CHAPTER 6. Cloud Development and Forms
Chapter Overview:
Processes of parcel lifting are described and related to static stability conditions. Frontal uplift,
convergence, orographic lifting are explained and associated to static stability. Stability
conditions are detailed with emphasis placed on parcel versus ambient air temperatures. Processes
that alter the environmental lapse rate are also detailed. Particular attention is paid to both the
causes and results of temperature inversions and cloud categorization.
Chapter at a Glance:
Clouds are instrumental to the Earth’s energy and moisture balances. Most clouds form as air
parcels are lifted and cooled to saturation.
• Mechanisms That Lift Air - Four primary mechanisms are responsible for lifting air to
saturation: orographic lifting, frontal lifting, convergence, and convection.
A. Orographic Uplift - Orographic lift occurs as air is displaced over topographic
barriers such as mountains and hills. On the windward side of the barrier, air is
displaced toward higher altitudes and undergoes adiabatic cooling, possibly to
saturation. On the leeward side, descending air warms through compression
leading to a dry rainshadow. This situation is readily exemplified by the Sierra
Nevada mountain range and the dry eastward interior.
B. Frontal Lifting - When boundaries between airs of unlike temperatures (fronts)
migrate, warmer air is pushed aloft resulting in adiabatic cooling and cloud
formation. Cold fronts occur when warm air is displaced by cooler air. The
opposite occurs in relation to warm fronts. In either case, however, the less dense
warmer air is lifted over the denser colder air. <Web>
C. Convergence - Atmospheric mass is non-uniformly distributed over the Earth.
Air advects from areas of more abundant mass to areas of less mass. Air moving
into these low pressure regions often converges with air entering from other
directions stimulating rising motions and adiabatic cooling.
D. Localized Convection - Localized surface heating may lead to spatially limited
free convection. Vertical motions are stimulated from the surface upward resulting
in towering clouds and a chance for intense precipitation over small spatial scales.
<ME6.1>
• Static Stability and the Environmental Lapse Rate - Static stability refers to
atmospheric conditions as they relate to vertical air motions. An atmosphere that supports
upward motions of air is statically unstable. If a parcel is pushed upward in this condition,
it will rise freely. In a statically stable situation the atmosphere resists vertical motions
such that a parcel pushed upward will return to its origination point. An atmosphere that
promotes neither situation is statically neutral and is exemplified by a displaced parcel
remaining at rest throughout the atmosphere. Static stability is related to temperature
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controlled buoyancy, with positive buoyancy indicating less density than surrounding
conditions, a statically unstable situation. The opposite describes the negative buoyancy
conditions common to statically stable situations. Temperature and saturation conditions
of rising parcels as they relate to ambient air lead to absolutely unstable, absolutely stable,
and conditionally unstable atmospheres.
A. Absolutely Unstable Air - When parcel temperature, considering the DALR, is
greater than the ambient air, taking into account the ELR, positive buoyancy occurs
resulting in an absolutely unstable environment. In this situation the ELR will
exceed the DALR . Vertical motions are supported throughout the atmosphere and
buoyancy increase with height as parcel cooling rates are less than that of the
ambient air whether the parcel is saturated or not.
B. Absolutely Stable Air - When the ELR is less than the SALR, negative
buoyancy results and vertical motions are discouraged. No matter where a parcel is
lifted it will always be cooler than the ambient air. These conditions typify an
absolutely stable atmosphere.
C. Conditionally Unstable Air - When the ELR is between the DALR and the
SALR, a rising parcel exists in a conditionally unstable situation. Initially, the
atmosphere will resist vertical displacement of a parcel, as parcel temperature will
be less than the ambient air, a negative buoyancy situation. However, if the parcel
is forced to rise through some lifting mechanism, and saturation occurs, parcel
cooling at the lesser SALR will eventually create a situation where the parcel
temperature will exceed that of the ambient air. In this new situation the parcel will
accelerate upward as it will experience positive buoyancy. So parcel buoyancy is
dependent on lifting to the level of free convection, <Web> the height at which a
parcel gains positive buoyancy.
• Factors Influencing the Environmental Lapse Rate - The ELR changes temporally
and spatially as surface temperatures change.
A. Heating or Cooling of the Lower Atmosphere - During the day, surface insolation
gains result in greater heating near the surface than aloft. This leads to a greater ELR
through the lower atmosphere. Thus, the ELR is greatest on clear days above dry, low
albedo surfaces. At night, the situation reverses as terrestrial radiation loss accounts for
near surface chilling. Temperatures at the surface become cooler than that aloft
resulting in a temperature inversion.
B. Advection of Cold and Warm Air at Different Levels - Vertical temperature profiles
may lead to varying wind directions with height. Advection of cold or warm air at
different levels promotes such conditions and affects the ELR accordingly.
C. Advection of and Air Mass with a Different ELR - Air masses are masses of air with
similar temperature and moisture characteristics. Those conditions are maintained as
the air mass advects to other locations, although modifications occur. As temperatures
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change with the advection, so does the ELR.
• Limitations on the Lifting of Unstable Air - Rising parcels are limited by atmospheric
stability and entrainment.
A. A Layer of Stable Air - A parcel rising through an unstable lower atmosphere
may reach a stable environment in the upper air, or eventually within the
stratosphere. In this situation, the parcel cooling rate will exceed that of the
ambient air. The parcel will slowly cease ascension and come to rest at some equal
temperature level. <F6.12;T51>
B. Entrainment - Entrainment refers to ambient air intrusions into parcels, which
limits vertical cloud development. Such impingements cause evaporation along
cloud boundaries. The evaporation process uses latent heat that cools the cloud
margins and reduces buoyancy.
• Extremely Stable Air: Inversions - The atypical situation when temperature increases
with height through the troposphere produces negative buoyancy conditions. These
inversions develop from a variety of situations with the most common relating to terrestrial
radiation loss on clear, calm nights. Near surface air diabatically chills more rapidly than
air aloft producing a radiation inversion. Radiation fogs are a common indicator of
radiation inversions. Frontal inversions frequently form along cold/warm air boundaries as
air associated with a front wedges into unlike air at some angle. Sleet and freezing rain are
common occurrences associated with an inversion aloft. Subsidence inversions occur
when air sinks toward the surface and undergoes adiabatic warming. Subsidence
inversions frequently occur on the eastern sides of either migratory or semi-permanent
high pressure areas and in lee of mountains.
• Cloud Types - Clouds show unlimited variety in size, shape, and composition, however,
virtually all may be easily subdivided into classes based on appearance and/or height. An
appearance scheme divides clouds into cirrus, stratus, cumulus, and nimbus. The height
scheme subdivides clouds as either high, middle, low, or vertical.
A. High Clouds - Clouds with bases above 6000 m (19,000 ft) and composed of ice
are classified as high clouds. The most common high cloud is cirrus, which has a
wispy appearance due to cold temperatures at such high heights and the associated
low water content. Fall streaks, comprised of falling ice crystals, may exist below
cirrus clouds. Cirrus clouds with horizontal swirls, or mares’ tails, occur in
somewhat turbulent conditions. Cirrostratus occurs when cirrus clouds thicken and
stretch horizontally across the sky. These clouds may form a halo about the sun or
moon as entering light is refracted 22o by the cloud’s ice crystals. Cirrocumulus
clouds occur when cirrus is thicker and exhibits a billowy appearance. Such
conditions often occur in association with wind shear which creates long rows of
cirrocumulus. Their resemblance to fish scales has garnered the moniker, mackerel
sky.
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B. Middle Clouds - Clouds with bases between 2000 and 6000 m (6,000-19,000 ft)
are largely composed of liquid drops. These middle clouds carry the prefix, alto.
Altostratuses are typically thick enough to almost fully obscure the sun or moon
and blanket the sky from horizon to horizon. Altocumuluses are usually typified by
a banded arrangement of billowy clouds.
C. Low Clouds - Low clouds have bases below 2000 m (6,000 ft) and are normally
composed of liquid water. They are best typified by stratus clouds. <Web>
Although stratus clouds typically blanket large spatial areas, their vertical extent is
shallow (0.5-1 km). Precipitation associated with nimbostratus clouds is usually
very light as dictated by their limited vertical extent. Stratocumulus clouds are low,
layered clouds which show some vertical development. <ME6.2>
D. Clouds with Vertical Development - When air is inherently unstable or
conditionally unstable, high vertical velocities are possible and thus, vertically
developed cumuliform clouds. Cumulus clouds may be subdivided based on the
extent of vertical development. Cumulus humilis, or fair weather cumulus which
develop primarily from convection, usually evaporate shortly after formation and
are, therefore, vertically limited. Cumulus congestus marks more organized
development as cloud towers appear. Each tower, indicative of uplift cells, has
relatively short life spans but is constantly replaced with each tower progressing
higher. Because of their vertical extent they are composed of both ice and liquid
water drops. Cumulonimbus, the most violent of all clouds, is indicative of
thunderstorms. <Web> They form in warm, moist, unstable air and from base to
top may extend fully through the troposphere and into the stratosphere. An anvil
top my occur in relation to cumulonimbus clouds with the anvil forming as ice
crystals are blown horizontally from the cloud by high speed upper atmospheric
winds. <ME6.3>
E. Unusual Clouds - Atypical clouds not easily categorized occur in a variety of
situations. Lenticular clouds form as a result of turbulence downwind of mountain
ranges. They are so named as they exhibit a lens shape. Banner clouds are similar
to lenticular clouds but are anchored to individual mountain peaks. Mammatus, or
sack like protrusions from the base of a cloud, indicates low level turbulence and
are most common in cumulonimbus clouds. <Web> Nacreous clouds, composed
of supercooled water or ice, are stratospheric clouds and are sometimes referred to
as mother of pearl clouds. Noctilucent clouds form even higher in the mesosphere
and are typically illuminated after sunset.
• Cloud Coverage - When clouds comprise more than 9/10th of the sky, the sky is
overcast. When coverage is between 6/10 and 9/10 the sky is said to be broken. Scattered
skies exist when clouds make up between 1/10 and ½ while skies with clouds occupying
less than 1/10 are said to be clear.
Chapter Boxes:
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6-1 Forecasting: Determining Stability from Thermodynamic Diagrams - Static stability of
air can be determined numerically by comparing the environmental lapse rate to the dry
and saturated adiabatic lapse rates. Thermodynamic diagrams accomplish this graphically.
Lines labeled moist adiabats and dry adiabats plot the change in temperature a parcel will
experience if lifted or lowered to particular heights. This is examined against a plot of the
ELR to determine stability through an atmospheric profile.
6-2 Special Interest: Radiation Inversions and Human Activities - Subfreezing temperatures
associated with terrestrial radiation loss exist mainly near the surface. Areas
unaccustomed to freezing temperatures, such as the southern United States, are particularly
prone to severe agricultural damage in these situations. Wind machines are sometimes
used to vertically mix the colder lower atmosphere with warmer air aloft in an effort to
prevent surface freezing. Smudgepots are used to inject energy directly into the lower
atmosphere to retard freezing. Spraying crops with water is another alternative. Freezing
water releases latent heat that insulates the crops from very low temperatures. Clear, cool
night inversions also prevent the vertical mixing of the lower atmosphere such that
atmospheric pollutants accumulate.
6-3 Focus on the Environment: The Unknown Influence of Clouds on Climatic Change General circulation models (GCMs) are currently used to estimate future climatic changes
given increases in anthropogenic gases. The models are limited in that current
understanding of atmospheric parameters is limited. One of the least understood aspects of
the atmosphere is the role of clouds on the energy budget. Some argue that increases in
certain gases will lead to higher temperatures, which would lead to greater cloud cover.
The type of cloud cover is still debated but nevertheless very important as certain types of
clouds act as reflectors of insolation while others absorb greater amounts of terrestrial
radiation. Therefore, particular clouds will encourage surface cooling while others will
contribute to warming. Many other related items are under investigation with regard to
clouds and Earth’s energy balance.
6-4 Special Interest: Why Clouds Have Clearly Defined Boundaries - Cloud bases are usually
flat as they define the lifting condensation level. Very small cloud droplets at the base also
contribute to the sharply visible base, as these drops are effectively scatter visible light. In
addition, evaporation rates of these small drops are very small, ensuring that the cloud base
is visibly distinct. Entrainment causes a similar effect on the sides of clouds resulting in
fairly sharp boundaries.
6-5 Physical Principles: The Surprising Composition of Clouds - The greatest amount of mass
in a cloud is air rather than ice or liquid water. This is due to the fact that the cloud drops
are exceedingly small and the total water vapor content of the atmosphere is quite limited,
even in clouds.
Related Web Sites:
Mammatus Clouds:
http://covisl.atmos.uiuc.edu/guide/clouds/other.clouds/html/mammatus.examples.html
Cumulonimbus: http://thunder.simplenet.com/photo/cb.htm
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Cold Front; Warm Front; Stratus: www.nws.noaa.gov
Level of Free Convection: http://twister.sbs.ohio-state.edu/helpdocs/lfc.html
Media Enrichment:
ME6.1 - Global cloud motions during 1999.
ME6.2 - Time lapse movie of stratus clouds.
ME6.3 - Time lapse movie of cumulus cloud development.
Key Terms:
orographic uplift
stratocumulus
cumuliform
cumulonimbus
contrails
convergence
banner clouds
stratus
altostratus
nimbus
coverage
saturated adiabatic lapse rate
rain shadow
level of free convection
warm front
anvil
inversions
static stability
cirrus
altocumulus
nimbostratus
contrails
high, middle, and low clouds
general circulation models
cold front
entrainment
lenticular clouds
cirrostratus
environmental lapse rate
cirrocumulus
noctilucent clouds
dry adiabatic lapse rate
level of free convection
Review Questions:
1. Describe the four mechanisms that lift air and promote cloud formation.
Air is lifted in one of four ways. Orographic uplift describes air that is displaced over
topographic barriers. Adiabatic cooling occurs on the windward side of the obstacle, which
initiates cloud formation. Frontal lifting describes a situation whereby less dense air is
forced aloft by more dense air as unlike air masses interact to displace one another.
Convergence refers to a situation where air converges upon a location from multiple
directions, forcing air to rise. Localized convection is the final forcing mechanism. This
occurs as air is heated from below, thereby decreasing density in the near surface air which
then rises. In all situations, the rising air expands and cools adiabatically to form clouds.
2. Explain how buoyancy affects the air’s susceptibility to uplift.
Atmospheric stability is related to temperature controlled buoyancy of a parcel. Positive
buoyancy indicates a situation whereby a parcel is less dense than the ambient air. As a
result, the parcel will rise. Opposite conditions, which cause a parcel to sink or be held at
the surface, describe a negative buoyancy environment.
3. Describe the situations that can cause air to be absolutely stable, absolutely unstable, or
conditionally unstable.
Stable, unstable, and conditionally unstable situations are dictated by ambient air versus
parcel temperatures. When a parcel temperature is cooler than the ambient air temperature,
the parcel exists in a negative buoyancy environment. As such, a parcel will sink as
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vertical motions in the atmosphere are discouraged. When a parcel’s temperature is
warmer than the ambient air temperature, the parcel exists in a positive buoyancy
environment. The parcel will rise freely. When a parcel exists in a negative buoyancy
situation, but is mechanically forced to rise and its temperature decreases at a rate less than
the ambient air temperature, parcel temperature will eventually exceed the ambient air
temperature. At this point, the point of free convection, the parcel gains positive buoyancy
and rises freely. This situation is indicative of a conditionally unstable atmosphere.
4. What two factors can ultimately stop rising parcels of air from continuing upward?
There are two factors that limit the ascension of parcels. The first is a layer of stable air. A
parcel rising through an unstable layer will eventually reach a stable layer. The parcel
cooling rate will exceed that of the ambient air and eventually cease ascension and come to
rest at some equal temperature level. The other factor is entrainment that refers to ambient
air intrusions into parcels which reduces buoyancy.
5. What will determine whether air that is conditionally unstable will become buoyant?
If a parcel is forced high enough to stimulate cooling at both the dry and wet adiabatic lapse
rates then, eventually, the parcel will become warmer than the ambient air. Once this
occurs, positive buoyancy will cause the parcel to rise freely.
6. What is the level of free convection?
This is the height at which a forced parcel will gain positive buoyancy and rise freely.
7. Describe the processes that bring about changes in the environmental lapse rates, and thus the
stability of the atmosphere.
The environmental lapse rate (ELR) is affected by changes in surface temperature. On
typical cloudless days, the ELR increases through the day, coincident with surface heating.
The condition reverses at night resulting in a morning inversion. The ELR is also affected
by changes in temperature induced by air mass advection. Varying wind direction with
height, a result of advection at various atmospheric levels, may also cause changes in the
ELR.
8. What is entrainment, and how does it affect the growth of clouds?
Entrainment refers to ambient air intrusions into parcels. Entrainment limits cloud
development by causing evaporation along the parcel (cloud) edge. This evaporation
induces net cooling which reduces parcel buoyancy, ultimately limiting cloud growth.
9. Define the term inversion, and describe the mechanisms that can cause the various types of
inversions.
Inversions refer to a reversed temperature profile with height. Instead of air temperature
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decreasing with height, an increase occurs. Inversions are caused by three primary
mechanisms. The first involves rapid chilling of surface air through radiation losses.
Secondly, frontal inversions are associated with air mass advection and may be affiliated
with sleet and freezing rain. Lastly, subsidence inversions occur when air sinks toward the
surface and adiabatically warms.
10. Describe the classification scheme for clouds based on their height and form.
Cloud classification is based on a cloud’s size, shape, and composition. Clouds are also
subdivided based on appearance and/or height. The appearance scheme divides clouds into
cirrus, stratus, cumulus, and nimbus while the height scheme separates clouds into high,
middle, low, or vertical.
11. List the major subtypes of high, middle, and low clouds.
High clouds, those with bases above 6000 m, are predominantly cirrus. These may be
subdivided into cirrus, cirrostratus, or cirrocumulus depending upon appearance. Middle
clouds, those with bases between 2000 m, carry the prefix alto. Altostratus and
altocumulus are common types. Low clouds, those with bases below 2000 m, are mainly
stratus. Stratus, stratocumulus, and nimbostratus are the main types.
12. What type of cloud produces a characteristic halo?
Typically, this occurs in relation to cirrostratus clouds. Ice crystals within the cloud refract
incoming light which produces a halo.
13. Other than height, what significant difference exists between altocumulus and cirrocumulus
clouds?
Cirrocumulus clouds occur when cirrus is thicker and exhibits a billowy appearance. This
is often associated with wind shear, which creates long rows of cloud producing a
“mackerel sky”. Altocumuluses are usually typified by a banded arrangement of billowy
clouds.
14. How do cumulus humilis and cumulus congestus clouds differ from each other?
Cumulus humilis develop primarily from convection and usually evaporate shortly after
formation. They are vertically limited. Cumulus congestus marks more organized
development as cloud towers appear. Each tower is indicative of uplift cells, each with a
relatively short life span. However, towers are constantly replaced which grows the
succeeding towers higher.
15. What distinctive feature characterizes a cumulonimbus cloud?
Cumulonimbus clouds typically have an anvil top that occurs as ice crystals are blown
horizontally from the cloud by high speed upper air winds.
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16. What conditions cause lenticular and banner clouds to form?
Lenticular clouds result from turbulence downwind of mountain ranges. Banner clouds
form in a similar manner but are seemingly anchored to individual mountain peaks.
17. What are mammatus?
Mammatus clouds are sack-like protrusions from the base of a cloud indicating low level
turbulence. They are most commonly associated with cumulonimbus clouds.
18. List and describe the types of clouds that exist above the troposphere.
Clouds of the stratosphere are predominantly nacreous clouds (a.k.a. mother of pearl
clouds). They are composed of supercooled water or ice. Noctilucent clouds form in the
mesosphere and are typically noticeable, illuminated, after sunset.
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