Geoscience Reference
In-Depth Information
Water drops
Snow flakes
Ice particles
Wind
Speed
(C)
50
15
(B)
°C
20m/s
10m/s
5m/s
40
-51
12
30
-26
9
(A)
20
6
-8
0
18
10
3
28
Surface
Surface
0
5
Heavy surface
rain
Light surface
rain
Horizontal scale
km
Figure 5.17
Classic view of the cycle of a local thunderstorm. The arrows indicate the direction and
speed of air currents. A: the developing stage of the initial updraft. B: the mature stage with updrafts,
downdrafts and heavy rainfall. C: the dissipating stage, dominated by cool downdrafts.
Source: After Byers and Braham; adapted from Petterssen (1969).
upward and downward movements of air are
both the principal ingredients and motivating
machinery of such storms. They occur: (1) due to
rising cells of excessively heated moist air in an
unstable air mass; (2) through the triggering of
conditional instability by uplift over mountains;
or (3) through mesoscale circulations or lifting
along convergence lines (see p. 255).
The life cycle of a local storm lasts for only a few
hours and begins when a parcel of air is either
warmer than the air surrounding it or is actively
undercut by colder encroaching air. In both
instances, the air begins to rise and the embryo
thunder cell forms as an unstable updraft of
warm air (
Figure 5.17
). As condensation begins
to form cloud droplets, latent heat is released and
the initial upward impetus of the air parcel is
augmented by an expansion and a decrease in
density until the whole mass becomes completely
out of thermal equilibrium with the surrounding
air. At this stage, updrafts may increase from
3-5m s
-1
at the cloud base to 8-10m s
-1
some
2-3km higher, and they can exceed 30m s
-1
. The
constant release of latent heat continuously injects
fresh supplies of energy, which accelerate the
updraft. The air mass will continue to rise as
long as its temperature remains greater (or, in
other words, its density less) than that of the
surrounding air. Cumulonimbus clouds form
where the air is already moist as a result of
previous penetrating towers from a cluster
of clouds, and there is persistent ascent.
Raindrops begin to develop rapidly when the
ice stage (or freezing stage) is reached by the
vertical build up of the cell, allowing the Bergeron
process to operate. They do not immediately fall
to the ground, because the updrafts are able to
support them. The minimum cumulus depth for
showers over ocean areas seems to be between 1
and 2km, but 4-5km is more typical inland. The
corresponding minimum time intervals needed
for showers to fall from growing cumulus are
about 15 minutes over ocean areas and
30
minutes inland. Falls of hail require the special
cloud processes, described in the last section,
involving phases of 'dry' (rime accretion) and
'wet' growth on hail pellets. The mature stage of
a storm (see
Figure 5.18B
) is usually associated
with precipitation downpours and lightning (see
Plate 5.16
). The precipitation causes frictional
