Geoscience Reference
In-Depth Information
feasible, the processes responsible for such effects
were unknown. A principal cause is the 'seeder-feeder'
('releaser-spender') cloud mechanism, proposed by
Tor Bergeron and illustrated in Figure 5.14. In moist,
stable airflow, shallow cap clouds form over hilltops.
Precipitation falling from an upper layer of altostratus
(the seeder cloud) grows rapidly by the wash-out
of droplets in the lower (feeder) cloud. The seeding
cloud may release ice crystals, which subsequently
melt. Precipitation from the upper cloud layer alone
would not give significant amounts at the ground, as
the droplets would have insufficient time to grow in the
airflow, which may traverse the hills in 15 to 30 minutes.
Most of the precipitation intensification happens in the
lowest kilometre layer of moist, fast-moving airflows.
moist, as a result of previous penetrating towers from a
cluster of clouds, and there is persistent ascent (Plate 6).
Raindrops begin to develop rapidly when the ice
stage (or freezing stage) is reached by the vertical
buildup 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 2 km, but 4 to 5 km is more
typical inland. The corresponding minimum time inter-
vals 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 5.17B) is usually
associated with precipitation downpours and lightning
(see Plate 12). The precipitation causes frictional
downdrafts of cold air. As these gather momentum, cold
air may eventually spread out in a wedge below the
thunder cell. Gradually, as the moisture of the cell is
expended, the supply of released latent heat energy
diminishes, the downdrafts progressively gain in power
over the warm updrafts, and the cell dissipates.
To simplify the explanation, a thunderstorm with
only one cell was illustrated. Storms are usually far
more complex in structure and consist of several cells
arranged in clusters of 2 to 8 km across, 100 km or
so in length and extending up to 10 km or more (see
Plate 11). Such systems are known as
squall lines
(see Chapter 9I).
≥
G THUNDERSTORMS
1 Development
In mid-latitudes the most spectacular example of
moisture changes and associated energy releases in the
atmosphere is the thunderstorm. Extreme 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 airmass; (2) through the trigger-
ing of conditional instability by uplift over mountains;
or (3) through mesoscale circulations or lifting along
convergence lines (see p. 201).
The lifecycle 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 equi-
librium with the surrounding air. At this stage, updrafts
may increase from 3 to 5 m s
-1
at the cloud base to
8 to 10 m s
-1
some 2 to 3 km higher, and they can
exceed 30 m s
-1
. The constant release of latent heat
continuously injects fresh supplies of energy, which
accelerate the updraft. The airmass 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
2 Cloud electrification and lightning
Two general hypotheses help to account for thunder-
storm electrification. One involves induction in the
presence of an electric field, the other is non-inductive
charge transfer. The ionosphere at 30 to 40 km altitude
is positively charged (owing to the action of cosmic and
solar ultraviolet radiation in ionization) and the earth's
surface is negatively charged during fine weather.
Thus cloud droplets can acquire an induced positive
charge on their lower side and negative charge on their
upper side. Non-inductive charge transfer requires
contact between cloud and precipitation particles.
According to J. Latham (1966), the major factor in
cloud electrification is non-inductive charge transfer
involving collisions between splintered ice crystals and
warmer pellets of soft hail (graupel). The accretion of
