Geoscience Reference
In-Depth Information
Figure 5.19
The electric charge structure in airmass storms in New Mexico, supercell storms and the convective elements of
mesoscale convective systems (see Chapter 9), based on balloon soundings of the electric field - 33 in updrafts and 16 outside them.
There are four vertical zones in the updraft region and six in the downdraft region, but the size, strength and relative positions of
the up- and downdrafts vary, as do the heights and temperatures shown.
Source
: Stolzenburg
et al
. (1998) Fig. 3, by permission of the American Geophysical Union.
carrying small hail aloft. Lightning commonly begins
more or less simultaneously with precipitation down-
pours and rainfall yield appears 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. The first
(leader) stage of the flash bringing down negative charge
from the cloud is met about 30 m 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 for only about 0.5 seconds (Plate
12). The extreme heating and explosive expansion of
air immediately around the path of the lightning sets
up intense sound-waves, causing thunder to be heard.
The sound travels at about 300 m s
-1
. Less commonly,
positive CG flashes occur from the upper positive region
(Figure 5.18B, case (i)), and they predominate in the
stratiform cloud sector of a travelling convective storm
(Chapter 9I). Positive charge can also be transferred
from a mountaintop or high structure towards the cloud
base (case (ii)). In the United States, over 20 per cent of
flashes are positive in the Midwest, along the Gulf Coast
and in Florida. Figure 5.18A represents a simple dipole
model of cloud electricity; schemes to address the com-
plexity shown in Figure 5.19 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 100 V m
-1
near the surface,
decreasing to about 1 Vm
-1
at 25 km, whereas beneath
a thundercloud it reaches 10,000 V m
-1
immediately
before a discharge. The 'breakdown potential' for light-
ning to occur in dry air is 3
10
6
V m
-1
, but this is ten
times the largest observed potential in thunderclouds.
Hence the necessity for localized cloud droplet/ice
