Textbook chapter 2, p. 3336 chapter 3, p. 7479 chapter 4, p. 122123

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Textbook chapter 2 p. 33-36 chapter 3 p.
74-79chapter 4 p. 122-123

• Stability and Cloud Development

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The Development of Clouds
Why does the atmosphere sometimes produce
stratus clouds while at other times cumulus
and/or cumulonimbus clouds form
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what caps the smoke plume and clouds
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Temperatures of Rising and Sinking Air Parcels
Recall that in the troposphere the temperature
typically decreases at an average lapse rate of
6.5C per kilometer .
This observed rate of temperature decrease is
called the Environmental Lapse Rate (ELR).
ELR -T/z
this equals 3.6 F/1000 ft
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Temperatures of Rising and Sinking Air Parcels
Rising air parcels undergo decompression as they
move upwards into regions of lower pressure.
Decompression causes adiabatic cooling.
For unsaturated (RH cooling known as the Dry Adiabatic Lapse Rate
(DALR) is about 10C per kilometer.
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Similarly sinking air parcels warm adiabatically
as they are compressed at lower altitudes.
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Note that rising parcels do NOT cool because they
are moving into regions of lower temperature!
Cooling results from decompression of the air.
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LCL and MALR
As a rising air parcels cools its relative
humidity increases eventually reaching 100 when
the temperature has declined to the parcels dew
point temperature. This level is called the
lifting condensation level.
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Above the LCL condensation takes place as the
rising parcel continues to cool adiabatically.
Condensation releases latent heat which
mitigates against the adiabatic cooling. Thus
above the LCL the parcel cools less rapidly.
This lower cooling rate is the Moist Air Lapse
Rate (MALR). Its value depends on the latent
heat release rate. It usually lies between
4C/km and 9C/km.
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Dry vs moist adiabatic ascent
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Below the LCL the parcel cools at the DALR.
Above the LCL the parcel cools at the MALR.
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Moist adiabatic processes

• High supersaturation is not observed. Instead RH
  in a cloud is never far from 100.
• This means that T and Td remain essentially equal
  to one another as the air continues to ascend.
• As water molecules condense from gas to liquid
  the vapor mixing ratio decreases.

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Stability and Instability
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Stability and Instability in the Atmosphere
In a stable atmosphere vertically displaced air
parcels tend to return spontaneously to their
original position.
In an unstable atmosphere vertically displaced
air parcels tend to move spontaneously further
away from their original position.
In a neutrally stable atmosphere vertically
displaced air parcels neither return to nor
continue to move away from their original
position.
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How can we determine the stability condition of
the atmosphere
The stability condition of the atmosphere can be
determined by comparing the density of an air
parcel with that of the surrounding air at the
same pressure (altitude) level.
Parcels that are more dense than their
surroundings will sink whereas those that are
less dense will rise.
Warmer parcels are less dense and tend to rise
while colder parcels are more dense and tend to
sink.
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Atmospheric Stability and Instability
At z 3 km the temperature (Te) is 10C. At
this altitude A parcel with a temperature (Tp)
of 30C will rise.
A parcel with a temperature (Tp) of -10C will
sink.
A parcel with a temperature (Tp) of 10C will
neither rise nor sink.
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Atmospheric Stability and Instability
Atmospheric stability or instability can be
determined by comparing an air parcels
temperature with that of the surrounding air at
the same pressure.
Our comparison requires that the temperature
profile be known. This profile known as the
environmental lapse rate (ELR) is measured twice
per day at 0000 and 1200 UTC at numerous
observing stations around the world.
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An Absolutely Stable Atmosphere
Lifted unsaturated air parcels cool adiabatically
at the DALR (d) (green line). Lifted saturated
parcels cool at the MALR (m) (red line).
When the ELR is small rising parcels are always
colder than the surrounding air so they tend to
resist lifting and sink down.
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An Absolutely Stable Atmosphere
An inversion is especially stable.
Stability at the surface inhibits vertical mixing
and fog dispersion
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An Absolutely Unstable Atmosphere
Lifted unsaturated air parcels cool adiabatically
at the DALR (d) (green line). Lifted saturated
parcels cool at the MALR (m) (red line).
When the ELR is large rising parcels are always
warmer than the surrounding air so they are
buoyant and rise spontaneously.
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Conditional Instability
Lifted unsaturated air parcels cool adiabatically
at the DALR (d) (green line). Lifted saturated
parcels cool at the MALR (m) (red line).
When the ELR lies between the DALR and the MALR
dry parcels will sink while saturated parcels
will spontaneously rise.
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An Example of Conditional Instability
Consider an atmosphere with an ELR of -8C/km.
An unsaturated parcel rises from the surface.
The initial parcel temperature is 30C with a
dew point temperature of 14C.
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An Example of Conditional Instability
As the parcel cools towards the dew point
temperature its relative humidity increases.
Note that the parcel is being lifted in a stable
atmosphere.
The parcel becomes saturated (RH 100) at 2 km.
This is therefore the LCL.
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An Example of Conditional Instability
Above the LCL the parcel cools less rapidly at
the MALR. Recall that this is due to latent heat
release from condensation partially offsetting
adiabatic cooling. Above the LCL the relative
humidity of the parcel is 100.
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An Example of Conditional Instability
At an altitude of 4 km the parcel temperature
becomes equal to the ambient temperature. Up to
this altitude the parcel has been lifted through
a stable atmosphere.
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An Example of Conditional Instability
Above 4 km the parcel becomes warmer than its
environment. It therefore now rises
spontaneously because it has entered a region of
the atmosphere that is unstable for saturated
parcels. The altitude where the parcel becomes
buoyant is the level of free convection (LFC).
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An Example of Conditional Instability
Cloud base is found at the LCL where the RH
first becomes 100.
Cloud top occurs at the altitude where the parcel
is no longer buoyant (9 km in this example).
Note that above the LFC cloud height is
determined by the depth of the unstable layer.
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Day-to-night change in lapse rate
near noon
near sunrise
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Growing a thunderstormtry this applet