Geoscience Reference
In-Depth Information
(A)
(B)
10km
10km
(i)
(ii)
Sur
f
ace
S
u
rface
Figure 5.19
Classic view of the vertical distribution of electrostatic charges in a thundercloud and at the
ground. A shows the common transfer of negative charge to the surface in a lightning stroke; B shows
other cases: (1) when positive charge from the upper part of the cloud is transferred towards a locally
induced area of negative charge at the surface; (2) positive charge transfer is from a summit or surface
structure towards the cloud base.
a traveling convective storm (Chapter 9). Positive
charge can also be transferred from a mountain
top or high structure towards the cloud base
(case (2)). In the United States, over 20 percent of
flashes are positive in the Midwest, along the Gulf
Coast and in Florida.
Figure 5.19
represents a
simple dipole model of cloud electricity; schemes
to address the complexity shown in
Figure 5.18
remain to be developed.
Lightning is only one aspect of the atmospheric
electricity cycle. During fine weather, the earth's
surface is negatively charged, the ionosphere
positively charged. The potential gradient of this
vertical electrical field in fine weather is about
100V m
-1
near the surface, decreasing to about
1V m
-1
at 25km, whereas beneath a thundercloud
it reaches 10,000V m
-1
immediately before a
discharge. The 'breakdown potential' for lightning
to occur in dry air is 3
ions conduct electricity from the ionosphere down
to the earth, and hence a return supply must be
forthcoming to maintain the observed electrical
field. A major source is the slow
point discharge
,
from objects such as buildings and trees, of ions
carrying positive charge (electrons) induced by
the negative thundercloud base.
Upward currents have recently been discovered
high above the stratiform regions of large
convective storm systems with positive CG
lightning. Brief luminous emissions, due to
electrical discharges, appear in the mesosphere
and extend downward to 30-40km. These so-
called
sprites
are red in the upper part, with blue
tendrils beneath. The red color is from neutral
nitrogen molecules excited by free electrons. In
the ionosphere above, a luminous expanding
ring (termed 'elve') may occur. High above the
lightning storm, a discharge takes place because the
imposed electric field of a vertical dipole exceeds
the breakdown potential of the low-density air. The
electrically conductive ionosphere prevents sprites
extending above 90km altitude.
10
6
V m
-1
, but this is ten
times the largest observed potential in thunder-
clouds. Hence, the necessity for localized cloud
droplet/ice crystal charging processes, as already
described, to initiate flash leaders. Atmospheric
×
