Guido Cervone EOS 121

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Chapter 7

• Guido CervoneEOS 121

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Chapter 6 Cloud Development and Forms
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Four mechanisms lift air so that condensation
and cloud formation can occur
1. Orographic lifting, the forcing of air above a
mountain barrier 2. Frontal lifting, the
displacement of one air mass over another 3.
Convergence, the horizontal movement of air into
an area at low levels 4. Localized convective
lifting due to buoyancy
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The upward displacement of air that leads to
adiabatic cooling is called orographic uplift (or
the orographic effect). When air approaches a
topographic barrier, it can be lifted upward or
deflected around the barrier. Downwind of a
mountain ridge, on its leeward side, air
descends the slope and warms by compression to
create a rain shadow effect, an area of lower
precipitation.
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Fronts are transition zones in which great
temperature differences occur across relatively
short distances. Air flow along frontal
boundaries results in the widespread
development of clouds in either of two ways. When
cold air advances toward warmer air (cold front),
the denser cold air displaces the lighter warm
air ahead of it (a). When warm air flows toward a
wedge of cold air (warm front), the warm air is
forced upward in much the same way that
the orographic effect causes air to rise above a
mountain barrier (b).
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Pressure differences set the air in motion in the
effect we call wind When a low-pressure cell is
near the surface, winds in the lower atmosphere
tend to converge on the center of the low from
all directions Horizontal movement toward a
common location implies an accumulation of mass
called horizontal convergence, or just
convergence for short.
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The airs susceptibility to uplift is called its
static stability Statically unstable air becomes
buoyant when lifted and continues to rise if
given an initial upward push statically stable
air resists upward displacement and sinks back to
its original level when the lifting
mechanism ceases Statically neutral air neither
rises on its own following an initial lift nor
sinks back to its original level it simply comes
to rest at the height to which it was displaced.
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When a parcel of unsaturated or saturated air is
lifted and the Environmental Lapse Rate (ELR) is
greater than the dry adiabatic lapse rate (DALR),
the result is absolutely unstable air.
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When a parcel of unsaturated or saturated air is
lifted and the Environmental Lapse Rate (ELR) is
less than the saturated adiabatic lapse rate
(SALR), the result is absolutely stable air and
the parcel will resist lifting.
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When the ELR is between the dry and saturated
adiabatic lapse rates the air is said to
be conditionally unstable, and the tendency for a
lifted parcel to sink or continue rising depends
on whether or not it becomes saturated and how
far it is lifted. The level of free convection is
the height to which a parcel of air must be
lifted for it to become buoyant and to rise on
its own.
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Assume the ELR is 0.7 C/100 m and the air is
unsaturated. As a parcel of air is lifted, its
temperature is less than that of the surrounding
air, so it has negative buoyancy.
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A parcel starts off unsaturated but cools to the
LCL, where it is cooler than the surrounding air.
Further lifting cools the parcel at the SALR. At
the 200-m level, it is still cooler than the
surrounding air, but if taken to 300 m, it is
warmer and buoyant.
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The ELR can be changed by the advection of air
with a different temperature aloft. In (a), the
winds at the surface and the 100 m level bring in
air with temperatures of 10 C and 9.5 C,
respectively, yielding an ELR of 0.5 C/100 m. In
(b), the surface winds still bring in air with a
temperature of 10 C. The wind direction at the
100 m level has shifted to northeasterly, and
the advected air has a temperature of 9.0 C.
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The ELR changes when a new air mass replaces one
that has a different lapse rate. Location A has
a steeper ELR than does B. As the air mass over
Location A moves over B, it brings to that
location the new temperature profile.
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Air that is unstable at one level may be stable
aloft. The solid line depicts a temperature
profile that is unstable in the lowest 500 m but
capped by an inversion. An unsaturated air parcel
displaced upward would cool by the DALR (dashed
line), making it initially warm and buoyant
relative to the surrounding level. After
penetrating the inversion layer, the rising air
is no longer warmer than the surrounding air, and
lifting is suppressed. The parcel continues
upward due to its momentum. It cools more rapidly
than the surrounding air and becomes relatively
dense. After stopping, the air parcel sinks and
eventually comes to rest at some equilibrium
level.
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An air parcel has no barrier to prevent it from
mixing with its surroundings. As air rises,
considerable turbulence is generated, which
causes ambient air to be drawn into the parcel.
This process, called entrainment, is
especially important along the edges of growing
clouds. Entrainment suppresses the growth of
clouds because it introduces unsaturated air into
their margins and thus causes some of the liquid
droplets to evaporate.
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Situations in which the temperature increases
with altitude are called inversions. Air parcels
rising through inversions encounter ever-warmer
surrounding air and have strong negative
buoyancy. Inversions are extremely stable and
resist vertical mixing. Radiation inversions
result from cooling of the surface. Frontal
inversions exist at the transition zone
separating warm and cold air masses. Subsidence
inversions result from sinking air.
Frontal Inversion
Subsidence Inversion
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The Basic Cloud Types
High clouds - cirrus, cirrostratus, and
cirrocumulus Middle clouds - altostratus and
altocumulus Low clouds - stratus, stratocumulus,
and nimbostratus Clouds with vertical development
- cumulus and cumulonimbus
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High clouds are generally above 6000 m (19,000
ft). The simplest of the high clouds are cirrus,
which are wispy aggregations of ice crystals.
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Cirrostratus clouds are composed entirely of ice
but tend to be more extensive horizontally and
have a lower concentration of crystals. When
viewed through a layer of cirrostratus, the Moon
or Sun has a whitish, milky appearance but a
clear outline. A characteristic feature
of cirrostratus clouds is the halo, a circular
arc around the Sun or Moon formed by the
refraction (bending) of light as it passes
through the ice crystals.
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Cirrocumulus are composed of ice crystals that
arrange themselves into long rows of individual,
puffy clouds. Cirrocumulus form during episodes
of wind shear, a condition in which the wind
speed and/or direction changes with height. Wind
shear often occurs ahead of advancing storm
systems, so cirrocumulus clouds are often a
precursor to precipitation. Because of their
resemblance to fish scales, cirrocumulus clouds
are associated with the term mackerel sky.
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Altostratus clouds are the middle-level
counterparts to cirrostratus. They are more
extensive and composed primarily of liquid
water. Altostratus scatter a large proportion of
incoming sunlight back to space. The insolation
that does make its way to the surface consists
primarily or exclusively as diffuse radiation.
When viewing the Sun or Moon behind altostratus,
one sees a bright spot behind the clouds instead
of a halo.
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Altocumulus are layered clouds that form long
bands or contain a series of puffy clouds
arranged in rows. They are often gray in color,
although one part of the cloud may be darker than
the rest and consist mainly of liquid droplets
rather than ice crystals.
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Low clouds have bases below 2000 m. Stratus are
layered clouds that form when extensive areas of
stable air are lifted. Usually the rate of uplift
producing a stratus cloud is only a few tens of
centimeters per second, and its water content is
low. Low, layered clouds that yield light
precipitation are called nimbostratus. Seen from
below, these clouds look very much like
stratus, except for the presence of precipitation.
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Stratocumulus are low, layered clouds with some
vertical development. Their darkness varies when
seen from below because their thickness varies
across the cloud. Thicker sections appear dark,
and thinner areas appear as bright spots.
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Cumuliform clouds are those that have substantial
vertical development and occur when the air is
absolutely or conditionally unstable.
Fair-weather cumulus (above) called cumulus
humilis, do not yield precipitation and they
evaporate soon after formation.
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Intensely developed clouds are cumulus congestus.
They consist of multiple towers, and each tower
has several cells of uplift. This gives them a
fortress-like appearance with numerous columns of
varying heights. Their strong vertical
development implies that these clouds form in
unstable air.
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Cumulonimbus are the most violent of all clouds
and produce the most intense thunderstorms. In
warm, humid, and unstable air, they can have
bases just a few hundred meters above the
surface and tops extending into the lower
stratosphere. A cumulonimbus is distinguished by
the presence of an anvil composed entirely of ice
crystals formed by the high winds of the lower
stratosphere that extend the cloud forward.
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An important characteristic of clouds is their
breadth or coverage. When clouds occupy more than
nine-tenths of the sky, conditions are said to be
overcast. When coverage is between six-tenths
and nine-tenths, it is called broken. Scattered
clouds occupy between one-tenth and one-half of
the sky, and less than one-tenth cloud cover is
classified as a clear-sky condition.
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The next chapter examines precipitation processes.