You can easily find beautiful photos of all the clouds mentioned below by pointing your webbrowser search engine at “cloud classification”, “cloud identification”, “cloud types”, or “International Cloud Atlas”. You can also use web search engines to find images of any named cloud. To help keep the cost of this book reasonable, I do not include any cloud photos.
Clouds that form in updrafts are called cumuliform clouds. The small and medium-size ones look like cotton balls, turrets on castles, or cauliflower. The largest cumuliform clouds — thunderstorms — have tops that look like an anvil or like the mushroom cloud of an atom bomb. The circulations that form cumuliform clouds have an aspect ratio of about one; namely, the ratio of cloud diameter to distance of cloud top above ground is roughly one.
Thicker clouds look darker when viewed from underneath, but when viewed from the side, the cloud sides and top are often bright white during daytime. The individual clouds are often surrounded by clearer air, where there is compensating subsidence (downdrafts).
Cumulus clouds frequently have cloud bases within 1 or 2 km of the ground (in the boundary layer). But their cloud tops can be anywhere within the troposphere (or lower stratosphere for the strongest thunderstorms). Cumuliform clouds are associated with vertical motions.
Cumuliform clouds are named by their thickness, not by the height of their base (Figure 6.3). Starting from the largest (with highest tops), the clouds are cumulonimbus (thunderstorms), cumulus congestus (towering cumulus), cumulus mediocris and cumulus humilis (fair-weather cumulus).
Cumuliform clouds develop in air that is statically unstable. This instability creates convective updrafts and downdrafts that tend to undo the instability. Cumuliform clouds can form in the tops of warm updrafts (thermals) if sufficient moisture is present. Hence, cumulus clouds are convective clouds. Internal buoyancy-forces (associated with latent heat release) enhance and support the convection and updrafts, thus we consider these clouds to be dynamically active.
If the air is continually destabilized by some external forcing, then the convection persists. Some favored places for destabilization and small to medium cumulus clouds are:
- behind cold fronts,
- on mostly clear days when sunshine warms the ground more than the overlying air,
- over urban and industrial centers that are warmer than the surrounding rural areas,
- when cold air blows over a warmer ocean or lake.
Cold fronts trigger deep cumuliform clouds (thunderstorms) along the front, because the advancing cold air strongly pushes up the warmer air ahead of it, destabilizing the atmosphere and triggering the updrafts (see Thunderstorm chapters).
Also mountains can trigger all sizes of cumulus clouds including thunderstorms. One mechanism is orographic lift, when horizontal winds hit the mountains and are forced up. Another mechanism is anabatic circulation, where mountain slopes heated by the sun tend to organize the updrafts along the mountain tops (Regional Winds chapter).
Once triggered, cumulus clouds can continue to grow and evolve somewhat independently of the initial trigger. For example, orographically-triggered thunderstorms can persist as they are blown away from the mountain.
Knowing the environmental sounding and a thermodynamic state of an air parcel starting near the ground, you can use a thermo diagram (Figure 6.4) to find cloud base (at the lifting condensation level, LCL) and cloud top (at the equilibrium level, EL). See the Atmospheric Stability chapter for an explanation on how to use thermo diagrams.
Use the sounding plotted here. Where is cloud base and top for an air parcel rising from the surface?
Find the Answer
Given: sounding above.
Find: Pbase = ? kPa, Ptop = ? kPa
First, plot the sounding on a large thermo diagram from the Atmospheric Stability chapter. Then conceptually lift a parcel dry adiabatically from the surface until it crosses the isohume from the surface. That LCL is at Pbase = 80 kPa.
However, the parcel never gets there. It hits the environment below the LCL, at roughly P = 83 kPa. Neglecting any inertial overshoot of the parcel, it would have zero buoyancy and stop rising 3 kPa below the LCL. Thus, there is no cloud from rising surface air.
Check: Units OK. Physics OK.
Exposition: This was a trick question. Also, the presence of mid-level stratiform clouds is irrelevant.
Stratiform clouds are horizontal cloud layers. They span wide regions (Figure 6.5), and have an appearance similar to a blanket or sheet. Stratiform clouds are often associated with warm fronts. Names of stratiform clouds, starting from the lowest, darkest layers, are nimbostratus, stratus, altostratus, altocumulus, cirrostratus, cirrocumulus, and cirrus.
Layered clouds are often grouped (high, middle, low) by their relative altitude or level within the troposphere. However, troposphere thickness and tropopause height vary considerably with latitude (high near the equator, and low near the poles, see Table 6-1). It also varies with season (high during summer, low during winter).
|z (km)||3 - 8||5 - 13||6 - 18|
|P (kPa)||70 - 35||54 - 16||47 - 8|
|z (km)||2 - 4||2 - 7||2 - 8|
|P (kPa)||80 - 61||80 - 41||80 - 35|
|z (km)||sfc. - 2||sfc. - 2||sfc. - 2|
|P (kPa)||Psfc - 80||Psfc - 80||Psfc - 80|
Thus, low, middle and high clouds can have a range of altitudes. Table 6-1 lists cloud levels and their altitudes as defined by World Meteorological Organization (WMO). These heights are only approximate, as you can see from the overlapping values in the table.
High, layered clouds have the prefix “cirro” or “cirrus”. The cirrus and cirrostratus are often wispy or have diffuse boundaries, and indicate that the cloud particles are made of ice crystals. In the right conditions, these ice-crystal clouds can cause beautiful halos around the sun or moon. See the last chapter for a discussion of Atmospheric Optics.
Mid-level, layered clouds have the prefix “alto”. These and the lower clouds usually contain liquid water droplets, although some ice crystals can also be present. In the right conditions (relatively small uniformly sized drops, and a thin cloud) you can see an optical effect called corona. Corona appears as a large disk of white light centered on the sun or moon (still visible through the thin cloud). Colored fringes surround the perimeter of the white disk (see the Atmospheric Optics chapter).
Altocumulus and cirrocumulus are layers of lumpy clouds. The edges of these small cloud lumps are often sharply defined, suggesting that they are predominantly composed of liquid water droplets. The lumpiness is not caused by thermals rising from the Earth’s surface, but instead are caused by smaller-diameter turbulent eddies generated locally within the clouds.
Low altitude stratiform clouds include stratus and nimbostratus. In North America, nimbostratus clouds often have low bases and are considered to be a low cloud. However, as we will see in the international cloud classification section, nimbostratus clouds are traditionally listed as mid-level clouds. In this book, we will treat nimbostratus as stratiform rain clouds with low cloud base.
The prefix “nimbo” or suffix “nimbus” designates a precipitating cloud. Nimbostratus usually have light to moderate rain or snow over large horizontal areas, which cause a diffuse-looking cloud base. Cumulonimbus (thunderstorm) clouds have heavy rain (or snow in winter) and sometimes hail in small areas or along narrow paths on the ground called swaths (e.g., hail swaths or snow swaths).
Layer clouds, especially those with cloud bases at low altitude, are sometimes called “cloud decks”.
Smooth-looking, non-turbulent stratiform clouds include stratus (low-altitude, thick, you cannot see the sun through it), altostratus (mid-altitude, modest thickness, you can see the sun faintly through this cloud) and cirrostratus (high altitude, diffuse, allows bright sunlight to cause shadows). None of these clouds (even the lowest ones) are associated with thermals rising from the Earth’s surface.
For most stratiform clouds, the external forcing is horizontal advection (movement by the mean wind), where humid air is blown up the gently sloping surface of a warm front. The source of these air parcels are often 1000 km or more from the clouds. In these warm-frontal regions the air is often statically stable, which suppresses vertical motions. Because these clouds are not driven by their own positive buoyancy, we consider them to be passive clouds.
This process is illustrated in Figure 5.20. Let the circle in that figure represent a humid air parcel with potential temperature 270 K. If a south wind were tending to blow the parcel toward the North Pole, the parcel would follow the 270 K isentrope like a train on tracks, and would ride up over the colder surface air in the polar portion of the domain. The gentle rise of air along the isentropic surface creates sufficient cooling to cause the condensation.
Stratiform clouds can be inferred from soundings, in the layers above the boundary layer where environmental temperature and dew point are equal (i.e., where the sounding lines touch). Due to inaccuracies in some of the sounding instruments, sometimes the T and Td lines become close and parallel over a layer without actually touching. You can infer that these are also stratiform cloud layers.
However, you cannot estimate cloud base altitude by the LCL of near-surface air from under the cloud, because stratiform clouds don’t form from air that rises from near the Earth’s surface. For altocumulus and cirrocumulus clouds, don’t let the suffix “cumulus” fool you — these clouds are still primarily advective, layer clouds.
Stratocumulus clouds are low-altitude layers of lumpy clouds, often covering 5/8 or more of the sky (Figure 6.3). They don’t fit very well into either the stratiform or cumuliform categories. They are often turbulently coupled with the underlying surface. Thus, their cloud bases can be estimated using the LCL of near-surface air.
Air circulations in “stratocu” can be driven by:
- wind-shear-generated turbulence (known as forced convection),
- IR radiative cooling from cloud top that creates blobs of cold air that sink (free convection), and
- advection of cool air over a warmer surface.
There are many beautiful and unusual clouds that do not fit well into the cumuliform and stratiform categories. A few are discussed here: castellanus, lenticular, cap, rotor, banner, contrails, fumulus, billow clouds, pyrocumulus, pileus, and fractus. You can find pictures of these using your web browser.
Other clouds associated with thunderstorms are described in the Thunderstorm chapters. These include funnel (tuba), wall (murus), mammatus (mamma), arc (arcus), shelf, flanking line, tail (cauda), beaver tail (flumen), and anvil (incus).
184.108.40.206. Clouds in unstable air aloft
Two types of clouds can be found in layers of statically or dynamically unstable air not associated with the ground: castellanus and billow clouds.
Castellanus clouds look like a layer of smalldiameter castle turrets (Figure 6.4). When a layer of relatively warm air advects under a layer of cooler air, the interface between the two layers aloft can become statically unstable. [The advection of air from different sources at different altitudes is called differential advection.]
If these castellanus form just above the top of the boundary layer, they are called cumulus castellanus. When slightly higher, in the middle of the troposphere, they are called altocumulus castellanus. Altocumulus castellanus are sometimes precursors to thunderstorms (because they indicate an unstable mid-troposphere), and are a useful clue for storm chasers.
Billow clouds (discussed in the Atmos. Stability chapter) are a layer of many parallel, horizontal lines of cloud that form in the crests of Kelvin-Helmholtz (K-H) waves (Figure 5.17). They indicate a layer of turbulence aloft caused by wind shear and dynamic instability. When similar turbulent layers form with insufficient moisture to be visible as billow clouds, the result is a layer of clear-air turbulence (CAT). Pilots try to avoid both CAT and K-H waves. Sometimes instead of a layer of billows, there will be only a narrow band of breaking wave clouds known as K-H wave clouds (fluctus).
220.127.116.11. Clouds associated with mountains
In mountainous regions with sufficient humidity, you can observe lenticular, cap, rotor, and banner clouds. The Regional Winds chapter covers others.
Lenticular clouds (lenticularis) have smooth, distinctive lens or almond shapes when viewed from the side, and they are centered on the mountain top or on the crest of the lee wave (Figure 6.7). They are also known as mountain-wave clouds or lee-wave clouds, and are passive clouds that form in hilly regions.
If a lenticular cloud forms directly over a mountain, it is sometimes called a cap cloud. A cap cloud can form when air that is statically stable is blown toward a mountain or other terrain slope. As the air is forced to rise by the terrain, the air cools adiabatically and can reach saturation if the air is sufficiently humid. As the air descends down the lee side of the mountain, the air warms adiabatically and the cloud droplets evaporate.
However, the static stability allows the air to continue to oscillate up and down as it blows further downwind. Lenticular clouds can form in the crest of these vertical oscillations (called mountain waves) to the lee of (downwind of) the mountain.
They are a most unusual cloud, because the cloud remains relatively stationary while the air blows through it. Hence, they are known as standing lenticular. The uniformity of droplet sizes in lenticular clouds create beautiful optical phenomena called iridescence when the sun appears close to the cloud edge. See the Regional Winds chapter for mountain wave details, and the Atmospheric Optics chapter for more on optical phenomena.
Rotor clouds are violently turbulent balls or bands of ragged cloud that rapidly rotate along a horizontal axis (Figure 6.7). They form relatively close to the ground under the crests of mountain waves (e.g., under standing lenticular clouds), but much closer to the ground. Pilots flying near the ground downwind of mountains during windy conditions should watch out for, and avoid, these hazardous clouds, because they indicate severe turbulence.
The banner cloud is a very turbulent streamer attached to the mountain top that extends like a banner or flag downwind (Figure 6.7). It forms on the lee side at the very top of high, sharply pointed, mountain peaks during strong winds. As the wind separates to flow around the mountain, low pressure forms to the lee of the mountain peak, and counterrotating vortices form on each side of the mountain. These work together to draw air upward along the lee slope, causing cooling and condensation. These strong turbulent winds can also pick up previously fallen ice particles from snow fields on the mountain surface, creating a snow banner that looks similar to the banner cloud.
18.104.22.168. Clouds due to surface-induced turbulence or surface heat
The most obvious clouds formed due to surface-induced turbulence are the cumuliform (convective) clouds already discussed. However, there are three others that we haven’t covered yet: pyrocumulus, pileus and fractus.
A pyrocumulus is a cumulus (flammagenitus) cloud that forms in the smoke of a fire, such as a forest fire or other wild fire. One of the combustion products of plant material is water vapor. As this water vapor is carried upward by the heat of the fire, the air rises and cools. If it reaches its LCL, the water vapor can condense onto the many smoke particles created by the fire. Some pyrocumulus can become thunderstorms.
Pyrocumulus clouds can also be created by geothermal heat and moisture sources, including volcanoes and geysers. Lightning can often be found in volcanic ash plumes.
The pileus cloud looks like a thin hat just above, or scarf around, the top of the rising cumulus clouds (Figure 6.8). It forms when a layer of stable, humid air in the middle of the troposphere is forced upward by cumuliform cloud towers (cumulus congestus) rising up from underneath. Hence, there is an indirect influence from surface heating (via the cumuliform cloud). Pileus are very short lived, because the cloud towers quickly rise through the pileus and engulf them.
Fractus clouds are ragged, shredded, often low-altitude clouds that form and dissipate quickly (Figure 6.8). They can form during windy conditions in the turbulent, boundary-layer air near rain showers, under the normal nimbostratus or cumulonimbus cloud base. These clouds do not need mountains to form, but are often found as stratus silvagenitus clouds along the sides of forested mountains during or after rainy weather. The falling rain from the cloud above adds moisture to the air, and the updraft portions of turbulent eddies provide the lifting to reach condensation.
Sometimes fractus clouds form in non-rainy conditions, when there is both strong winds and strong solar heating. In this case, the rising thermals lift air to its LCL, while the intense turbulence in the wind shear shreds and tears apart the resulting cumulus clouds to make cumulus fractus.
22.214.171.124. Anthropogenic Clouds
The next two clouds are anthropogenic (manmade). These are contrails and fumulus.
Fumulus (cumulus homogenitus) is a contraction for “fume cumulus”. They form in the tops of thermal plumes rising from cooling towers or smokestacks (Figure 6.8). Modern air-quality regulations often require that industries scrub the pollutants out of their stack gasses by first passing the gas through a scrubber (i.e., a water shower). Although the resulting effluent is much less polluted, it usually contains more water vapor, and thus can cause beautiful white clouds of water droplets within the rising exhaust plume. Similarly, cooling towers do their job by evaporating water to help cool an industrial process.
Contrail (cirrus homogenitus) is a contraction for “condensation trail”, and is the straight, long, narrow, horizontal pair of clouds left behind a high-altitude aircraft (Figure 6.8). Aircraft fuel is a hydrocarbon, so its combustion in a jet engine produces carbon dioxide and water vapor. Contrails form when water vapor in the exhaust of high-altitude aircraft mixes with the cold environmental air at that altitude (see mixing subsection earlier in this chapter). If this cold air is already nearly saturated, then the additional moisture from the jet engine is sufficient to form a cloud. On drier days aloft, the same jet aircraft would produce no visible contrails.
Regardless of the number of engines on the aircraft, the exhaust tends to be quickly entrained into the horizontal wing-tip vortices that trail behind the left and right wing tips of the aircraft. Hence, jet contrails often appear initially as a pair of closelyspaced, horizontal parallel lines of cloud. Further behind the aircraft, environmental wind shears often bend and distort the contrails. Turbulence breaks apart the contrail, causes the two clouds to merge into one contrail, and eventually mixes enough dry ambient air to cause the contrail to evaporate and disappear.
Contrails might have a small effect on the globalclimate heat budget by reflecting some of the sunlight. Contrails are a boon to meteorologists because they are a clue that environmental moisture is increasing aloft, which might be the first indication of an approaching warm front. They are a bane to military pilots who would rather not have the enemy see a big line in the sky pointing to their aircraft.
Although almost all our clouds occur in the troposphere, sometimes higher-altitude thin stratiform (cirrus-like) clouds can be seen. They occur poleward of 50° latitude during situations when the upper atmosphere temperatures are exceptionally cold.
These clouds are so diffuse and faint that you cannot see them by eye during daytime. These clouds are visible at night for many minutes near the end of evening twilight or near the beginning of morning twilight. The reason these clouds are visible is because they are high enough to still be illuminated by the sun, even when lower clouds are in the Earth’s shadow.
Highest are noctilucent clouds, in the mesosphere at heights of about 85 km. They are made of tiny H2O ice crystals (size ≈ 10 nm), and are nucleated by meteorite dust. Noctilucent clouds are polar mesospheric clouds (PMC), and found near the polar mesopause during summer.
Lower (20 to 30 km altitude) are polar strato- spheric clouds (PSC). At air temperatures below –78°C, nitric acid trihydrate (NAT-clouds) can condense into particles. Also forming at those temperatures can be particles made of a supersaturated mixture of water, sulphuric acid, and nitric acid (causing STS clouds).
For temperatures colder than –86°C in the stratosphere, pure H2O ice crystals can form, creating PSCs called nacreous clouds. They are also known as mother-of-pearl clouds because they exhibit beautiful iridescent fringes when illuminated by the sun. These stratospheric clouds form over mountains, due to mountain waves that propagate from the troposphere into the stratosphere, and amplify there in the lower-density air. They can also form over extremely intense tropospheric high-pressure regions.