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Clouds
Article and Photos



Left: Cloud Pictures. Middle: Cirrus, cirrocumulus, circumhorizon arc. Right: Whale and baby swimming by in sky.

Clouds are cool, and often add depth and ambiance to an image. Over the years, I've unintentionally developed quite the library of cloud photos. Hence the inspiration for this page, which shows photos (most are my own unless otherwise referenced) and gives brief descriptions of the ten major cloud types. I also give photos of some other cool clouds and optical phenomena that fall outside of these major categories.

Scroll down the page or jump to specific sections via the links below.

Thanks to Doug McKeever for his help identifying the clouds in the photos.

10 MAJOR CLOUD TYPES

High-Level Clouds Mid-Level Clouds Low-Level Clouds Vertically-developed Clouds Other clouds and phenomena
1. Cirrus (Ci)
2. Cirrostratus (Cs)
3. Cirrocumulus (Cc)
4. Altostratus (As)
5. Altocumulus (Ac)
6. Stratus (St)
7. Stratocumulus (Sc)
8. Nimbostratus (Ns)
9. Cumulus (Cu)
10. Cumulonimbus (Cb)
a. Contrails
b. Fog
c. Mammatus
d. Lenticular
e. More Oddball clouds
f. Optical effects

High-Level Clouds

(The following is excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, PhD)

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.


1. Cirrus (Ci) (High-Level)
Cirrus clouds form above 23,000 feet (about 7,000m), in the cold region of the troposphere. They are denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent. Isolated cirrus clouds often do not bring precipitation, however, large amounts of cirrus clouds can indicate an approaching storm system and cool and fair weather. There are several variations of cirrus cloud.

Cirrus clouds above cumulus humilis clouds, Wind Rivers, WY. (I took this photo when I was 11 yrs old on a family backpacking trip.)

Wispy cirrus clouds caused by winds high over the North Cascades (often called "mare's tails" or "horse tails").

Cirrus clouds in the North Cascades (and a contrail upper left).

2. Cirrostratus (Cs) (High-Level)
Cirrostratus clouds consist of a continuous, wide layer of cirrus that covers a large area of the sky. They are formed when moist air cools to saturation at a high altitude, forming ice crystals.

Cirrostratus clouds (and halo).

Cirrostratus clouds (and contrails) over Joshua Tree NP.

Cirrostratus clouds above Mt. Blanc, France.

3. Cirrocumulus (Cc) (High-Level)
Cirrocumulus clouds form when moist air at a high altitude reaches saturation, creating ice crystals. Instability at the cloud level gives the cloud its cumuliform appearance. Cirrocumulus clouds are not too common.

Cirrocumulus clouds (and also cirrostratus) above the Alpine Lakes Wilderness, WA.

Cirrocumulus clouds above Stanford University, CA. (Could potentially be altocumulus, which are lower; it's hard to tell how high they are.)

Cirrocumulus clouds against a brilliant dawn sheet of cirrostratus, over the Alpine Lakes Wilderness, WA. The rippling is classic cirrocumulus (forming "mackerel sky"), which form due to episodes of wind shear, either directional or speed related with height.

Mid-Level Clouds

(The following is excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, PhD)

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.

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.


4. Altostratus (As) (Mid-Level)
Altostratus clouds form when a large lifted air mass is condensed, usually from a frontal system, and can bring rain or snow.

Winter light on altostratus clouds over Mt. Shuksan and Mt. Baker, WA.

Altostratus clouds (with a bank of low cumulus) at Death Valley, CA.

Altostratus clouds inflamed by the sunset.

Altostratus undulatus in the North Cascades, WA. Photo by Doug McKeever.

5. Altocumulus (Ac) (Mid-Level)
Altocumulus clouds are not usually associated with a front but can still bring precipitation.

Altocumulus clouds over the North Cascades, WA.

Altocumulus reflected in tarn with altostratus at top of image, North Cascades, WA.

Altocumulus clouds and midair snowboarder at Mt. Seymour, BC.

Low-Level Clouds

(The following is excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, PhD)

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.


6. Stratus (St) (Low-level)
Stratus clouds are horizontal layer-like clouds having a uniform base, which is associated with widespread precipitation or ocean air, and often produce drizzle.

Drizzle in the Treasure Lake area of the Sierras, CA.

A bank of stratus clouds in the North Cascades. This is what they look like from above!

Stratus clouds (with crepuscular rays) in the San Juan Islands, WA.

An unusual variety of stratiform cloud: the dark wisps are stratus fibratus clouds and the gray clouds behind are typical stratus clouds.

7. Stratocumulus (Sc) (Low-level)
Stratocumulus clouds are lumpy, layered clouds often following a cold front, and they can produce rain or drizzle.

Sunset colors on stratocumulus clouds over the North Cascades, WA.

Stratocumulus clouds over Mt. Challenger, North Cascades.

Stratocumulus in the North Cascades as seen from above on the summit of Mt. Fury.

8. Nimbostratus (Ns) (Low-level)
Nimbostratus clouds tend to bring constant precipitation and low visibility.

Nimbostratus clouds (?) over Squamish, BC. Thick clouds with low cloud bases directly overhead of the observer can be difficult to recognize unless you are in a thunderstorm, which indicated they are indeed cumulonimbus clouds.

Nimbostratus clouds over the San Juan Islands, WA.

Nimbostratus clouds (and rainbow) in the North Cascades, WA.

Vertically-developed Clouds

(The following is excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, PhD)

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.


9. Cumulus (Cu) (Vertical development)
Cumulus clouds are sometimes called fair weather clouds but can grow into more storm-condition clouds (cumulonimbus, for example), and continued upward growth suggests showers later in the day. "Cumulus" means "heap" in Latin, and all cumuliform clouds show vertical development. Cumulis humilis are no higher than their base width, cumulus congestus grow vigorously to heights several times their base width, while cumulonimbus clouds really get out of control!

Cumulus humilis (fair weather cumulus) below cirrus clouds, Wind Rivers, WY. (I took this photo when I was 11 yrs old on a family backpacking trip.)

A variety of cumulus called cumulus castellanus ("castles"). Cumulus castellanus indicate instability and often develop into cumulus congestus (towering cumulus) or even into cumulonimbus ("thunderclouds"). It often rains not too many hours after seeing clouds like this. (Photo also shows crepuscular rays). Taken over Saturna Island, BC.

Orographically-produced cumulus clouds over the Grand Tetons, WY. Orographic refers to an effect of mountains on rising air. As air is forced to rise over the mountains or high hills, it cools due to expansion and is more likely to form clouds.

Scattered short-lived cumulus over the Matterhorn. As is common in the mountains, by mid-day these clouds had grown much larger (see next photo).

Unorganized cumulus with a banner cloud on the left side of the upper Hornli Ridge. Clouds like this are so common in the mountains due to variable wind and convection coupled with variable temperature and moisture content of the turbulent air. This photo was taken not long after the previous photos.
    
Cumulus humilis growing into cumulus congestus (blow up on right) over some foothills in Switzerland. Note how the cumuliform clouds are well developed over the hill and not over the lake. This indicates there is an anabatic wind (wind blowing upslope). As an air parcel rises, it cools adiabatically (due to expansion into an area of lower pressure at higher altitude). When the parcel cools to the dew point, the air becomes saturated and the cloud forms. If the air within the cloud is warmer than the air outside the cloud at the same altitude, the cloud continues to rise due to being less dense. This image subtly illustrates the concept of stability and instability rather nicely and explains why it is very often cloudier over mountains than over lowlands and valleys.

10. Cumulonimbus (Cb) (Vertical development)
Cumulonimbus are the giants among clouds.They can grow to immense size, commonly reaching heights of 30,000' or even 50,000' in the tropics. Violent weather including thunder and lightning, gusty winds, strong downdrafts (microbursts), intense rain or snow showers (cloudbursts), hail, and in severe thunderstorms including supercells, even tornadoes can occur.

Thunderheads developing near Mt. Shasta, CA.

Cumulonimbus incus (incus = "with anvil"). Photo by Steve Maciejewski.

Cumulonimbus clouds behind Little Tahoma, Mt. Rainier. Photo by Doug McKeever.

Cumulonimbus clouds over Goat Rocks Wilderness, WA. Photo by Doug McKeever.

Other clouds and phenomena

Not all clouds fall within the 10 categories described above.

a. Contrails (other)
Contrails (short for "condensation trails") or vapor trails are basically artificial clouds that are the visible trails of condensed water vapor made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals. Depending on moisture conditions in air at altitude, contrails may be visible for only a few seconds (indicating low relative humidity aloft), or for an hour or more (indicating high relative humidity).

Contrails (and cirrostratus clouds) over Joshua Tree NP.

Contrail (and cirrus fibratus intortus clouds) over Mt. Baker, WA. The contrail appears to be sheared out a bit by differential westerly winds aloft.

Contrail (and halo and cirrocumulus clouds).

Contrails caused by condensation of engine exhaust. The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor; the local increase in water vapor can push the water content of the air past saturation point, forming contrails. Photo by Peter Mackenzie.

b. Fog (other)
Lots of kinds of fog, which is essentially a cloud in contact with the ground. Some examples below.

Advection fog over the San Juan Islands, WA. Advection fog forms when the temperature of moist air drops as it moves over a cooler surface.

This is upslope fog, formed by near-surface moist air moving up to higher elevations. As the air rises it cools adiabatically, reaches the dew point and becomes saturated, whereupon fog develops. The upper clouds are stratus fractus ("fractured").

Steam fog on Lake Samish, WA. Photo by Doug McKeever.

Radiation (valley) fog and a marine layer over the Pacific NW coast.

c. Mammatus clouds (other)
Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air. These clouds often precede thunderstorms and sometimes even tornadoes.

Cumulus mammatus clouds over Death Valley.

Mammatus clouds can get really cool. This photograph taken by Jorn Olsen shows spectacular mammatus clouds over Hastings, Nebraska.


d. Lenticular clouds (other)
Lenticular clouds, technically known as altocumulus standing lenticularis, are stationary lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction.

Lenticular altocumulus cloud (or "altocumulus lenticularis" or "cap cloud") over Mt. Shasta, CA.

Lenticular altocumulus cloud (or "altocumulus lenticularis" or "cap cloud") over Mt. Shasta, CA.

Lenticular cloud over the summit of Mt. Baker, WA (aerial photo).

Lenticular clouds above clouds (aerial photo).

e. More Oddball clouds (other)
There are lots of cool oddball formations of clouds. The first half of the photos in the following section were taken by me, while the second half contains some REALLY cool oddball cloud photos that I found online (references given).

Fall streaks seen while climbing Mt. Shasta, CA. Fall streaks are caused by snow falling from a small cumulus cloud that sublimates before reaching ground. This is called virga if it is rain that evaporates while falling (see next photo). Note the wind shear on the lower half of the fall streaks.

Sunset-lit Virga over the San Juan Islands, WA. Virga is caused by rain that evaporates while falling.

Weakling banner cloud with fog at summit, positioned perfectly to obscure the view.

Now, that's a banner cloud! A typical sight on the Matterhorn by mid-day.

An unusual variety of stratiform cloud: the dark wisps are stratus fibratus clouds and the gray clouds behind are typical stratus clouds.

Nacreous clouds are located in the stratosphere between 9 and 16 miles high. Their "mother of pearl" colors come from sunlight striking tiny ice crystals inside the clouds. Very low temperatures near -85o C are required to form the crystals, which is why nacreous clouds are seen mainly during winter over places like Alaska, Iceland, and Scandinavia. This photo was taken from the Antarctic Peninsula. Reference

Billow clouds above Mount Baker. Billow clouds are created from instability associated with air flows having marked vertical shear and weak thermal stratification. The common name for this instability is Kelvin-Helmholtz instability. These instabilities are often visualized as a row of horizontal eddies aligned within this layer of vertical shear.

Noctilucent cloud, Photo by Pekka Parviainen, on Astronomy of Ice in the Mesosphere website. Made up of crystals of water ice and located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi), noctilucent clouds are the highest clouds in the Earth's atmosphere. They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no evidence that they were observed before 1885. t is theorized that this increase is connected to climate change. Reference.
     

These three photos show a rare phenomenon called cirrostratus lacunosus ("having holes") or "Hole Punch" cloud. Left: From Mt. Washington, New Hampshire. Photo by Doug McKeever. Middle: A less dramatic version, over the North Cascades, WA. Photo by me (Steph Abegg). Right: A simply amazing "Hole Punch" cloud observed in Mobile, Alabama on December 11, 2003. There is an article about it on the NOAA website.

A leading hypothesis holds that the hole-punch cloud is caused by falling ice-crystals. The ice-crystals could originate in a higher cloud or be facilitated by a passing airplane exhaust. If the air has just the right temperature and moisture content, the falling crystals will absorb water from the air and grow. For this to happen, the water must be so cold that all it needs is a surface to freeze on. The moisture lost from the air increases the evaporation rate from the cloud water droplets so they dissipate to form the hole. The now heavier ice crystals continue to fall and form the more tenuous wispy cloud-like virga (explained above) seen inside and just below the hole. Water and ice from the virga evaporates before they reach the ground.




These photos show clouds produced by a sonic boom from an aircraft. In the first photo, rapid condensation of water vapor due to a sonic shock produced at sub-sonic speed creates a vapor cone (known as a Prandtl–Glauert singularity), which can be seen with the naked eye (photo and article found on Wikipedia). The second photo shows a turbulence effect. So cool!

A sequence of photos taken by my friend Mark Thomas of a hole in the clouds forming over Shasta during some high winds.

f. Optical effects (other)
Various interesting and beautiful optical effects in the atmosphere can be produced due to reflection, refraction, diffraction (rainbows, halos, coronas, and others), and slight variations in light speed due to temperature differences in closely-spaced layers of air (mirages). Below are some photos of some of the optical effects I've seen....

(A great website that shows and explains a whole host of cool optical effects is http://www.atoptics.co.uk/.)


Crepuscular rays (and cumulus castellanus clouds), Saturna Island, BC. Crepuscular rays, also called sun rays, are parallel columns of sunlit air separated by darker cloud-shadowed regions. The rays appear to diverge because of perspective effects. Airborne dust, salts, aerosols, water droplets, and air molecules themselves scatter the sunlight and make the rays visible. Reference.

Anti-crepuscular rays (and stratocumulus clouds). Both crepuscular and anti-crepuscular rays appear to diverge toward the viewer, but the difference is crepuscular rays radiate downward, whereas anti-crepuscular rays appear to diverge in an upward direction.

Crepuscular rays around sun while climbing Glacier Peak, WA.

  Cloud shadows are the inverse of crepuscular rays, and can make for dramatic effects.
  
These two photos show 22° halos on cirrostratus clouds. Halos are formed when sunlight passes through ice crystals. The 22° radius is caused by refraction through a given crystal shape. Reference.

(The right photo also shows a halo and cirrocumulus clouds.)

     
These three photos show sundogs, or parhelia. Sundogs are formed when sunlight passes through plate crystals in cirrus clouds. All crystals refract the sun's rays but we see only those that glint their light towards our eyes. They are the crystals that, to us, are 22° or more from the sun and at the same altitude. Their collective glints form sundogs. (Are Puny sundogs called sunpuppies?) Reference.

(The left photo also shows a couple of other cool optical effects: Circumzenithal arc, faint upper tangent arc, 22° halo, sundog, parhelic circle. Read about these at this Reference.)


Circumhorizon arc (and cirrus and cirrocumulus) at Yosemite NP. These are formed when sunlight passes through ice crystals. It is a very large halo and always parallel to the horizon. They are formed when the sun is very high in the sky - higher than 58°. Reference.

Rainbow (and cirrus clouds on left and edge of cumuliform cloud on right) over Moraine Meadows in Rocky Mountain NP. Rainbows are formed by refraction and reflection of sunlight by raindrops.

Double rainbow (and altostratus clouds) in the North Cascades, WA. Double rainbows are caused when light is reflected twice inside a raindrop. The secondary has a radius of 51º and lies some 9º outside the primary bow. It is broader, 1.8X the width of the primary, and its colors are reversed so that the reds of the two bows always face one another. The secondary has 43% of the total brightness of the primary but its surface brightness is lower than that because its light is spread over its greater angular extent. The primary and secondary are concentric, sharing the antisolar point for a center. Reference.

These photos show the phenomenon of shadows in the sky. The left photo, taken from the summit of Mt. Whitney, shows two forms of shadows: (1) The triangular feature is the shadow of the mountain on the skyline. Mountain shadows at sunrise and sunset are immensely long tunnels of unlit air, crepuscular rays in fact. From the summit, perspective effects cause the shadow to appear triangular regardless of the mountain's profile. You are standing at the top edge of the shadow tunnel and looking out along its length which can be several miles long. (2) The dark band on the horizon is the shadow of the earth on the atmosphere. Immediately above, where the air is lit, glows a pink band called the anti-twilight arch or "Belt of Venus". The pink arises from scattered and deeply reddened sunlight mingling with the deep blues of the sky. A similar effect can be seen at twilight too, as shown in the right-hand photo taken just after sunset in a wintertime North Cascades. Reference.
  
These two photos show sun pillars. The left photo is a faint sun pillar over the Gulf Islands, BC (photo by me). The right photo is a much more defined sun pillar, photographed by Joe Owen. A sun pillar is a vertical shaft of light extending upward or downward from the sun. Typically seen during sunrise or sunset, sun pillars form when sunlight reflects off the surfaces of falling ice crystals associated with thin, high-level clouds (like cirrostratus clouds). (The left photo also shows the reflection of the sun on the water, which is really multiple distorted images of the sun reflected from the ocean surface.) Reference.

Shadows of the trees on the clouds.

Faint circumzenithenal arc in moonlit sky. Circumzenithenal arcs are ethereal rainbows wrapped improbably about the zenith. Reference.
A glory (multiple-colored ring) surrounds an aircraft's shadow and my shadow. A glory is always directly opposite the sun and is formed when light is scattered backwards by individual water droplets. The right-hand photo also demonstrate a Brocken spectre, which is a magnified shadow of an observer, cast when the Sun is low on clouds below the mountain the viewer is standing on.
 Reference.

Primary rainbow, Secondary rainbow, Alexander's dark band (between), Supernumeraries (fringes inside primary bow). Read more here: http://www.atoptics.co.uk/bows.htm

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