SCIENCE1 - The Story of Clouds

Pat and I live atop a ridge overlooking Tucson and the Catalina Mountains and I spend a lot of time appreciating the view, including frequent fascinating cloud formations.  So this blog is my attempt to understand how clouds work, the various types of clouds, and how they change.  My goal was to take all the cloud photos myself, from my backyard; but that’s going to take a while longer as targets of opportunity.  I’ll post this article as is and update it later when I get more of my photos.  

A sweeping cloud panorama with Tucson in the distance at left and the Catalina Mountains at right.
(Courtesy of Bob Ring)

How Clouds Work

Clouds are made up of tiny colorless water droplets or ice crystals, sometimes a mixture of both.  The water droplets form around tiny particles of dust; sea salt; soot from wildfires, volcanoes, and fossil fuel burning; pollution; and certain plant bacteria.  These droplets are exceedingly small, averaging about 0.0004 inch in diameter.

The water and ice scatter all light, making clouds appear white.  Gray color in clouds occurs when clouds are so dense or thick that their top parts absorb most of the sunlight, making them dark on the bottom, or reflect sunlight upwards, so things aren't as bright below.  Storm clouds are the thickest clouds, and look the darkest from down below, though they still look bright white if we see them from above.  Gray color can also be caused by higher clouds casting their shadow on lower-based clouds.

Countless billions of water droplets inside clouds either grow on their own or join up with other droplets to become drops. Millions of drops form in clouds that can only hold so much water before they become too heavy to hold more and precipitation occurs as rain, snow, freezing rain, sleet, or hail. 

Amazingly, individual clouds may weigh hundreds of tons!  Clouds float in the sky because they are less dense than the surrounding air and are also supported by updrafts.

The shape of clouds is influenced by the wind.  Large fluffy clouds are formed when air moves vertically; layered cloud shapes are formed when air moves horizontally.

How Clouds Form.  Here is a simplified explanation of how clouds form: 

Clouds are formed from rising warm, moist air:  Air temperature generally decreases with altitude above the ground.  Water from the Earth’s surface moves into the air by evaporation and becomes water vapor.  Warm air can hold more water vapor than cold air.  Warm air rises because it is less dense than cold air. When the rising air is saturated with water vapor (dew point), further cooling causes the airborne water vapor to condense to form water droplets and clouds. 

There are three ways that clouds form in Earth’s atmosphere:

Some clouds form from Earth’s surface heating.  As solar radiation heats the ground and the air immediately above it, the warming air expands and becomes lighter and flows upward.  As the air rises, its temperature decreases and so does the amount of water vapor that the air can hold. This vapor rapidly condenses and soon clouds result.  

Some clouds form from sun-heated warm air rising from the Earth's surface.

Other clouds develop from air blowing over hills or mountains that is forced upward, cooling the air and causing water droplets to condense as clouds. 

Clouds can also form when cold and warm air masses collide; two different air masses can’t mix unless they are very similar in temperature and moisture content.  The less dense warmer air is forced upward where it cools and forms clouds.

Clouds Effect on Climate.  From an article from “How Stuff Works” by Jessika Toothman: 

“Clouds have many effects on our climate besides simply … [providing precipitation]. For example, they serve as barriers for heat moving both in and out of the Earth's atmosphere. Researchers estimate that clouds' current net effect on our planet's atmosphere is to cool it slightly. This is, however, something researchers are examining and modeling closely, as part of efforts to gather information on possible climate change.” 

“Clouds generally affect the temperature in two ways. Over the surface of the planet, clouds reflect about 20 percent of the incoming heat back into space. Clouds, water vapor and other atmospheric gasses also absorb about 20 percent of this incoming solar radiation. Low-level clouds reflect the greatest amount of heat, which is why we enjoy cooler temperatures during a cloudy day. Conversely, a cloudy night is warmer than a cloudless night because clouds also create a blanketing effect. Clouds partially absorb outgoing heat (such as the heat released in the evenings, as the ground cools) and reradiate a portion of that heat back towards the Earth's surface. High-level clouds typically absorb this outgoing heat.”

Types of Clouds

According to the World Meteorological Organization’s International Cloud Atlas, there are more than 100 types of clouds.  Meteorologists use a nomenclature system for differentiating cloud types (derived from Latin) that was proposed in 1802 by Luke Howard, a British manufacturing chemist and amateur meteorologist. 

Basic Cloud Types.  The many variations of cloud types can be grouped into ten basic types, on the basis of their general shape and the height above ground level at which they occur.  Without going into all the derivations from Latin, here are the nomenclature roots for the ten basic cloud types:

Clouds form in three basic patterns:

1.      Cumulus:  heaped or piled.

2.      Stratus:  sheets or layers.

3.      Cirrus:  curly or fibrous.

There are three other words that are associated with these three cloud patterns as prefixes or suffixes to designate the altitude range above the ground or to identify precipitation clouds.  The prefix cirro is used to designate high altitude clouds and the prefix alto is used to designate middle altitude clouds.  Nimbus is used as a prefix and suffix to designate clouds that produce precipitation.

The ten basic cloud types are shown in the figure below.  Note the low, mid, and high altitude ranges.  An explanation of each cloud type follows the figure.

The ten basic types of clouds are defined by their shape and height above ground.

Low-Level Clouds

Stratus:  The lowest forming of all cloud types.  Stratus clouds are flat, hazy, featureless clouds varying in color from dark gray to nearly white.  They give the sky an overcast appearance with horizontal layering that sometimes produces light rain or drizzle.  Stratus clouds can cover hundreds of square miles but are usually thinner than 1,500 feet of altitude.  They occur when large areas of air rise, usually due to an incoming front, or wind encountering a large air mass.  As the air is pushed up, it spreads out to form a layer or layers.  Fog is a stratus cloud that reaches down to, or forms on, the ground.

Cumulus:  Fair weather large brilliant white fluffy clouds, often seen on bright blue days, with a flat relatively dark bottom and distinct edges that resemble different shapes. Cumulus clouds can form in lines stretching over 300 miles. They can cover vast areas and may be broken or continuous.  Cumulus clouds are formed as air warmed at the surface begins to rise.  As the air is rising, some cooler air aloft is sinking around the clouds, keeping the air between clouds clear.  Generally formed a few hours after sunrise, cumulus clouds tend to dissipate before sundown as ground heating from sun decreases.

Stratocumulus:  Mixture of stratus and cumulus, low and lumpy.  Stratocumulus clouds are clumps or patches of clouds varying in color from bright white to dark gray.  Some parts are often darker than others. They usually have gaps between them, but they can also be joined together. Stratocumulus clouds can be diffused over wide distances, resembling cumulus clouds with less-defined edges. They usually form from a layer of stratus clouds breaking up. They are indicators of a change in the weather and are usually present near a weather front. 

Mid-Level Clouds

Altostratus:  Usually appear across the sky as a uniform gray layer of variable thickness.  Altostratus clouds most often take the form of a featureless sheet but can be wavy as a result of wind shear.  They can also be fragmented with clear sky visible.  Altostratus clouds are formed by the lifting of a large mostly stable air mass that causes water vapor to condense into clouds.  It can produce light precipitation.  Altostratus clouds usually arrive ahead of storms and can turn into a nimbostratus clouds.

Altocumulus:  Appear as sheets of little round clouds or as parallel striped clouds.  Altocumulus clouds feature shading on their textured surfaces.  They are formed from heated air rising from the ground or transformations of other clouds including altostratus, stratocumulus, or nimbostratus.

High-Level Clouds

Cirrus:  The highest of the high-level clouds; composed entirely of ice crystals.  Cirrus clouds appear as white or light gray, delicate, wispy strands or fans that often curve with wind.  They are formed when water vapor develops at high altitudes where the atmospheric pressure is low.  These conditions commonly occur at the leading edge of a warm front.  Because humidity is low at such high altitudes, these clouds tend to be very thin.

Cirrostratus:  May cover the sky with a very thin, generally uniform, milky blanket of ice crystals, allowing some weak sunlight and moonlight to filter through.  Cirrostratus clouds are the best of the high clouds at producing a halo effect around the Sun or Moon.  They are often seen 12 to 18 hours in advance of an unsettled weather system.  Cirrostratus clouds form as a result of slowly rising air.

Cirrocumulus:  Usually create patterns of white patchy cotton balls in the sky.  Cirrocumulus clouds may also form in cloudlet bands, creating a wavy appearance. They are formed from sun-heated air rising from the ground.

Rain-Producing Clouds

Nimbostratus:  A vertical stratus cloud that appears as a low, gray, often dark, formless, nearly uniform cloud that usually produces continuous rain, snow, or sleet and no lightning or thunder - usually produces precipitation over a wide area.  Nimbostratus clouds can creep from low-level well into mid-level heights.  They occur along a warm front or when a cold front overtakes a warm front, where the slowly rising warm air mass creates nimbostratus.

Cumulonimbus:  A dense, towering vertical cloud, formed from water vapor carried by powerful upward air currents.  Cumulonimbus clouds produce rain and hail.  They may be referred to as thunderheads.  Cumulonimbus can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes. The tops of cumulonimbus clouds may reach 60,000 feet or more and be sheared off by strong winds aloft forming an anvil shape.

Stratus and altostratus clouds in an approaching front over Tucson in the distance at left.
(Courtesy of Bob Ring)

Cumulus clouds over the Santa Catalina Mountains northeast of Tucson in the distance at left.
(Courtesy of Bob Ring)

Cirrus clouds photo from the internet;  Iwill replace later with my own photo.

Special Clouds.  Here are a few rare, unusual, or in my opinion just interesting cloud types. 

Sunsets/Sunrise:  At sunset and sunrise, sunlight takes a much longer path through the atmosphere than during the middle part of the day when the Sun’s rays have a higher “incidence angle” with the atmosphere.  During this longer path, the atmosphere scatters out of the light beam much more of the violet and blue colors in the light spectrum than it does orange and red, thus the light that reaches our eye is noticeably reddened.  Clouds catch the last red-orange rays of the setting sun (or the first red-orange rays of the rising sun) and reflect this light to the ground.  Some of the most beautiful sunsets and sunrises feature mid-level or high clouds that cover a large portion of the sky except for a narrow clear strip near the horizon.  High altitude cirrocumulus clouds are often present in beautiful sunsets and sunrises.

Lenticular Clouds:  Lenticular clouds form in stable air along mountain ranges as a result of an area of turbulence below.  They are stationary, hovering in the strongest winds.  They can occur at altitudes up to 40,000 feet, and are shaped like a lens or saucer, often looking like a flying saucer.  Lenticular clouds occur in three main types:  altocumulus, stratocumulus, and cirrocumulus.

One of my favorite photos:  Two lenticular clouds over the Tucson Mountains at sunset.
(Courtesy of Bob Ring)

Contrail Clouds:  Produced by the exhaust of jet planes, form in the wake of the exhaust when the upper atmosphere’s cold air freezes the vapors in the exhaust.  Contrails appear as a white stream following the jet aircraft; they usually fade quickly after the jet passes.

Noctilucent Clouds:  Very thin, fine cloud-like phenomena that occur at altitudes higher than any other cloud in the atmosphere:  250,000 to 300,000 feet.  Noctilucent clouds are too faint to be seen in daylight, and are visible only at twilight when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in Earth’s shadow, exhibiting eerie blueish-white colors.  These clouds, with the first confirmed record of observation in the 1880s, are not well understood; possible contributing factors include:  volcanic eruptions, space vehicle booster exhausts, pollution, and meteoroids.

Noctilucent clouds over Kuresoo bog, Wiljandimaa, Estonia. 
(Courtesy of Wikimedia)

A Final Word

It is not uncommon to have five or six different cloud types visible at the same time.  The most varied cloud conditions are when thunderstorms occur over mountains, when a trained weather observer might be able to identify 10-20 different cloud types and sub types at any given moment.

It’s also not uncommon to have no clouds in the sky.  Clear skies occur when air is moving downward instead of rising, usually in a high pressure situation, or when the air is simply too dry to form water droplets.

Clouds form, dissipate, and change from one type to another constantly, depending on atmospheric conditions.  Slight differences in wind speeds, temperature, moisture, updrafts, and pressure systems moving over and under each other can produce different clouds in the same region.  The clouds we see at one place might have been formed at a different place and traveled long distances due to upper winds that might be blowing from a different direction.  You can sometimes see one layer of clouds moving in a different direction (or speed) than another layer of clouds.  As weather patterns progress, the air mass might reach a point where the clouds dissipate. It's only a matter of time before that water vapor joins another cloud, and the process begins again.

If you want to know more about the different cloud types or weather, the following four sources have all kinds of good data and photographs:  World Meteorological Organization’s International Cloud Atlas, The Book of Clouds by John A. Day, The Cloud Collector’s Handbook by Gavin Pretor-Pinney, and National Geographic’s Pocket Guide to the Weather of North America, by Jack Williams.

Personal note:  After this fun exercise, I will never look at clouds in the same way!  Cloud spotting and identification is way more complicated than I thought; I’ll be honing my skills for a long time.