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Chapter 6 Cloud Development and Forms. 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

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slide1

Chapter 6

Cloud Development and Forms

slide2

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

slide3

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.

slide4

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).

slide5

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.

slide6

The air’s 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.

slide7

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.

slide8

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.

slide9

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.

slide10

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.

slide11

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.

slide12

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.

slide13

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.

slide14

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.

slide15

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.

slide16

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 inversionsresult from cooling of the surface.

Frontal inversions exist at the transition zone separating warm and cold air masses. Subsidence inversionsresult from sinking air.

Frontal Inversion

Subsidence Inversion

slide17

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

slide18

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.

slide19

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.

slide20

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.”

slide21

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.

slide22

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.

slide23

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.

slide24

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.

slide25

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.

slide26

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.

slide27

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.

slide28

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.

slide29

The next chapter examines

precipitation processes.