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Stability & Cloud development
Chapter 6
Importance of Clouds
• Release heat to atmosphere
• Help regulate energy balance
• Indicate physical processes
• Adiabatic processes: change in temperature
without giving or removing heat energy with
surroundings
• Due to compression or expansion
 Adiabatic temperature changes
 When air is compressed
 Motion of air molecules increases
 Air will warm
 Descending air is compressed due to increasing air
pressure
 When air expands
 Air parcel does work on the surrounding air
 Air will cool
 Rising air will expand due to decreasing air pressure
Rates of Adiabatic Heating and
Cooling
 Dry adiabatic lapse rate
• No condensation occurring, air not saturated
•
10°C/1000m (5.5F/1000ft)
– Moist (wet) adiabatic rate
 Begins at the condensation level
 Air has reached the dew point
 Condensation is occurring and latent
heat is being liberated
 Heat released by the condensing
water reduces the rate of cooling
 varies, but use:
 6°C/1000m (3.3F/1000ft)
Atmospheric Stability
• Stability is a state of equilibrium in terms
atmospheric movement; no vertical
movement occurs
Determining Stability
Warm air rises or is unstable
Cool air sinks or is stable
To determine stability we compare air parcel
lapse rate to environmental lapse rate
 The temperatures of the surrounding air
 What stability conditions can exist?
 Neutral stability
 Absolute stability
 Absolutely unstable
 Conditionally unstable
• Stable environment
– Environmental lapse rate less than moist lapse
rate
– If an air parcel is forced it will spread horizontally
and form stratus clouds
– Usually due to cooling of surface air by:
• Night time radiational cooling
• Cold air advection at surface
• Air moving over a cold surface
– Inversion: warm over cool.
The initial environmental lapse rate in diagram (a) will become more stable (stabilize)
as the air aloft warms and the surface air cools, as illustrated in diagram (b).
An absolutely stable atmosphere occurs when the environmental lapse rate is less
than the moist adiabatic rate.
In a stable atmosphere,a rising air parcel is colder and more dense than the air
surrounding it, and, if given the chance, it will return to its original position.
FIGURE 6.5 Cold surface air, on this
morning, produces a stable atmosphere that
inhibits vertical air motions and allows the fog
and haze to linger close to the ground.
The layer x–y is initially 1400 m thick.
If the entire layer slowly subsides, it shrinks in the more-dense air near the surface.
As a result of the shrinking, the top of the layer warms more than the bottom, and the
entire layer (x’–y’) becomes more stable, and in this example forms an inversion. –
SUBSIDENCE INVERSION
A strong subsidence inversion along the coast of California.
The base of the stable inversion acts as a cap or lid on the cool, marine air below.
An air parcel rising into the inversion layer would sink back to its original level because
the rising air parcel would be colder and more dense than the air surrounding it.
This acts to concentrate pollution !
• Absolutely Unstable Atmosphere
– Environmental lapse rate greater than
the dry adiabatic lapse rate
– As air parcel rises it forms a vertical
cloud
– Convection, thunderstorms, severe
weather
An absolutely unstable atmosphere occurs when the environmental lapse rate is
greater than the dry adiabatic rate.
In an unstable atmosphere, a rising air parcel will continue to rise because it is warmer
and less dense than the air surrounding it.
• Conditionally Unstable Atmosphere
– Moist adiabatic lapse rate is less than the
environmental lapse rate which is less
than the dry adiabatic lapse rate
– Stable below cloud unstable above cloud
base
– Atmosphere usually in this state
Conditionally unstable atmosphere. The atmosphere is stable if the rising air is
unsaturated (a) ,but unstable if the rising air is saturated (b).
A conditionally unstable atmosphere occurs when the environmental lapse rate is
between the moist adiabatic rate and the dry adiabatic rate.
When the environmental
lapse rate is greater than
the dry adiabatic rate, the
atmosphere is absolutely
unstable.
When the environmental
lapse rate is less than the
moist adiabatic rate, the
atmosphere is absolutely
stable.
And when the
environmental lapse rate
lies between the dry
adiabatic rate and the
moist adiabatic rate
(shaded green area), the
atmosphere is
conditionally unstable.
The initial environmental lapse rate in diagram (a) will become more unstable (that is,
destabilize) as the air aloft cools and the surface air warms, as illustrated in diagram (b).
The warmth from this forest
fire in the northern Sierra
Nevada foothills heats the
air, causing instability near
the surface. Warm, lessdense air (and smoke)
bubbles upward, expanding
and cooling as it rises.
Eventually the rising air
cools to its dew point,
condensation begins, and a
cumulus cloud forms.
• Causes of Instability
– Cooling of air aloft
• advection
• radiation cooling in clouds
– Warming of surface
• daytime heating of the surface
• warm air advection
• air moving over a warm surface
• Mixing may also increase instability
Mixing tends to increase the lapse rate.
Rising, cooling air lowers the temperature toward the top of the layer,
while sinking, warming air increases the temperature near the bottom.
The lifting of an entire layer of air tends to increase the instability of the layer.
The initial stable layer (x–y) after lifting is now a conditionally unstable layer (x’–y’).
Convective instability.
The layer a–b is initially absolutely stable.
The lower part of the layer is saturated, and the upper part is “dry.”
After lifting, the entire layer (a’–b’) becomes absolutely unstable.
Cloud Development
• Clouds develop as an air parcel rises and
cools below the dew point.
• Usually a trigger or process is need to
initiate the rise of an air parcel.
Cloud Development
• Convection
– Differential land surface heating creates areas
of high surface temperature.
– Air above warm land surface heats, forming a
‘bubble’ of warm air that rises or convection.
– Cloud base forms at level of free convection.
Stepped Art
Fig. 6-16, p. 152
The development of a cumulus cloud.
The air’s stability greatly influences the growth of cumulus cloud
Cloud Development
• Topography
– Orographic uplift
– Orographic clouds
– Windward, leeward, rain shadow
– Lenticular clouds
Dry lapse rate Moist lapse rate Dew point lapse rate
10 deg/1000m
6 deg/1000m
2 deg/1000m
Satellite view of
wave clouds
forming many
kilometers
downwind of the
mountains in
Scotland and
Ireland.
Cloud Development
• Changing cloud forms
– Stratus clouds can change to cumulus clouds
if the top of the cloud cools and the bottom of
the cloud warms.
– Alto cumulus castellanus: towers on alto
stratus
– If moist stable air without clouds is mixed or
stirred it can form stratocumulus clouds.
Satellite view of cloud streets, rows of stratocumulus clouds forming over the warm
Georgia landscape.
Billow clouds forming in a region of rapidly changing wind speed, called wind shear.
An example of altocumulus castellanus.
The mixing of a moist layer of air near the surface can produce a deck of
stratocumulus clouds.
• Topic: Adiabatic charts
– Adiabatic charts show how various
atmospheric variables change with
height: pressure, temperature, humidity.
– Pressure/Altitude and Temperature
Dry adiabats added
Moist adiabats
Mixing ratio
The adiabatic chart. The arrows illustrate the example given in the text. The cloud
on the right side represents the base and height of the cloud given in the example.