Geoscience Reference
In-Depth Information
produces an irregular surface, which is warmed as
the droplets release latent heat on freezing. The
impacts of ice crystals on this irregular surface
generate negative charge, while the crystals
acquire positive charge. Negative charge is usually
concentrated between about -10
place. Above/below the altitude of this threshold,
graupel pellets charge negatively/positively. The
lower area of positive charge represents larger
precipitation particles acquiring positive charge at
temperatures higher than this threshold. The
origin of the uppermost zone of negative charge
is uncertain, but may involve induction (so-called
'screening layer' formation) since it is near the
upper cloud boundary and the ionosphere is
positively charged. The non-updraft structure
may represent spatial variations or a temporal
evolution of the storm system. The origin of the
positive area at the cloud base outside the updraft
is uncertain, but it may be a screening layer.
Radar studies show that lightning is associated
with both ice particles in clouds and with rising air
currents carrying small hail aloft. Lightning
commonly begins more or less simultaneously
with precipitation downpours and rainfall yield
appearing to be correlated with flash density. The
most common form of lightning (about two-
thirds of all flashes) occurs within a cloud and is
visible as
sheet lightning.
More significant are
cloud-ground (CG) strokes. These are frequently
between the lower part of the cloud and the
ground which locally has an induced positive
charge (
Figure 5.19A
). The first (leader) stage of
the flash bringing down negative charge from the
cloud is met about 30m above the ground by a
return stroke, which rapidly takes positive charge
upward along the already formed channel of
ionized air. Just as the leader is neutralized by the
return stroke, so the cloud neutralizes the latter
in turn. Subsequent leaders and return strokes
drain higher regions of the cloud until its supply
of negative charge is temporarily exhausted.
The total flash, with about eight return strokes,
typically lasts about 0.5 seconds (
Figure 5.19
). The
extreme heating and explosive expansion of air
immediately around the path of the lightning sets
up intense soundwaves, causing thunder to be
heard. The sound travels at about 300m s
-1
. Less
commonly, positive CG flashes occur from the
upper positive region (
Figure 5.19B
, case (1)), and
they predominate in the stratiform cloud sector of
C in a
thundercloud, where ice crystal concentrations
are large, and due to splintering of crystals at about
the 0
°
and -25
°
C level and the ascent of the crystals
in up-currents. The separation of electrical charges
of opposite signs may involve several mechanisms.
One is the differential movement of particles under
gravity and convective updrafts. Another is the
splintering of ice crystals during the freezing
of cloud droplets. This operates as follows: a
supercooled droplet freezes inward from its surface
and this leads to a negatively charged warmer core
(OH-ions) and a positively charged colder surface
due to the migration of H+ ions outward down the
temperature gradient. When this soft hailstone
ruptures during freezing, small ice splinters
carrying a positive charge are ejected by the ice shell
and preferentially lifted to the upper part of the
convection cell in updrafts. However, the ice-
splintering mechanism appears to work only for a
narrow range of temperature conditions, and the
charge transfer is small.
The vertical distribution of charges in a
thundercloud, based on balloon soundings, is
shown in
Figure 5.18
. This general scheme applies
to air mass thunderstorms in the southwestern
USA, as well as to supercell storms and mesoscale
convective systems described in Chapter 9. There
are four alternating bands of positive and negative
charges in the updraft and six outside the updraft
area. The lower three bands of the four in the
updraft are attributed to collision processes.
Ice crystals carried upward may explain why
the upper part of the cloud (above the -25°C
isotherm) is positively charged. Negatively
charged graupel accounts for the main region of
negative charge. There is a temperature threshold
around -10° to -20°C (depending on the cloud
liquid water content and the rate of accretion on
the graupel) where charge-sign reversal takes
°
to -5
°
