CHAPTER 5
CLOUDS AND STABILITY
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Remember that rising air expands and cools, and
sinking air compresses and warms
When this happens in unsaturated conditions, it is
called an adiabatic process – no heat is gained or lost
This is reversible – the air parcel has the same
temperature at the beginning and end
But what happens if at some point along its path the
parcel becomes saturated, and a cloud forms? (more
on this in a minute…)
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Simple definition: the condition is stable if
an object, when forced to move, will tend to
move back to its original position
If it tends to continue moving away from its
original position, it is unstable
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In atmospheric science, we often consider the
motions of air parcels, which can heat and cool
but do not mix in air from the outside
Rising parcels expand and cool, sinking parcels
are compressed and warm
At the same height, warm air is less dense than
cool air
An unsaturated (dry) parcel will always rise and
sink at 9.8°C per km – the dry-adiabatic lapse
rate
For the sake of discussion today, we’ll just use
the round value of 10 °C per km for the dryadiabatic lapse rate
Stepped Art
Fig. 5.2, p. 123
Moist parcels
• When parcels rise and cool, they will
eventually become saturated
• As they continue to rise, the water vapor
will condense into liquid water – a cloud
• Remember what happens during
condensation: latent heat is released
inside the parcel
• So, instead of cooling at 10°C per km, it
cools slower – usually around 6°C per km
– this is the moist-adiabatic lapse rate
Table 5.1, p. 123
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Parcels (individual bubbles of air) always rise
and sink at either the dry or moist adiabatic
rate
The environment (the air surrounding the
parcel) can have a variety of temperature
profiles
This environmental rate is measured with
radiosondes (on weather balloons)
Since the adiabatic lapse rates are known, we
can compare them to the environmental lapse
rate to determine whether the parcel will be
stable or unstable
◦ If a parcel is forced to rise, and it’s warmer than its
environment, it will continue to rise: unstable
◦ If it is forced to rise and is cooler than its
environment, it will sink back down to the original
position: stable
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When the environmental lapse rate is less
than the moist adiabatic rate (6°C/km)
◦ All parcels will be colder than their environment
and sink back to their original position
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Example: temperature inversion (temperature
increases with height) – acts as a lid
Often occurs at night due to radiational
cooling
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When environmental lapse rate equals the
dry-adiabatic lapse rate
Unsaturated parcels will not tend to rise or
sink in this condition – they tend to stay
where they are
If environmental lapse rate equals moistadiabatic lapse rate, then the atmosphere is
neutrally stable for saturated parcels
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When the environmental lapse rate is greater
than the dry-adiabatic lapse rate
◦ All parcels will be warmer than their environment
and continue to rise
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This condition usually only occurs near the
surface, and not for very long – the rising
parcels will bring the warm air up and cool air
will mix down around them
No need for a helicopter here!
But this would also never happen!
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When the environmental lapse rate is between
the dry and moist adiabatic lapse rates
◦ Unsaturated parcels will sink back to their original
position, saturated parcels will continue to rise
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There is instability, on the condition that the
parcel is saturated
On average, the atmosphere is in this state
(average environmental lapse rate =
6.5°C/km)
This helps to explain why cumulus clouds can
grow very tall (into cumulonimbus) – only
saturated parcels continue to rise
Need the helicopter here
But not here
Fig. 5.10, p. 130
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Warming at low levels or cooling aloft:
increases lapse rate, makes atmosphere less
stable
◦ “Warm air advection” at low levels will destabilize
the atmosphere
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Mixing
◦ If absolutely unstable, mixing will make atmosphere
more stable
◦ If stable, mixing can make atmosphere unstable
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Lifting or subsidence in layers
If a whole layer sinks, it compresses in the
denser air near the ground, top warms
more than bottom, and becomes more
stable
If a whole layer rises, it stretches in the lessdense air aloft, and it tends to become more
unstable, especially when the bottom is
saturated and the top is dry – convective
instability
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In real life, we don’t have helicopters pulling
parcels of air up and down…so what causes
air to rise and sink?
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Lifting Condensation Level
(LCL) : level at which
condensation occurs – this
represents the cloud base
◦ Force an unsaturated parcel to rise
from the surface
◦ Temperature will decrease by 10 °C
per km, dew point by 2 °C per km
until saturated
◦ At the point where the T and Td are
equal, the parcel is saturated, water
can condense, and a cloud can
form (T and Td will fall at the same
rate above this)
◦ Handy formula: LCL = 125 (T-Td),
where T and Td are in Celsius, and
LCL will be in meters
Stepped Art
Fig. 5.16, p. 134
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Level of Free Convection
(LFC) : level at which air
will continue to rise
without forcing
◦ Force an unsaturated parcel
to rise from the surface
◦ Calculate how the
temperature cools with
height (using dry adiabatic
rate when unsaturated, moist
adiabatic when saturated)
◦ Find the point where the
parcel’s temperature
becomes warmer than its
environment – above this
point, it will rise freely
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Equilibrium level (EL): level above the LFC at which
the air will stop rising
◦ Continue the process that you used for the LFC
and find the point at which the parcel’s
temperature becomes equal to or cooler than the
environmental temperature
If the atmosphere is very unstable, this level can be
very high, and this situation is conducive to deep
cumulonimbus development
Nashville, TN
North Platte, NE
Nashville, TN
North Platte, NE
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Mountains can provide the lifting required to
bring air to its LCL and/or LFC
On the windward side, clouds often form and
rain/snow is common
On the leeward side there is sinking motion –
a “rain shadow”
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If unsaturated, temperature will be the
same at start and finish
But if saturation is reached…this is no
longer reversible
Heat is released in condensation
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In stable conditions above
a mountain, waves can set
up in the airflow
Clouds can form on the
upward parts of the
waves, and they appear to
be stationary (standing
wave clouds)
They sometimes take a
“flying saucer” appearance
Lenticular wave clouds
SPECIAL TOPIC
• Subsidence Inversions
– Strong subsidence exacerbates air pollution due to the lack of
vertical motion
– Pollution is not diluted
FOCUS
• Lake Effect Snow (Great Lakes)
– Snowstorms that form on the downwind side
of one of these lakes are known as lake effect
snows
– Cold air moves over the lakes when they are
relatively warm and not quite frozen
– Water vapor absorbed
– Becomes snow on edge of lake
• Rising motion due to heat addition and
slowing/terrain
Lake Effect Snow
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
– Altocumulus castellanus: castle-like towers on
altocumulus
– If moist stable air without clouds is mixed or
stirred it can form stratocumulus clouds.
Fig. 5.25, p. 140
Fig. 5.26, p. 141
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dry-adiabatic lapse rate