AS 120 Principles of Aeronautical Science

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Cloud Development - Introduction
to Atmospheric Stability

Ever wonder why clouds form on some
days and not on others?


Why does the atmosphere sometimes produce
stratus clouds (thin layered) while other times
we get cumulus, or cumulonimbus clouds to
form?
The answer depends on concept of
atmospheric stability.....
Stable Environment

Consider a marble in the bottom of a bowl
 If you push the marble up the side of the bowl, it
will roll back down to the bottom, to its original
position
Stable Atmosphere

Parcels in a stable environment will not rise
Vertical motion is inhibited
 If clouds form, they will be shallow, layered
clouds like stratus

Unstable environment

If the marble is on
the top of the bowl
and you give it a little
push, it rolls off the
bowl.... does NOT
come back to it's
original position
 This is an unstable
situation
Unstable Atmosphere


Unstable air (parcel) - vertical motion occurs
 Commonly produces Cu, Cb clouds
So, how do we determine the stability of the
atmosphere?
Rising air parcels and adiabatic
cooling
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Consider a rising parcel of air
As the parcel rises, it will adiabatically
expand and cool
Adiabatic - a process where the parcel
temperature changes due to an expansion
or compression alone, (no heat is added
to or taken away from the parcel)
The parcel expands since the lower
pressure outside allows the air molecules
to push out on the parcel walls
Since it takes energy for the parcel
molecules to "push out" on the parcel
walls, they use up some of their internal
energy in the process.
Therefore, the parcel cools since
temperature is proportional to molecular
internal energy
Sinking air parcels and adiabatic
warming

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

A sinking parcel of air:
As the parcel sinks, it will
adiabatically compress and
warm
The parcel compresses since
it is moving into a region of
higher pressure
Due to the parcel’s
compression, the air
molecules gain internal
energy
The mean (average)
temperature of the parcel
increases
Dry adiabatic lapse rate
Dry adiabatic cooling=
10oC/1000m
What will the
temperature of the
parcel be if it is raised
to 1 km?
30oC
Moist Adiabatic Lapse Rate

At 2 km, the temperature
and the dew point
temperature lines intersect



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The parcel has become
saturated
After saturation is reached,
the parcel will cool at a
smaller rate
A saturated parcel of air,
cools at the moist
adiabatic lapse rate =
6°C/km (3.5oF/1000ft)
What will be the parcel's
temperature be at 3 km?
Moist Adiabatic Rate

What will be
the parcel's
temperature
be at 4 km?
14oC
Moist Adiabatic Rate

Why does the parcel
cool at a slower rate
(6°C/km) when it is
saturated and a faster
rate (10°C/km) when
it is unsaturated?
8oC
Dry versus Moist-Adiabatic Process

The moist adiabatic
lapse rate is less
than the dry
adiabatic lapse rate
because as vapor
condenses into
water in a saturated
parcel, latent heat is
released into the
parcel--partially
offsetting the
adiabatic cooling
Applying this to determine the
stability of the atmosphere

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To this point, we’ve learned that a parcel
of air will cool at either the dry or moist
adiabatic rate when it is lifted.
We now have to compare the
temperature of the parcel to the
temperature of the atmosphere that
surrounds it

If the parcel is warmer (lighter, less dense)
than the atmosphere surrounding it--it will
rise


Unstable atmosphere
If the parcel is cooler (heavier, more dense)
than the atmosphere surrounding it—it will
sink

Stable atmosphere
Assessing Atmospheric Stability

The bottom line To determine whether or not a parcel will
rise or sink in the atmosphere, we must
compare the parcel’s temperature (Tp)
with that of the environment (Te) at some
altitude:
 if Tp > Te what will the parcel do?
Te
 if Tp = Te what will the parcel do?
 if Tp < Te what will the parcel do?

How do we find the temperatures in
the atmosphere above us?

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Vertical profiles of
atmospheric
temperature, winds
and dew point are
collected at 12 and
00 UTC every day
from over 1000
locations worldwide
by launching weather
balloons
(rawinsondes)
Temperature, dew
point and winds are
plotted on a diagram
called a Skew-T, LogP diagram
RAWINSONDE
LAUNCH--UZBEKISTAN
100 gram balloon
Combat Weather HUAH
Rawinsonde
1. Absolute Stability
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If a parcel is lifted
from the surface
it will cool either
dry or moist
adiabatically
But in either case,
the parcel will be
cooler than the
environment.
This is an
example of
absolute stability
Absolutely Stable
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
So an absolutely stable
parcel (whether it is
unsaturated or saturated)
will always be cooler than
the environment and will
sink back down to the
ground
The condition for absolute
stability is: Γe<Γm<Γd
(Γ is “gamma”=lapse
rate)



Γd is the dry adiabatic
lapse rate (10°C/km)
Γm is the moist
adiabatic lapse rate
(6°C/km)
Γe is the environmental
lapse rate (In this
example= 4°C/km)
Stability of Inversion Layers

How would you
characterize the stability
of an inversion layer?


stable
Note that the absolute
stability criteria:


Inversions are absolutely
Γe<Γm<Γd
How do stable layers
form in the atmosphere?
Formation of Stable Layers



How does the
atmosphere form stable
layers?
1. Radiational Cooling radiation inversion
2. Cold air advection at
low levels


Behind a cold front (over
land)
3. Warm air moving over
cold ground

Fog forming over snow
fields
2. Absolute Instability



This is an example of
absolute instability
Everywhere on this
diagram an unsaturated
or a saturated parcel will
always be warmer than
the environment and will
continue to rise
The condition for
absolute instability is:
Γe> Γd >Γm
 Γe is the


environmental lapse
rate (12°C/km)
Γd is the dry
adiabatic lapse rate
(10°C/km)
Γm is the moist
adiabatic lapse rate
(6°C/km)
3. Conditional Instability




This is an example of conditional
instability
An unsaturated parcel will be
cooler than then environment and
will sink back to the ground
The saturated parcel will be
warmer than the environment
and will continue to ascend
The condition for conditional
instability is: Γd > Γe > Γm
 Γd is the dry adiabatic lapse
rate (10°C/km)
 Γe is the environmental lapse
rate (8°C/km)
 Γm is the moist adiabatic
lapse rate (6°C/km)
Conditional Instability - example
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Let’s start with a parcel
on the surface with a
temperature and dew
point of 30 °C and 10°C,
respectively
The parcel is initially
forced to rise in an
environment where the
environmental lapse rate
(Γe) is 8°C/km up to 8
km.
Let's follow the parcel
upward…
Conditional Instability - 1km
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The parcel must rise
dry adiabatically
(10°C/km) because it
not saturated

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The parcel
temperature < Te, so
something is forcing
the parcel upward
Onward to 2km .....
Conditional Instability - 2km

The parcel reaches
saturation at 2km

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The temperature of
the parcel is still < Te,
so something is still
forcing the parcel
upward...
Onward to 3km .....
Conditional Instability - 3km
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The parcel now rises
moist adiabatically
(6°C/km)


The parcel
temperature is still
cooler than the
environment, so
something is still
forcing it upward....
Upward to 4km .....
Conditional Instability - 4km
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The parcel continues to
rise moist adiabatically
(6°C/km)
 Notice that now parcel
temperature = Te
What happens if the
parcel is pushed upward
just a little???
Conditional Instability - 4km+
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The height where the parcel temperature
becomes equal to or larger than it’s
environment is level of free convection
The parcel is still rising moist adiabatically
(6°C/km)
The parcel will continue to rise until the parcel
becomes cooler than the environment (at 9km
above the ground)

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Above that point, parcel temperature < Te , so
the parcel will rise no further
Below 4 km where parcel is cooler than the
environment, the atmosphere is stable and
there will be no upward parcel movement
(something must push the parcel upwards to
the level of free convection)
Above 4 km where parcel is warmer than the
environment, the atmosphere is unstable, and
the parcel will rise on it’s own
This is an example of a conditionally unstable
atmosphere... the condition is lifting the parcel
above 4 km where it can then rise on it's own
The “real atmosphere”
What are the
stabilities of
each layer?
(Recalling that:
Γd=10oC/km
Γm=6oC/km)

Γd=10oC/km
Γm=6oC/km)
Processes that destabilize the
atmosphere
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1. Cold air advection
aloft

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This often occurs when an
extratropical cyclone (a
winter low pressure
system) passes overhead
2. Surface Heating

Tells us that the
atmosphere will be most
unstable at time of
maximum surface heating
Processes that destabilize the
atmosphere
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3. Warm air advection
at low levels


This often occurs
ahead of a cold front
4. Cool air moving
over a warm surface

A common example is
after a cold front
passes us and goes
into the Gulf of Mexico
Atmospheric Instability and Cloud
Development

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Where will the base
(bottom) of a cloud form?
What determines the
height to which the cloud
will grow?

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Using the previous example
of a rising air parcel
On this diagram, where is
cloud base?
On this diagram, where is
cloud top?
Atmospheric Instability and Cloud
Development
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On this diagram, where is
cloud base?
 Where the parcel
reaches saturation -- 2
km
On this diagram, where is
cloud top?
 Where the parcel will
no longer be able to
rise -- 9 km
Abs Stable
Abs Unstable
Atmospheric Instability and
Cloud Development - lifting
mechanisms
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Two questions should
arise at this point:
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1. How are vertical parcel
motions that create clouds
generated naturally in the
atmosphere?
2. What kind (if any)
clouds will you visually
observe in:



an absolutely stable
environment?
a conditionally unstable
environment?
an absolutely unstable
environment
Atmospheric Instability and Cloud
Development - Convection
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Convection usually occurs
when the surface is heated
and a surface parcel
becomes warmer than the
environment
The vertical extent of the
cloud is largely determined
by the stability of the
environment
In an absolutely stable
environment, no clouds will
likely form
Atmospheric Instability and Cloud
Development - Shallow Convection

In a shallow
conditionally unstable
or absolutely unstable
environment, we
might expect clouds
to develop, but their
vertical growth will be
limited...

We may observe
Cumulus Humilis
(shallow cumulus)
 Stratocumulus

Atmospheric Instability and Cloud
Development - Deep Convection

In a deep conditionally
unstable or absolutely
unstable environment, we
would expect clouds with
significant vertical
development to form

We might observe:
Cumulus congestus
 Cumulonimbus

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