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slows down to the saturated lapse rate as long as the
bubble's temperature is still higher than that of the
environment. In our example, the temperature difference
between the parcel (SALR) and the environment (ELR)
becomes greater as the parcel rises above condensation
level. Under these conditions we get large convec-
tional clouds building up that will probably bring rain
( Plate 4.2 ).
Whether the atmosphere is still stable or not will
depend upon the relative rates of cooling of the dry
bubbles, the saturated bubbles and the environment. We
can summarize this in Figure 4.4 . If the environmental
lapse rate is cooling more rapidly than the dry adiabatic
lapse rate we have absolute instability, as bubbles of air,
even if they cool at their maximum rate (the DALR), will
be cooling more slowly than the environment (area c).
If the environmental lapse rate is cooling more slowly
than the saturated adiabatic lapse rate we have absolute
stability (area a). If the environmental lapse rate is between
the DALR and the SALR we have conditional instability
(area b); in other words, instability depends upon the
air reaching saturation point.
Stability has a considerable effect upon the degree to
which convective activity will take place. If the air is
unstable it will rise and may produce clouds, whereas if it
Temperature
of
parcel
environment
1500
6
2
SALR
1000
8
11
10
Condensation level
12
Cloud base
ELR
DALR
500
15
14
21
20
0
10
20
Temperature (°C)
Figure 4.3 The effect of condensation on the rate of cooling
of an air parcel. It is assumed that the parcel starts slightly
warmer, owing to localized heating.
Plate 4.2 An unstable atmosphere with cumulonimbus clouds. The fibrous nature of the higher part of the cloud indicates ice
crystal formation.
Photo: Peter Smithson
 
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