The initial cause of a lightning strike is a separation of
charge within a cloud. Positively charged particles accumulate at the top of the
cloud while negatively charged particles concentrate themselves at the bottom of
the cloud. Unfortunately, it is currently unknown why this charge separation
occurs. This model illustrates a widely held theory in this respect. The water
cycle itself is the primary cause. As water vapor rises it often acquires a
charge as it interacts with dirt, ice, and other materials during its ascent
towards the cloud. Once in the cloud the heavier vapor particles have a negative
charge and thus sink to the cloud's bottom. The actual mechanism behind the
creation of charge separation is unimportant in the actual strike and so the
water cycle and charge separation are included in this model solely to
demonstrate how the charge separation may occur. Vapors rise off of the water's
surface with a frequency specified by the EVAPORATION-RATE slider. The vapors
then rise toward the sky until they reach the cloud, at which time they spawn
opposing charges in the upper and lower regions of the cloud. The ELECTRIC-FIELD
is measured by the number of negatively charged particles (electrons) in the
cloud. Since the strength of an electric field is directly proportional to the
amount of charge in that field, this is a reasonable measure.
Within the cloud the charges move in a random manner. However, they are
constrained to their charge regions. The positive charges do not try to reach
the negatives nor vice versa. In this way the charge separation and electric
field strength is maintained. Occasionally charge may dissipate from the cloud.
While the actual mechanism for this dissipation is also unknown, it is
illustrated in this model by precipitation in the same manner as the charge
buildup. Charge leaves the cloud at a rate specified by the PRECIPITATION-RATE
slider. When the EVAPORATION-RATE equals the PRECIPITATION-RATE the strength of
ELECTRIC-FIELD will remain fairly constant.
When ELECTRIC-FIELD has strength of 40 or greater the charges in the ground
begin to be affected. The negative charges in the bottom of the cloud have such
a high concentration that they force the electrons on the Earth's surface deep
into the ground. This also has the affect of pulling the positive charges
(positrons) in the Earth's surface to a very shallow depth. Thus, the ground has
a strong positive charge and the cloud has a strong negative charge. The two
surfaces act like two parallel plates (for those who know a bit of electrical
physics). This charge disparity cannot continue forever, and lightning is
nature's solution to the problem.
Charge continues to build until the ELECTRIC-FIELD hits the critical value of
80. With field strength of such a large magnitude the electrons in the cloud
begin "breaking down" the air around the cloud and converting it into plasma.
Plasma is the same material that comprises stars. Without going into too much
detail, plasma is matter (in this case air) that has been ionized. The positive
and negative components of the air itself are pulled apart and separated from
each other. This separation allows the electrons to flow through the plasma much
more easily than they could through normal air. The plasma is created in a
chain-reaction manner as electrons flowing through the plasma force the air
around them to become plasma in turn. In this way electrons in the cloud "burn
out" paths towards the earth. The paths are known as "step leaders" and grow
from cloud to ground in a tentacle-like manner. The chain reaction that allows
the step leaders to grow in size does not have to take place in a straight line.
Air is not homogenous and is littered with impurities that affect the formation
of step leaders. These impurities cause the air to not ionize equally in all
directions. Impurities in the air may cause some patches of air to turn into
plasma more easily than others. Rather than direct lines from cloud to ground,
lightning takes the path of least resistance. This is what gives lightning
strikes the forked and jagged patterns that everyone can recognize. In this
model this is illustrated by assigning each patch of air a RESISTANCE value.
Electrons will ionize air and progress towards the ground always trying to
follow the path of least resistance.
As the step leaders progress to the ground the ELECTRICAL-FIELD likely will
still be increasing in magnitude. Once it reaches a value of 120 the charges in
the ground react by letting off what are known as "streamers". Streamers are the
positive equivalent of the negative step leaders created by clouds. However,
streamers are not self-sufficient and thus do not grow indefinitely towards the
cloud. All objects on the Earth's surface will emit a streamer, though depending
on the size and material of the object the streamer's length may vary. A tall
building will release a larger streamer than a person on the sidewalk. The
streamer's release is what causes people to often feel a tingling sensation
before a lightning strike. The positive charges in the ground are reaching out
to the negative charges in the sky. Streamers are released much more quickly
than a step leader since they are much smaller and only extend a very limited
distance.
Step leaders progress towards the ground until they encounter either the ground
or a streamer. In both cases the "circuit" is completed and charge may flow
freely between cloud and ground. Since step leaders and streamers grow in an
irregular way the strike may occur at an otherwise unexpected place. It is not
uncommon for lightning to strike a 6-foot tall person instead of the 40-foot
tree nearby. Once a connection has been made charges begin to move through the
path. Contrary to popular understanding, the actual lightning strike flows from
the ground up. The large concentration of positive charges on the earth's
surface flow very quickly through the plasma stream towards the sky and
neutralize the electrons in the cloud. The flash of light that is seen is the
rapid movement of charge through the air, exactly the same as the light you see
during a spark of static electricity. This massive movement of charge results in
an enormous current that generates an equally enormous amount of heat around the
strike. The current and heat are what cause damage to people and objects during
a lightning strike. When the strike occurs the air surrounding the lightning's
path reaches temperatures hotter than the surface of the sun.
When a step leader completes the path between cloud and ground, the charges that
were forming other step leaders quickly flow back up towards the cloud. This
rapid movement of charge causes secondary strikes that usually accompany a
lightning strike. While the step leader that actually makes contact is known as
the primary strike, several secondary strikes are often visible for short
periods of time in the sky surrounding the cloud. These strikes do not extend
all the way to the ground, but can still be impressive to watch. There may be
several primary strikes within a given lightning strike, but only one set of
secondary strikes. In this model you may observe secondary strikes as the
electrons in the step leaders move back into the cloud once contact is made.
Once the massive negative charge in the cloud has been neutralized the flow of
charge through the air comes to a stop. The plasma becomes de-ionized and once
again becomes air as the step leaders are destroyed. The environment is now
ready to begin the process anew.