shine through cirrostratus and often form a halo. Cirrus clouds can also thicken into small, rounded cotton-ball-like masses, the cirrocumulus clouds. Sometimes these look like scales of a fish, so a sky filled with cirrocumulus is called a "mackerel sky." When a storm advances, these high clouds are first to arrive. They are great clues to impending weather changes.
Warm air often streams ahead of storms. This warm, less dense air is forced to rise over colder surface air. That warmer air can extend a thousand miles from a particular storm center, and as it rises to great heights, it cools and forms the high clouds. First those thin, wispy cirrus clouds appear. Then they thicken to cirrostratus and cirrocumulus. At that point, halos and rings form around the sun and moon and storms can be just 24 hours away. Later, as the storm grows closer, the clouds thicken and begin to lower.
Generally speaking, middle clouds form at elevations ranging from about 6,500 feet (2,000 meters) to 20,000 feet (6,000 meters). Because these clouds are lower than the high-flying type, they can consist of both water droplets and ice crystals. The "alto" clouds fall in this category: altostratus and altocumulus.
The altostratus clouds are similar to cirrostratus except that they are thicker and lower. More of the sun or moon will be obscured. Unlike cirrostratus, these clouds do not produce halos and obscure enough light to produce few, if any, shadows on the ground.
There is a proverb that says "Every cloud has a silver lining."
Likewise, altocumulus clouds are thicker and lower versions of cirrocumulus. The tiny cotton ball appearance thickens into larger round masses. Because of the thickness, the sky appears gray rather than white. After the high cirrus clouds give way to this middle group of clouds, we know that rain is only a few hours away.
These clouds form at elevations below 6,500 feet (2,000 meters) and consist mostly of water, except during the winter when snow becomes a possibility. Stratus clouds, which are the fog clouds, fall into this category. When low stratus clouds begin to deliver rain, they are called nimbostratus. This cloud is dark, gray, and appears flat at the base. The storm has arrived when nimbostratus appears. The precipitation is steady, not showery. Usually, the rain will come down at a light to moderate rate, and it will last a good part of the day, or even longer. When nimbostratus appears, it's time to curl up with a big, thick Russian novel.
Sometimes, stratus clouds form rounded, puffy masses, and these clouds are called stratocumulus. These differ from altocumulus because there are larger round masses, and can make the sky appear very dark and ominous. These clouds are the most difficult to determine—whenever I can't exactly figure out what a particular cloud is, it turns out to be stratocumulus. It can be confused with other low cloud types. Stratocumulus will often form when a stratus layer is heated, and the atmosphere begins to overturn. That process of overturning from heating is called convection. It becomes an important factor in the next category of clouds.
First, though, let's look at a unique cousin of the cloud. Fog is a form of stratus cloud. Although all fog can creep along on little cat feet, many types of fog exist.
The basic mechanism that causes fog is simply anything that can bring the relative humidity up to 100 percent. Usually, when the air cools to the dew point, fog will roll in. That cooling may occur during a clear, calm night. The accumulated heat of the day will radiate from the earth, and the temperature near the ground cools. It could cool all the way down to the dew point. That type of fog is called radiation fog because it is brought about by radiational cooling—by cooling caused by the radiating of heat from the ground. This type of fog frequently forms in valleys, because there the wind has the best chance of being light. If the wind is gusty, that type of fog will not form because the atmosphere remains mixed, and will not likely cool as easily to the dew point.
"When clouds appear like rocks and towers,
Another type of fog occurs when warm air is brought over a colder surface. The warm air will be cooled on contact with the colder surface, and as its temperature lowers to the dew point, fog will develop. This fog often appears during a winter thaw when warm air streams over a frozen or snow-covered surface. Fog is often said to be a great snow-eater. But it doesn't come along and chomp away at the snow. The warm air, which contributes to the fog, melts the snow away.
Fog that develops over the ocean and in coastal areas often forms in a similar way. Warm air streams over a colder ocean surface. As soon as the air is cooled to its dew point, fog will shroud the ocean surface and adjacent shores. If the air is tropical with plenty of water vapor and a high dew point, it will not take much to cool it to its dew point. (Of course, if the water is cold, that also helps.) Early summer is a favorite time for coastal fog. This entire fog category is called advection fog because warm air is advected, or brought to, colder regions.
Clouds with Vertical Development
So far, we have looked at clouds that can be found at specific levels of the atmosphere. There are others that can be found extending through all elevations. These are the ones associated with strong upward atmospheric currents. These updrafts spread the moisture through a large column of the atmosphere, and the clouds appear to have a puffy, even tower-like structure. Convection plays a big role in delivering these updrafts. In fact, these clouds are often called convective clouds. The rain that falls from them is frequently referred to as convective precipitation. Just like a boiling pot of water, the atmosphere can cook on a hot summer's day, causing these convective clouds to appear. The precipitation is often heavy, but not necessarily long-lasting. As they say, "The sharper the rain, the shorter the shower."
The basic cumulus clouds fall into this category. Sometimes these puffy clouds are limited in vertical development. They look innocent enough and take on different shapes. These are fair-weather clouds. Just the normal heating of the day is enough to set the stage for these clouds to pop up overhead. But on other occasions, the upward motions are large, and the cumulus clouds develop towers, which can cluster and grow into a full-blown thunderstorm within an hour. Those towering cumulus clouds that deliver rain, lightning, and thunder are called cumulonimbus. They can extend from just a few thousand feet above the ground to levels of 50,000 feet or higher. The bigger they grow, the more violent the weather becomes. Hail will often fall from these clouds. Even tornadoes are possible when cumulonimbus clouds appear.
Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.