Met Office College - Course Notes
Atmospheric stability
Contents
1. What is atmospheric stability?
2. Stability in dry air
3. Stability in cloudy air
4. Conditionally unstable air
5. Stability and air masses
6. Further reading
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1. What is atmospheric stability?
On some days the weather is characterised by vigorous vertical motions,
producing deep cumulus or cumulonimbus clouds, heavy showers and
thunderstorms. At other times the sky is overcast with a continuous
layer of grey, featureless stratus cloud. Why does the atmosphere exhibit
such marked variations in cloud formation from day to day?
The nature of vertical motions in the atmosphere depends on two
factors:

the vertical temperature profile;

the availability of moisture to form clouds.
Most of us are familiar with the idea that warm, less dense air will rise,
while cool, heavy air will sink. From this idea it follows that if the air
aloft is very cold, warmer air near the earth's surface will tend to rise.
Indeed, it will continue to rise until it reaches a level where it is no
longer warmer than the surrounding atmosphere. This type of
atmosphere is termed unstable (see box, below). If there is sufficient
moisture for clouds to form, an unstable atmosphere is characterised by
convective clouds – cumulus and cumulonimbus.
If, however, the air aloft is warmer than the air below, any cold air at the
surface that is pushed upwards will find itself colder than its
surroundings, and will sink back to the surface. Hence, vertical motions
are inhibited, and clouds tend to form in broad, flat layers. This type of
atmosphere is termed stable (see box, below). If there is sufficient
moisture for clouds to form, a stable atmosphere is characterised by
stratiform clouds such as stratus, altostratus and nimbostratus.
For a fuller explanation of atmospheric stability, there is one additional
factor to be taken into account: as air moves up or down in the
atmosphere its temperature does not remain constant.
Stable, unstable and neutral equilibrium
A body is said to be in stable equilibrium if, when given a small
disturbing impulse, it then returns to its original position.
A body is said to be in unstable equilibrium if, when given a small
disturbing impulse, it continues to move in the direction of the impulse.
A body is said be in neutral equilibrium if, when given a small
disturbing impulse, it then remains in the disturbed position.
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Atmospheric Stability
2. Stability in dry air
As air rises in the atmosphere, it experiences a steady decrease in
pressure. This causes its temperature to fall at a constant rate of 3°C for
every 1000 ft of ascent. This rate of cooling is known as the dry adiabatic
lapse rate1 (DALR). It is constant throughout the atmosphere, as long as
the air remains cloud-free (or unsaturated).
Now, consider the temperature change experienced by a ‘bubble’ of air
rising from the earth’s surface. If its initial temperature is 10°C, by the
time it reaches a height of 3000 ft it will have cooled to 1°C. At this
height, the temperature of the surrounding air will depend on the
general lapse rate in that location (called the environmental lapse rate, or
ELR). Three cases can be considered:
1. The environmental lapse rate is less than the DALR. If the ELR is
2°C per 1000 ft, the temperature 3000 ft above the surface will be
4°C. However, the bubble of air at this level has cooled to 1°C. Since
it is cooler than its surroundings it will tend to sink. This case is,
therefore, a stable atmosphere.
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of dry air
rising from the surface and
cooling at 3°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 1. Temperature profiles in a dry, stable atmosphere.
2. The environmental lapse rate is equal to the DALR. If the ELR is
3°C per 1000 ft, the temperature 3000 ft above the surface will be
1°C. The bubble of air at this level also has a temperature of 1°C.
Since it has the same temperature as its surroundings there will be
no forcing on it, either up or down. This case is described as a
neutral atmosphere.
1. The term lapse rate is used to describe the rate of temperature fall with height in the
atmosphere. Hence a positive lapse rate indicates a decrease of temperature with
increasing height.
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Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of dry air
rising from the surface and
cooling at 3°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 2. Temperature profiles in a dry, neutral atmosphere.
3. The environmental lapse rate is greater than the DALR. If the ELR
is 4°C per 1000 ft, the temperature 3000 ft above the surface will be
–2°C. However, the bubble of air at this level has only cooled to 1°C.
Since it is warmer than its surroundings it will continue rising. This
case is, therefore, an unstable atmosphere.
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of dry air
rising from the surface and
cooling at 3°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 3. Temperature profiles in a dry, unstable atmosphere.
These three stability states in dry air can be summarised as follows:

ELR < DALR :
stable atmosphere

ELR = DALR :
neutral atmosphere

ELR > DALR :
unstable atmosphere
3. Stability in cloudy air
The three cases above describe vertical motions in the atmosphere in the
absence of cloud. When air rises within cloudy air, the cooling causes
water vapour to condense onto the cloud droplets. This condensation
process gives out heat, changing the adiabatic lapse rate. Thus a ‘bubble’
of air rising within a cloud in the lowest few thousand feet of the
atmosphere cools at only around 1.5°C. This is known as the saturated
adiabatic lapse rate (SALR). It is not constant, but increases with height in
the atmosphere.
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Atmospheric Stability
In considering the stability of cloudy air, the atmospheric temperature
profile should be compared to the SALR. Consider the temperature
change experienced by a 'bubble' of air rising from the earth's surface
through cloud. If its initial temperature is 10°C, by the time it reaches a
height of 3000 ft it will have cooled to approximately 5.5°C. At this
height, the temperature of the surrounding air will depend on
environmental lapse rate. Again, three cases can be considered:
1. The environmental lapse rate is less than the SALR. If the ELR is
1°C per 1000 ft, the temperature 3000 ft above the surface will be 7°C.
However, the bubble of cloudy air at this level has cooled to 5.5°C.
Since it is cooler than its surroundings it will tend to sink. This is a
stable atmosphere.
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of cloudy air
rising from the surface and
cooling at ~1.5°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 4. Temperature profiles in a cloudy, stable atmosphere.
2. The environmental lapse rate is equal to the SALR. If the ELR is
1.5°C per 1000 ft, the temperature 3000 ft above the surface will be
5.5°C. The bubble of cloudy air at this level also has a temperature
of 5.5°C. Since it has the same temperature as its surroundings there
will be no forcing on it, either up or down. This is a neutral
atmosphere.
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of cloudy air
rising from the surface and
cooling at ~1.5°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 5. Temperature profiles in a cloudy, neutral atmosphere.
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3. The environmental lapse rate is greater than the SALR. If the ELR
is 2°C per 1000 ft, the temperature 3000 ft above the surface will be
4°C. However, the bubble of cloudy air at this level has only cooled
to 5.5°C. Since it is warmer than its surroundings it will continue
rising. This is an unstable atmosphere.
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of cloudy air
rising from the surface and
cooling at ~1.5°C per 1000 ft
2000
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 6. Temperature profiles in a cloudy, unstable atmosphere.
These three stability states in dry air can be summarised as follows:

ELR < SALR :
stable atmosphere

ELR = SALR :
neutral atmosphere

ELR > SALR :
unstable atmosphere
4. Conditionally unstable air
From sections 2 and 3 it can be seen that the effects of vertical motion
depend upon whether the air is dry or cloudy. For example, consider
again a ‘bubble’ of air at the surface with a temperature of 10°C, and an
environmental lapse rate of 2°C per 1000 ft (see Section 2, case 1 and
Section 3, case 3):
The environmental lapse rate is less than the DALR and greater than
the SALR. With an ELR of 2°C per 1000 ft, the temperature 3000 ft above
the surface will be 4°C. If the bubble remains dry and rises to this height,
it will cool to 1°C. Since it is cooler than its surroundings it will sink
back to the surface (i.e. stable). However, if the bubble is cloudy, it will
only cool to 5.5°C. Since this is warmer than its surroundings it will
continue rising (i.e. unstable). This case, where the stability depends
upon whether the air is dry or cloudy, is called conditionally unstable air.
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Atmospheric Stability
Height
(ft)
Actual temperature
of the atmosphere
3000
Temperature of cloudy air
rising from the surface and
cooling at ~1.5°C per 1000 ft
2000
Temperature of dry air
rising from the surface and
cooling at 3°C per 1000 ft
1000
–2
+1
+4
+7
+10
Temperature
(°C)
Figure 7. Temperature profiles in a conditionally unstable atmosphere.
5. Stability and air masses
Polar maritime and arctic maritime air masses travel from their cold
source regions towards Western Europe over a comparatively warm sea
surface. Such air masses tend to be conditionally unstable, with strong
vertical motions associated with cumulus or cumulonimbus clouds.
Tropical maritime air travels from the sub-tropical Atlantic over a
comparatively cold sea surface towards Western Europe. This air is
stable in both the dry and cloudy cases (termed absolutely stable) and is
characterised by layer clouds (stratus or stratocumulus). Stable, tropical
maritime air is also associated with deep layer clouds (alto-stratus,
nimbo-stratus and cirro-stratus) when subject to mass ascent within
depressions and frontal zones.
Polar continental and tropical continental air masses may be absolutely
stable or conditionally unstable, depending on the time of year and the
surface temperature en route from their source region.
It is unusual for the atmosphere to be unstable for both the dry and
cloudy cases. This absolutely unstable condition arises near the surface
under strong sunshine, but only extends to significant heights in the
atmosphere over hot desert regions.
6. Further reading
Further information on atmospheric stability may be found in the
following publications, both of which are available in the Met. Office
College library.
HMSO (1994) Handbook of Aviation Meteorology, pp. 35–36.
Barry, R.G. and Chorley, R.J. (1992) Atmosphere, Weather and Climate,
pp. 66–69.
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