Cloud

All of us are familiar with the beautiful and wonderful -- and occasionally menacing -- shapes of the clouds which flow across heaven in an endless ballet of textures, colors, and volumes. From fat, friendly-looking cumulus clouds on a warm summer afternoon to delicately-waving streaks of cirrus, from the marble summits and brooding underbellies of thunderstorms to the fine shadings of mist streaked with golden sunlight on an autumn morning, clouds are a daily part of our scenery and affect our everyday lives as well as the great cycles of the world's atmosphere in many ways.

There are many types of clouds, existing at different heights, forming different shapes and patterns, and each produced by different processes. By learning about clouds and the processes that form them, you can both increase your enjoyment of these marvelous, airy sculptures, and also become more proficient at 'reading' the information they have to tell you and making guesses about the current and near-future weather.

What clouds are and how they form

Clouds are visible masses of water droplets or ice crystals which occur anywhere from ground level to the upper layers of the atmosphere, close the the fringes of space. Most of the clouds we are familiar with -- leaving out fog, which is a cloud but will be described separately -- are woven from the interactions of water, wind, sunlight, and air masses. Their appearance depends on the processes that formed them, and their development is governed by the changes of the local atmosphere, which is the environment in which they exist.

There are several different processes that form clouds, but the underlying principle is the same for all. Air can only hold so much moisture before it becomes 'saturated' -- humidity is at 100% -- and the water begins to condense out of it into droplets or ice crystals, depending on the height and temperature. Warm air can hold more water vapor than cold air can, so the basic mechanism that begins to 'squeeze' visible water droplets or ice crystals out of the air is the cooling of a mass of air.

You can see this in the winter when you exhale outside and a cloud of steam streams out of your mouth momentarily. This is because the hot, moist air you're breathing out is cooling suddenly, and can't hold all the water vapor that your lungs put into it. So that water condenses out of the air into droplets, or visible steam. Your breath is on such a small scale that this water quickly dissipates into the huge mass of air around it, but when the water vapor and the cooling is on a much more massive scale, clouds can form.

One of the main ways that air is cooled is by rising or being lifted. Since the higher the air is, the less weight is on it -- since less of the rest of the atmosphere is resting on top of it -- it expands as it rises, filling more space with the same amount of air molecules. With lessening density, the air molecules also become farther apart and the air can't hold heat like it did when it was more compressed, so it also cools as it rises. Eventually, it cools enough so that the water vapor condenses out into clouds.

How many clouds are formed, what their height above the ground is, and what type are created by the process are all determined by a complex interaction of overall temperature, the amount of water vapor available in the atmosphere, wind, conditions at different levels of the atmosphere, season, weather patterns, temperature differences at various heights, and so on. Although it needs a trained meteorologist to understand all the possible variables, there are several main ways that clouds form that we can look at more closely here.

• Clouds can form through convectional lifting, which happens when the sun heats the earth and the air just above it enough for a mass of warm air to start to rise. This air streams skyward, and as it does, the water vapor in it condenses into clouds. If the air mass is very dry, you may only see a few small puffs of cumulus cloud form. If there's a bit of moisture, then the classic fair-weather cumuli will dot the sky, gleaming white in the sun against the sapphire dome beyond. If there is a lot of moisture, like on a hot, sticky, humid summer afternoon, the clouds may build into thunderheads, and scattered thunderstorms will move across the landscape -- typically in the late afternoon, when the air has had a long time to be heated by the sun and is fountaining skyward strongly, creating the updrafts needed for a thunderstorm. These thunderstorms often happen daily in the tropics, where humidity is intense and the sun is searingly hot year-round, but they also happen in the temperate summer in the interior of continents.
• The collision of air masses results in cloud formation through frontal lifting, where one air mass shoves in over another, or creeps in beneath it. Cold air masses tend to be heavy and their edge lies close to the ground, so they wedge their way in violently, shoving warm, humid air quickly skyward. This means that cold fronts usually have a band of very intense thunderstorms along their forward edge, and may be powerful enough to trigger dangerous storms hundreds of miles ahead of the main front as well. The edge is fairly sharp, though, so the cloud band usually isn't too deep unless the warm air mass puts up extraordinary resistance. Warm fronts are more buoyant and press in from above, squeezing the cooler air out gradually, and thus tend to be less violent. They are still lifted by the cool air as they push it out, however, with the warm, moist air 'riding up over' the cooler air mass, cooling, and causing clouds. Deep bands of rain clouds (nimbostratus) often mark a warm front.
•  Air lifted by mountains cools and can condense into clouds in a process called orographic uplift. This is why there are huge amounts of snow in the westward-facing Sierra Nevada range in California, while to the east of the Rockies, the land is dry and even desert. The air there is lifted, condensed into clouds and then into rain or snow, and 'wrung dry' before it moves further east. This phenomenon of dry, rainless areas on the downwind side of a mountain range is called the "rain shadow."
• The exception to these general rules of air rising, cooling, and condensing out water vapor into cloud is radiative cooling, when the sun sets and the ground and air touching it begin to cool rapidly through longwave radiation. The air isn't rising, but it cools rapidly and as a result any water vapor in it soon condenses out. If there's enough moisture in the air, this leads to the formation of fog.

If the condensation is strong enough, the droplets will merge until they form drops too large to be help up by the updrafts of the cloud. These drops then fall as rain, or crystallize into snow.

Now that we have a good idea of how and why clouds form, it's time to take a closer look at the many other interesting aspects of clouds -- including what the different kinds are and what they mean, what effect clouds have on the overall weather of our world, and the many other fascinating things we can learn about clouds.

High, wispy clouds -- the cirrus family

These clouds form high in the atmosphere, where the air is thinner than at the ground and the fierce cold condenses moisture into sparkling ice crystals that trail out into shimmering, gauze-like forms of great delicacy and beauty. These clouds form at between three and eight miles into the sky in the temperate latitudes, but in the tropics, they may form as high as twelve miles above the earth. The usual types are cirrus, cirrocumulus, and cirrostratus.

• Cirrus clouds are the long, slender streamers that are sometimes called "mare's tails" -- they may be as fine as the most careful, delicate brush strokes or have a dense, almost shaggy appearance. These clouds may form under any conditions. If they are very thick, or get steadily thicker, they may well be foreshadowing the approach of storms -- moisture sprayed out into the upper atmosphere by mighty updrafts in the approaching weather front.
• Cirrocumulus clouds are masses of high cumulus clouds, mostly made of ice crystals but with a small amount of supercooled droplets as well. From the ground, they appear as a rippled sheet made up of many small puffs. These clouds are created when a sheet of cirrus or cirrostratus is affected by a light, warm updraft. This updraft melts some of the ice crystals into droplets and causes the cloud to develop vertically, bunching up into many 'cloudlets.' Like cirrus, they indicate that the high atmosphere is unstable, and further that updrafts are occurring at a considerable height, so they may foretell wet weather.

• Cirrostratus clouds resemble a fine veil of gauze stretched over the sky, without blocking the sunlight or moonlight. Sometimes, they can only be seen near the sun or moon as a glowing halo where light reflects from their surface; at other times, they are thick enough to be visible as a filmy layer over the sky. These clouds indicate a lot of moisture in the upper atmosphere, but are more likely to herald a warm front's approach than a cold front.

As can be seen from these descriptions, cirrus-type clouds don't cause ground-level weather effects themselves. Instead, they are more likely to be the signal of wet weather on the way in the near future. If you see these clouds, noting whether they are getting thicker or thinner over time will be a good indicator of whether they are indeed the forerunners of a storm, while observing their direction of movement will tell you which way the rain is likely to come from if it does.

Middle clouds -- more moisture, lower formation

The middle clouds, altocumulus and altostratus, occur anywhere from one to four miles above the Earth's surface. Being both more substantial in volume -- made up of water droplets rather than ice crystals, and so having vertical development -- and closer to most human observers, they have a much more robust look than the etherial, fairyland forms of the high clouds. Generally speaking, they don't cause rain or snow themselves, but they indicate large amounts of moisture through a deep layer of the atmosphere and may well mean that rain will arrive within a few hours or less.

• Altocumulus appear as large masses of individual flattened, but puffy, clouds, often arranged in rippling rows as if they had been arranged by the stroke of some colossal garden rake across the sky, like the gravel in a Japanese garden. They are often brilliantly white clouds with a small grey shadow on the underside. They are usually attendant on systems that produce thunderstorms, so seeing a mass of altocumulus flowing out of the west is a sign that an afternoon picnic or golf game might be suddenly interrupted by the onrush of a storm.
• Altostratus is a grey, sheet-like cloud that covers the sky like a stratus cloud, but is higher and somewhat thinner. The sun usually shows through it, and this is one of the ways you can distinguish it from stratus (one of the low clouds). Altostratus indicates both a lot of moisture in the atmosphere and the meeting of a warm, moist air mass and a colder, drier one. Rain, snow, or other frontal weather may follow on the appearance of altostratus, especially if the altostratus thickens over time. There is also some chance of the altostratus thickening and developing into rain clouds -- nimbostratus.

The color of these middle clouds is darker than the higher clouds, and their forms are thicker, heavier, and less delicate, making it easy to identify them as different with only a little practice. Like their higher counterparts, they indicate moisture in the atmosphere -- but also show more vigorous instability in atmosphere, greater amounts of water vapor, and a higher chance of rain or snow in the near future.

Low clouds -- wet weather on the way?

The lowest clouds are those which range from the level of the ground itself -- fog, mist, and haze -- to a little over a mile from the surface. These clouds include several of the smaller kinds of cumulus, as well as stratocumulus and stratus. Several of these clouds forms are closely involved with precipitation such as rain and snow, while others are simply the result of normal cloud development.

• Stratus is the most recognizable low-level cloud a sheet of white or grey clouds covering the sky, and thick enough to make the sun invisible unless a small chink of thinner cloud passes across it for a moment. They can produce rain showers, but if anything more intense than a light drizzle falls from them, they are classified as nimbostratus instead. These clouds are formed by the rising of moist air into a cooler air mass, resulting in condensation into clouds. Sometimes, they indicate coming rain, while at other times, they have no significance other than showing that there is a lot of moisture close to the ground.
• Nimbostratus looks very much like stratus, but is accompanied by rain or snow. The bottom of nimbostratus usually appears to be smoother than stratus, because falling rain or snow obscures the underbelly of the clouds. These clouds usually occur as part of a warm front during the spring, summer, or autumn, whereas cold fronts are usually marked by thunderstorms.
• Stratocumulus are lumpy or rolled masses of cloud, often with cumulus-like puffiness in some areas. They are created by a layer of wet, unstable air under a layer of dry, stable air that 'caps' their vertical development, and are often seen near large bodies of water such as the oceans or America's Great Lakes. These clouds can also indicate the approach of storms, although they can't produce more than light precipitation.
• Several small cumulus cloud varieties count as low clouds humilis and mediocris. Cumulus humilis are typical fair-weather cumulus clouds fluffy, white, and not very tall. Their small size and lack of height are what make them count as low clouds. They are indicators of continued good weather, since their size is being limited by dry, stable air aloft. Mediocris are larger and taller than humilis, with cauliflower-like tops and grey undersides. They indicate that vertical development is possible and might eventually grow and merge into a thunderstorm.

Vertical clouds fortresses of lightning and thunder

The towering shapes of cumulus congestus and cumulonimbus are the two most familiar kinds of vertical clouds. They are formed by huge updrafts that cause them to mushroom from low bases to enormous heights, and are able to support some of the most violent weather on Earth thunderstorms, lightning, hail, and tornadoes.

• Tall cumulus clouds are called cumulus congestus and can be four miles or more tall. If you watch a cumulus congestus on a summer afternoon as it towers, white and solid-looking, into the sky, you can often see the potent updrafts at work. The top of the cloud can be seen to be rising and expanding, bulging upwards and churning slightly from within. If this process is strong enough, the cumulus congestus can turn into a cumulonimbus, or thunderhead.
• The tallest clouds are cumulonimbus, or thunderheads the monarchs of the temperate sky. These huge towers of cloud can be smooth-topped (calvus) or form the classic anvil (incus). A calvus cloud will result in a thunderstorm, while an incus cloud has reached the stratosphere and is unable to rise further, and can develop into a supercell thunderstorm that can produce tornadoes.

Other clouds

There are yet more clouds that form in Earth's atmosphere beyond the types we've already looked at, although these other types are rare and often occur only under special conditions. These clouds are very interesting, but most people are unlikely to see them, and they often have little effect on the weather.

One of these odd cloud types is the pyrocumulus, which forms over large forest fires, volcanoes, and similar events. The most famous pyrocumulus is the massive mushroom cloud over a nuclear explosion. Forest fires can cause pyrocumulus clouds by creating violent updrafts of hot air, which raises moisture released from burning wood and leaves until it condenses into a cloud. This cloud can produce lightning which may further spread the fire, or it may rain and douse the very blaze that spawned it.

A far more serene and beautiful cloud is the noctilucent cloud, an extremely delicate cloud of tiny ice crystals that forms up to 50 miles above the Earth's surface and then may drift southwards. Its name means giving light at night, because it is so high that it may still glow in the sunlight long after the sun has set and the land has darkened below. These clouds are still rather mysterious to scientists, since they are difficult to study up close, except through the use of rockets.

Clouds the engines and results of weather

Clouds, as we have seen, both result from and create weather processes. They are created by the cooling of air that rises through various mechanisms, whether that rising is simply the result of sunlight on a warm meadow, the boisterous advance of a cold front, or the column of hot air spurting up over a major forest fire. This cooling causes moisture to condense out into drops, and if the condensation is strong enough, this leads to rain or snow, and brings those weather conditions to the Earth's surface.

Clouds are therefore both the result of the planet's weather patterns and their vehicle, carrying moisture from one place to another, shading the world from the sun's rays in some places by reflecting them back into space, keeping it warmer in others by holding in heat that would otherwise be radiated back into space after sundown. Regardless of the weather, the presence or absence of clouds is an important part of what defines it and what makes its processes work.