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Ground Current
A
typical strike injects 20,000 amps into the Earth so
past lightning education tended to dwell on direct
strikes as the primary mechanism of injury. Yet direct
strikes account for only 3-5% of lightning fatalities.
Since the Earth resists electrical flow, large voltage
differences appear in the ground all around the strike
point. The closer you are to the strike, the stronger
the ground current.
More
important than the total voltage is the distance between
your feet. If one part of your body contacts one voltage
and another part of your body contacts a different
voltage, the greater the difference in voltage.
Side Flash
When
lightning hits a tree or other tall object, the main
current follows the tree trunk to the ground. Some of
the current often arcs across the air to a path of
lesser resistance (like people). This “side flash” is
more common with trees than with towers. Since side
flash emanates from tall objects when they are struck,
never seek shelter near a tree, other tall
object, or tall vertical surface.
Side
flash contributes to 20-30% of lightning fatalities and
is one of the reasons that the “cone of protection” is a
myth. Images of groups of dead animals that were
touching fences when lightning struck the wire are
common nowadays on the Internet. You surely don’t want
to be one of them.
Get Inside
There really is no such thing as a surprise storm. Yet
every year in the US, about 40 people die and another
400 are injured by lightning. You can reduce your
chances by a few simple safety procedures.
Set
turnaround times that will get you off of exposed
terrain before a storm hits. Observe changing weather
and discuss it with your group. Change your plan rather
than summit a peak or cross open ground during a
thunderstorm. And get inside a substantial building or
metal-topped vehicle when you hear thunder.
If
there is NO chance to get to a building or vehicle
before a storm hits:
·
Find
safer terrain as soon as you hear thunder. In a flat,
quiet, windless location, most people can hear thunder
for about 10 miles; in windy conditions, 5 miles, and in
hard rain, just 1 mile. Some people have been struck
before they heard thunder at all. Of course, wearing
iPods or other musical devices may preclude hearing
thunder as well. You can also try the flash-bang system
to guess how far away a thunderstorm is, mindful
that you might not be able to tell which flash is
associated with which bang. Count the number of seconds
between the obvious flash and the obvious bang, divide
by 5; the result is approximately the distance in
miles between you and the storm.
·
Avoid peaks, ridges, and high ground. If you have a
choice, descend a mountain on the side that has no
clouds, since strikes will be less frequent on that side
until the clouds move in.
·
Avoid trees and long conductors (wire fence, etc.) once
lightning gets close; ground current accounts for 40-50%
of lightning fatalities. If you need to move through a
forest while seeking safer terrain, stay away from tree
trunks as you move. Avoid open areas that are the width
of a football field or wider. Lone trees are especially
dangerous: you are hundreds of times safer in a forest
with hundreds of trees than you are near a lone tree in
an open space.
·
In
wide, open ground, look for a dry ravine or other
significant depression. If you’re in a group, spread out
at 50’ intervals to reduce ground current. If you are
the tallest thing for 150’, the next stepped leader
could be headed to you.
·
Naturally wet ground, like damp ground next to a stream,
isn’t any more dangerous than dry ground. Wet ground
actually dissipates ground current faster. Standing
in water, however, is very dangerous.
·
Avoid cave entrances. Small overhangs can allow arcs to
cross the gap. Natural caves that go well into the
ground can be struck, either at the entrance or through
the ground. If you are near an entrance to a cave, don't
stand in water, avoid metal conductors, and avoid
bridging the gap between ceiling and floor. Move quickly
through the entrance (in or out) to minimize your
exposure.
·
If
you are boating, monitor the weather and get off before
the storm arrives. When you get to shore, look for
protective terrain. Be especially cautious of trees at
the edge of water because they might be the tallest
objects around. Boats that can't get off the water in
lightning-prone areas should have lightning protection (http://nasdonline.org/document/209/d000007/boating-lightning-protection.html)..
·
Get
in the lightning position if lightning is striking
nearby but consider it may do more harm than good if you
stop moving to a less risky location. Limit your body’s
tendency to launch upward leaders by crouching into a
tight ball as close to the ground as possible. Keep your
feet close together to limit the voltage difference.
Wrap your arms around your legs. Do NOT lie down. Do not
kneel.
·
If
you feel hairs on your head, leg, or arms tingling and
standing on end, you are in an extremely high electric
field. The response to any of these signs should be to
instantly (seconds matter) move away from long
conductors, tall trees, or high points, spread out, and
adopt the lightning position. Do not ignore these signs
and do not try to run to safety, unless safety is
literally seconds away. If any of these signs are
detected, the probability of a close discharge is very
high. Every effort should be made to minimize injuries
and the number of injured.
·
Avoid tents. Tent poles conduct ground current and may
generate upward leaders. If you are in a tent in "safer
terrain" and you hear thunder, get in the lightning
position. If your tent is in an exposed location, such
as on a ridge, in a broad open area, or near a tall
tree, get out of the tent and move to safer terrain.
Determine a meeting spot, have rain gear and flashlights
accessible, and have a plan for managing your group as
well.
·
Stay
in a safe location for 30 minutes or more after hearing
the last thunder.
I
love being in the woods. I love watching thunder and
lightning. But from now on, I think I’ll do my watching
indoors! |
