Every once in a while, you'll get a perfectly clear day — "not a cloud in the sky," as they say. While this makes for an ideal picnic setting, clouds have interesting stories to tell. Clouds are an integral part of the weather on earth. In many ways, they are the weather. Observant cloud watchers can even use them for forecasting.
How it all works
Clouds play a critical role in the Earth's water budget. Water evaporates primarily from the oceans and becomes water vapor in the atmosphere. To keep the cycle going, we need to get this water back down to Earth's surface. When air is cooled sufficiently, water vapor condenses into water droplets or ice crystals depending on the temperature. This is cloud formation. These droplets and ice crystals are so small that gravity is negligible and air currents keep them floating; therefore clouds don't fall to the ground. If the droplets or crystals become large enough, they fall in the form of rain, snow, or hail, thereby annoying some people but replenishing the Earth's water supply.
To produce the condensation that forms clouds, the air must be cooled. As pressure decreases with height, as air is lifted, it expands, and this causes a cooling effect. Cool air holds less water vapor than warm air, so eventually some of the water vapor condenses. Technically, condensation occurs when the air temperature reaches the dew point temperature.
The drop and lift
There are four major causes of "lifting" in the atmosphere and, therefore, four places we find cloud formation. "Orographic" lifting occurs when air is physically forced up over a topographic feature. Clouds are therefore prevalent in mountainous regions. When air heats, it rises. This is called "convection" and is common in many locations in the warm season. "Convergence" means that airflows meet or converge in one spot or along a line. If this happens near the surface, the air is forced to rise. Convergence is associated with low pressure areas, and lows typically have clouds.
"Fronts" are boundaries between relatively warmer and cooler air masses. With warm air being lighter, it typically rises over the cold air, and if moisture is present, clouds will form. Fronts typically have clouds due to this frontal lifting. Of course, once clouds form, they can move with the wind. Whether or not they dissipate (evaporate) depends on the amount of moisture in the air.
To a certain extent, you can use the clouds to forecast the weather. Long before there were professional meteorologists — and even today — people who spent a lot of time outdoors either developed this skill or it was passed down from previous generations. This is especially true of farmers, to whom weather is critical.
Observing sky conditions over time is the best way to develop this skill. Note changes in clouds. Are the clouds descending and getting thicker, indicating oncoming precipitation? Which way are the clouds moving? With experience, you can relate direction of movement with typical weather. Are clouds at different heights moving in different directions? This often means a front is near and weather changes are to be expected.
Class of its own
Scientists love to classify things. It was an amateur meteorologist, Luke Howard, who in 1803 devised the cloud classification scheme still used today. He classified clouds by their appearance and height. Latin terms were used to describe how different clouds look. Clouds in layers were called "stratus." Clouds with some vertical development that often look like cotton balls were called "cumulus." They are composed of liquid water droplets (liquid water can even occur with subfreezing temperatures). "Cirrus" from the wispy or fibrous appearance of clouds formed of ice crystals. Rain-producing clouds were called "nimbus."
Clouds can occur from near the Earth's surface to more than 12 miles above the surface. In Polar Regions, there are even clouds (called "nacreous" and "noctilucent" clouds) that occur 30 miles high. On the other end of the spectrum, fog is a cloud that touches the ground. It is made up of water droplets or ice crystals but usually is formed by cooling from below not lifting.
To classify clouds by height, there are three main groups. "Low clouds" (surface to 6,500 feet) include stratus, stratocumulus, cumulus, and cumulonimbus. "Middle clouds" (6,500 to 23,000 feet) include altocumulus, altostratus, and nimbostratus. "High clouds" (over 23,000 feet) are cirrus, cirrostratus, and cirrocumulus.
Time for some disclaimers. Mother Nature really doesn't care about man's classification schemes. Cloud heights vary with location and season. Some clouds typical for one height category can occur at other heights. Oftentimes, the clouds you'll see are "hybrids" — some combination of cloud types.
Identifying cloud types
So now, let's look at the basic 10 cloud types, starting with high clouds. We have cirrus, cirrostratus, and cirrocumulus.
Cirrus — ice crystal clouds — look like whitish wisps, and are very fibrous or filamentous. They often are surrounded by very blue sky and typically indicate fair weather probably for a few days.
Cirrostratus is a pale, whitish sheet of cloud that can cover the whole sky. The layer is thin and transparent. The ice crystals that make up cirrostratus can interact with rays of sunlight to produce optical phenomena like halos around the sun or moon. This cloud indicates an increase in moisture aloft, a precursor of more significant moisture and possible rain or snow in the following day or two.
Often occurring in a whitish patch or sheet, cirrocumulus has small but clearly defined individual cloud cells (the cumulus part). This is usually a fair-weather cloud.
Next, there are three mid-level clouds: altocumulus, altostratus, and nimbostratus.
Altocumulus occurs in patches or sheets, like cirrocumulus, but the individual cloud elements are larger and lower. Altocumulus itself indicates good weather at least in the near term.
Altostratus, on the other hand, often precedes precipitation by 24 hours or less in the winter. It is a grayish sheet that covers the whole sky. The sun can be seen vaguely through it, but there is no halo effect.
Nimbostratus is officially the cloud associated with heavy rain or snow. However, heavy precipitation often makes the sky not discernible at all. Actually, in these storm situations, the atmosphere is completely saturated to well aloft, and the whole layer contains cloud of some type. It's probably a stretch to say this is a separate cloud type, as it straddles the low- and mid-level cloud line.
Now to the low clouds: stratus, stratocumulus, cumulus, and cumulonimbus.
Stratus is a low, grayish sheet that covers the entire sky. If no more substantial clouds are occurring above it, at most it can produce light drizzle, rain, or snow.
A grayish or whitish patch or sheet of low clouds with large, prominent cloud elements is Stratocumulus. This can cover part or all of the sky. Depending on the vertical development and moisture availability, stratocumulus can produce showers of rain or snow.
Finally, we have the cumulus family of clouds. They are included with low clouds since their bases are at these levels. But they can develop vertically and even extend to high levels of the atmosphere.
Cumulus Humilis, or better known as "fair-weather cumulus," are your typical big, white puffy clouds. From the ground, they look like big balls of cotton. Looking down on them from a plane shows cloud tops that look like cauliflower. The tops are often bright white, but the bottoms can be dark due to blockage of sunlight. With limited vertical development, good weather is to be expected.
At times especially with the heat and humidity of summer, cumulus clouds will continue to develop vertically reaching heights of tens of thousands of feet above the ground. If a thunderstorm develops, the cloud is then called a Cumulonimbus. Officially, a cumulonimbus has a flattened top or anvil often made of ice crystals. However, even without this structure,a well-developed cumulus cloud can produce rain and even lightning. (These are often called "towering cumulus.")
The main cloud types also have subsets, variations of the particular cloud. For example, "mammatocumulus" appears as dark, rounded protrusions below a higher cloud deck. Although it is often linked with severe thunderstorms, it can in fact occur with fair-weather clouds, too. When including these variations, we can easily identify more than 100 different clouds.
Besides the primary cloud types, there are other clouds out there. "Lenticular" clouds are rounded or lens shaped. They are typically produced when the wind blows over a mountain or mountain range. They can move with the wind or get locked in place just on the lee side of the mountain. "Pileus" clouds are formed in a similar fashion, but in this case the wind actually blows over the top of another cloud. They typically form over cumulus-type clouds, especially thunderstorms. "Contrails" are man-made clouds, formed from the condensed moisture in the exhaust of jet engines.
Reading the weather
How important are clouds to meteorologists? For standard surface observations which are taken every three hours at thousands of stations around the world, cloud type and estimated height are always given. If you look at a fully plotted station model on a surface weather map, you will see symbols that stand for the various cloud types. ("Evil" meteorology teachers, like myself, used to make students memorize 30 different cloud symbols.) Cloud observations are even more important for aviation meteorology where cloud obstructions to vision pose great danger. Amount and height of cloud cover are reported frequently and are highlighted in aviation forecasts.
Ever wonder exactly what "partly cloudy" means? The NWS (National Weather Service) has official criteria for its sky cover forecast. Following longtime convention, the sky is broken into eighths, or "octants." Cloud cover implies opaque, non-transparent clouds. Clear or sunny means less than 1/8 of the sky contains clouds. "Mostly clear" or "mostly sunny" means 1/8 to 1/4 sky coverage. "Partly cloudy" or "partly sunny" (for you glass-half-full folks) means 3/8 to 1/2 coverage. "Mostly cloudy" or "considerable cloudiness" means 5/8 to 7/8 coverage. And "cloudy" or "overcast" means total sky coverage. The term "fair" is mainly used at night indicating less than 1/2 sky coverage.
If you want the official last word on clouds, the latest copy of the International Cloud Atlas, a product of the World Meteorological Organization, is available online at https://CloudAtlas.wmo.int/home.html. It was first published in the late 1800s and its most recent update was just last year. This is the first time a digitized version is out there free for everyone to see. Aside from basic information on clouds, there are more than 600 images to satisfy your cloud cravings. And, yes, they even added 12 new cloud types. These aren't new clouds, they've just been officially recognized.
And let's not forget about weather satellites. Although I don't have time to get into detail, since 1960, we've had a view of clouds from above. So far above, that we can make out entire storm systems, watching them evolve and move. Satellite meteorology has greatly improved our knowledge of the weather and our ability to forecast it. To check out the latest spectacular images from space, go to www.Weather.gov/satellite#geocolorGOESe.
For more than 30 years, Ed Brotak taught thousands of college students about the weather and helped hundreds of them pursue a career in meteorology. He lives in Asheville, North Carolina, with his wife (also a meteorologist) and his two daughters (who vow never to be "weather weenies"). Ed still goes outside when he hears thunder.