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A clear view of clouds

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Clouds are indicators of atmospheric conditions. Identifying and understanding their formation can make flight operations safer.


By Karsten Shein
Comm-Inst. Climate Scientist

Altocumulus standing lenticular clouds above Torres del Paine, Chile in December 2020. Lenticulars form as part of atmospheric waves generated by air flowing over terrain obstacles such as mountains. Such clouds indicate strong to extreme turbulence.
For millennia, people have looked to the sky for an indication of what weather may be approaching. While it is likely that ancient civilizations recognized that different types of clouds were associated with different types of weather, there are few examples of clouds being given different names based on their characteristics until the 1800s, when scientists created a taxonomy of clouds to distinguish them, by shape, altitude, and even vertical extent.

This allowed meteorologists to categorize similar types and explore their formative processes and associated weather. Until relatively recently, many people believed that clouds were solid objects floating in the sky. Fog was not considered a cloud, even though one might walk through a fog on a mountainside that appeared as a cloud to someone in a valley below.

Early balloon flights and experiments with water changed that perception. These determined that not only were clouds not solid masses, they were made up of water droplets that would form and dissipate as temperature, humidity, and pressure around them changed.

While individual water droplets were only visible under the microscope, there were enough of them scattering light that the effect was a visible mass of water.

Cloud formation

Although it may seem like clouds are everywhere, it is difficult for the atmosphere to form clouds, especially in clean air.

At any given time and place, there is a constant process of evaporation and condensation taking place. A water molecule will transition between liquid and gas frequently during its time in the air, and any time the air is unsaturated (which is most of the time), there is more evaporation than condensation, so any water that condenses will reevaporate quickly.

This means the odds are stacked against cloud formation. What changes that and allows clouds to form when the environment is less than 100% humidity is the presence of aerosols and tiny particulates in the air, such as sulfates, dust, and sea salts.

These aerosols attract condensing water, gathering many into microscopic droplets and creating a solution that has a lower evaporation rate than a droplet of pure water. With such aid, liquid water molecules can exist in air with relative humidity as low as 75%.

Cumulus including a distant cumulonimbus beneath an altocumulus deck, suggesting a humid atmosphere with the potential for storms and turbulence at all levels.

With evaporation slowed and the solution continuing to attract condensed molecules, the droplet grows. The growth itself reduces evaporation from the droplet, until it reaches a diameter of around 0.02 mm, when it is large enough to be considered a cloud droplet. At this diameter, the droplet will effectively scatter the light it intercepts.

As more and more droplets of this size are present, light scattering becomes sufficient to be apparent to the human eye. It doesn’t take a lot of droplets to reach that level. In fact, around 15 droplets per cubic centimeter (cc) can be enough to create a haze.

Most thicker clouds, however, have droplet densities from 250 for stratus clouds to around 1300 for cumulonimbus. That’s less than 0.001% of the air’s volume occupied by water, even in the thickest clouds. The volume in ice crystal clouds is far lower, with a cirrus cloud becoming visible with fewer than 1 crystal per cc.

Generally, this low density of water droplets means that even thick clouds are unlikely to produce airframe or intake icing, but, any time liquid water is present and the temperature is below freezing, ice accretion should be anticipated. This goes for cloud droplets as well as rain drops.

Clouds and their meanings

All clouds are categorized by their altitude and by their vertical development, and may be further distinguished by their shape or behavior. In terms of vertical development, clouds can be either stratiform or cumuliform. Stratiform clouds have limited vertical development, and are formed in generally stable air as humid air is forced to rise and cool, but is unable to achieve buoyancy and rise of its own accord.

This is often the case ahead of warm fronts or along coasts with a cool onshore flow of moist air. Thus, stratiform clouds are characterized primarily by horizontal development.

Conversely, cumuliform clouds are the product of unstable air that rises and cools through free convection, and so they are more vertical than horizontal in their growth.

Secondary categories

Secondary categories apply to the altitude at which the cloud forms – high, middle, or low altitude. Thus, there are 6 major categories of cloud. Clouds forming above 20,000 ft in the tropics to 10,000 ft at the poles are high clouds, and, with the exception of cirrus, are given the prefix “cirro.” High-level clouds are good indicators of approaching weather and high-altitude moisture and turbulence.

Cirrus, cirrostratus, and cirrocumulus are ice crystal clouds, as they almost always form above the freezing level. Cirrocumulus decks are often described as a popcorn sky, and come from small atmospheric waves created by approaching convective weather.

Cirrocumulus may occasionally produce virga. Cirrostratus is produced by an intrusion of moisture into the upper troposphere, as frequently happens ahead of a warm front. Rather than individual clouds, cirrostrati appear as a wide area of thin, almost transparent cloud that, due to the presence of ice crystals, may produce halos around the sun or moon.

Cirrus is a third subtype of high-altitude cloud that doesn’t fit the stratus or cumulus categories, despite sometimes originating as one of those cloud types. High winds aloft shear the cloud to create a streaky appearance with wispy tails due to strong and turbulent winds aloft.

The ragged tails occur downwind and can give a pilot an indication of wind direction and severity of turbulence aloft. In addition, cirrus clouds have often been sheared from the anvil of an approaching thunderstorm. Mid-level clouds are stratus or cumulus with bases from around 6500 ft up to the high level.

In the summer, they may be a mix of water and ice, but in winter they are usually ice clouds. Higher temperatures in this level allow for more moisture and thus thicker clouds than those at the high level. Like cirrostratus, altostratus clouds form as moisture penetrates the area, often due to warmer air riding over a warm front.

Cumulus congestus, towering cumulus, and cumulonimbus over Panama at sunset. Convective clouds and embedded storms are common in many parts of the tropics. The presence of large cumuli often indicates the potential for storms, icing, and turbulence.

Altostratus is usually translucent,, with the sun or moon appearing blurry or washed out. Similar to cirrocumulus, altocumulus frequently covers large areas of sky and usually forms as a result of atmospheric waves moving through the middle atmosphere ahead of fronts or downwind of terrain obstacles.

Lenticular (lee wave) clouds are a type of altocumulus (altocumulus lenticularis). Often, the waves form the clouds into rows, or “cloud streets,” with clear air between them. Altocumulus clouds are indicators of moderate or greater turbulence aloft and may precede a frontal squall line.

Altitudes at or below lee wave cloud formations should be avoided, as they may be accompanied by severe turbulence, especially closer to the terrain that produced them. The lowest level of cloud is a type often associated with adverse weather.

These are clouds that form within 6500 ft of the surface, known as the stratus layer. Given that this is the warmest and most humid level of the 3, with ample heating from the surface to produce convection, clouds here are more common than those higher up. They tend to be opaque and may even block enough light from above that their bases appear a dull gray.

Because it would otherwise be redundant, stratiform clouds in the low level are simply called stratus. Stratus forms a thick sheet of cloud (stratocumulus) as warm and humid air is lifted in stable air, such as by a warm front or where humid air flows onshore or uphill along gradually rising terrain.

Stratus is termed fog if the cloud contacts the ground or near-surface visibility drops below 0.5 sm (~1 km). In some cases, usually near to a warm or stationary front’s surface boundary, the uplifted humid air saturates a deeper layer and the stratus can thicken sufficiently to where cloud droplets coalesce into rain droplets (or, in colder air, ice crystals into snowflakes).

These clouds are termed nimbostratus to signify a precipitating stratus cloud. Although they have some vertical development, nimbostratus clouds are still usually only a few thousand feet thick and produce widespread and steady rain showers.

Like cirrocumulus and altocumulus, stratocumulus clouds are sheet clouds that form over a wide area as a result of limited convection, often in the warm sector of a low pressure system ahead of a cold front. There is usually some clear air between them, but the deck is more or less continuous and can be thicker in places where a “hot spot” has enhanced convection or moisture available for the cloud to grow vertically.

In some cases, stratocumulus can produce brief rain showers, and low-level atmospheric waves pushing ahead of a front can form the stratocumulus into cloud streets.

Vertical clouds

Sheet of stratocumulus extends to the horizon, with occasional hot spots where heat and moisture build the clouds to higher levels. If these clouds are able to break through the temperature inversion, they may develop into cumulonimbus. Light to moderate turbulence should be expected when passing through this cloud deck.

There is a special category of clouds for which vertical development may result in the cloud spanning several height categories. These are cumulus clouds that develop from free convection – the unimpeded rising of warm air into a colder atmosphere. The size and vertical extent of these clouds depend on the temperature and moisture of the rising air and the air through which it rises.

These clouds range from fair weather cumulus to supercell thunderstorms. Cumulus humilis are “fair weather” cumulus clouds that often dot the lower atmosphere.

They form when the warm and humid air rises convectively, but vertical development of the cloud is limited by a capping temperature inversion, dry air aloft, or limited surface moisture that can be drawn into the rising air current.

These clouds should be viewed as indicators of mild to moderate low-level thermal turbulence at lower levels. In the summer, these clouds also suggest that there is enough heat and moisture to fire up an afternoon airmass thunderstorm. A greater supply of heat and moisture produces larger clouds if the air at higher altitudes is favorable.

Cumulus congestus are thick clouds that may rise several thousand feet until the air either reaches an equilibrium temperature with the environment and stops rising, or sufficiently dry air aloft evaporates the cloud top as it builds. These clouds can produce rain showers and icing, and may hide embedded thunderstorms.

The largest clouds are towering cumulus. These clouds have enough energy and moisture along with a favorable environment to grow into the upper troposphere. Some towering cumulus can reach heights above FL600 in the tropics, but tops between FL200 and FL400 are most common.

Towering cumulus often develops from cumulus congestus decks, appearing as “turkey necks” to pilots flying above a cumulus deck. More so than cumulus congestus, towering cumulus clouds are frequently storms waiting to happen and have updrafts that can exceed 100 ft/sec along with moderate to severe turbulence.

If they develop an internal vertical circulation and begin to precipitate, they become cumulonimbus. If they produce lightning, they become thunderstorms. Towering cumulus and cumulonimbus should be avoided at all costs, with a minimum separation of 1 mile per 1000 ft of cloud top height.

Further, any cloud in the vicinity of the freezing level should be viewed as a potential for ice accretion, so it should be avoided. If clouds or precipitation can’t be avoided, flying in temperatures above freezing or below –15 °C is recommended to minimize the risk of icing.

Reporting clouds

Most cloud reporting in metars is provided in terms of ceiling and sky coverage, because automated ceilometers don’t generally distinguish cloud type. Sky cover may be reported in eighths or tenths, but is often provided in terms of category.

Thus, clear is SKC, less than 25% cover is FEW, scattered (26–50%) is SCT, broken (51–88%) is BKN, and overcast is OVC. Cloud base heights are given in hundreds of feet (or meters), following the coverage code. In some places where manual reports or augmentations are available, cloud type may be shown as a code, such as AC for altocumulus, SC for stratocumulus, or CI for cirrus.

A few cloud types are commonly reported, such as cumulonimbus (CB), thunderstorm (TS), towering cumulus (TCU), altocumulus standing lenticular cloud (ACSL), or cumulonimbus with mammatus (CBMAM) – an indicator of a severe storm.

The presence of thunderstorms can also be determined by reports of lightning (LTG). Satellite imagery is also a good source of cloud coverage information. A visible satellite image can show cloud streets, stratus, cirrus, and even the shadows and anvils of thunderstorms.

However, enroute pilot reports are often the most reliable source of cloud information, and it can be very helpful to fellow pilots if you include your observation of clouds – including cloud type – when you file a pirep. In this age of instant weather information at a pilot’s fingertips, it is easy to forget that simply looking out the window at the clouds can give pilots a lot of knowledge about the state of the atmosphere and what may be coming their way.


SheinKarsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and an NOAA and NASA climatologist. Shein holds a comm-inst pilot license.