LIGHTNING INJURIES - Medicine for Mountaineering and Other Wilderness Activities - page 440

particles tend to separate under the influences of updrafts and gravity until the upper por-
tion of the cloud acquires a net positive charge and the lower portion of the cloud be-
comesnegatively charged.Theseparation ofchargesproducesanenormouselectric poten-
tial—millions of volts—both within the cloud and between the cloud and ground. Even-
tually the electrical resistance of the air breaks down and a flash begins. Lightning is an
electrical discharge between positive and negative regions of a thundercloud.
Withtheinitialbreakdownoftheaerialresistance,astreamerbeginstopropagatedown-
ward toward Earth's surface. The streamer, which cannot be seen, moves in discrete steps
of about 50 meters, in what is called a stepped leader. As it grows, it creates an ionized
path, depositing charge along the channel. As the stepped leader nears Earth's surface, a
large potential difference is generated between the end of the leader and the surface. Typ-
ically,astreamer islaunchedfromthesurfaceandintercepts thedescending steppedleader
just before it reaches the ground. Once a connecting path is achieved, a return stroke flies
upthealreadyionizedpathathalfthespeedoflight.Thisreturnstrokereleasestremendous
energy,brightlight,andthunder.Theaveragenumberofstrokesineachboltoflightningis
four, each lasting about thirty milliseconds, which causes lightning to flicker.
Occasionally, when a thunderstorm grows over a tall, earth-grounded object, such as
a radio antenna, an upward leader may propagate from the object toward the cloud. This
“ground-to-cloud” flash generally transfers a net positive charge to earth and is character-
ized by upward-pointing branches.
Theinitialbreakdownandpropagationaresimilarforcloud-to-cloudorintracloudlight-
ning, but the discharge occurs between regions of opposite charge. Without the benefit of
air conducting the strike to Earth's surface, intracloud lightning does not produce a return
stroke. Nevertheless, tremendous energy, bright light, and thunder are still produced by in-
tracloud lightning.

Table 29-1 Facts about Lightning

The spot where lightning is going to strike is not determined until the bolt is about 30
meters fromthegroundorobject. Shorterobjects maybestruckeventhoughataller object
isnearby.Associated stepvoltages andsurface arcsmaybefatal morethan40meters from
the initial strike point. Lightning is unpredictable!
Lightning channels as long as 10 miles have been observed. These can produce a “bolt
from the blue,” a lightning strike that occurs without thunderclouds overhead or even
nearby. Lightning is very unpredictable! ( Table 29-1 )