Geoscience Reference
In-Depth Information
Absolute instability in the atmosphere is infrequent
except very close to the ground - the convection it initi-
ates helps to transfer heat upwards and so reduces the
environmental lapse rate. What is much more common is
for the environmental lapse rate to lie between the dry
adiabatic lapse rate and the saturated adiabatic lapse
rate (
Figure 4.4
- area b
). In this situation of conditional
instability the atmosphere is stable for air which has not
reached saturation point, but is unstable for saturated air.
If the air can be forced to reach the condensation level,
either by ascent over hills or mountains, or by convergence
associated with low-pressure systems, it will become unsta-
ble and assist vertical motion. The former process is one
of the mechanisms which leads to higher rainfall over
mountains.
(a) absolute stability
(b) conditional instability
(c) absolute instability
(a)
SALR
(b)
ELR
ELR
DALR
ELR
(c)
Causes of condensation
Clouds are one of the most interesting aspects of the sky
(see additional case study 'Use of clouds as indicators
of the state of the atmosphere' on the support website
at
www.routledge.com/textbooks/9780415395168
). Their
shape and form change constantly to reflect the processes
of formation and the environment in which they are devel-
oping. To produce clouds, we need the air to reach satu-
ration point, either by cooling the air or by adding water
to air (
Figure 4.2
).
It is by cooling of the air that the major-
ity of clouds are formed. Orographic lifting, convergent
uplift near depressions or within air streams, and con-
vection will all produce vertical motion which may be suf-
ficient to produce clouds. The second process of adding
Temperature
mental lapse rate (ELR), the dry adiabatic lapse rate (DALR)
and the saturated adiabatic lapse rate (SALR).
is stable convection will be reduced. Sometimes, especially
when air pressure is high, the temperature will increase
with height - a situation known as an inversion of
temperature. If the air beneath the inversion is fairly
moist, a layer of cloud may develop here. Moist air will
have been brought to the inversion by convection and, as
it cannot rise further, it may spread out beneath the
inversion to form a sheet of cloud (
Figure 4.5
).
Parcel warmer
than environment
to this level
Cloud
Inversion
SALR
Condensation level
Cloud base
DALR
ELR
Figure 4.5
The effect of an inversion of
temperature on cloud
development with moist air.
Temperature
