As the particles within a cloud (called hydrometeors) grow
and interact, some become charged possible through collisions. It is thought
that the smaller particles tend to acquire positive charge, while the larger
particles acquire more negative charge. These particles tend to separate under
the influences of updrafts and gravity until the upper portion of the cloud
acquires a net positive charge and the lower portion of the cloud becomes
negatively charged. This separation of charge produces enormous electrical
potential both within the cloud and between the cloud and ground. This can
amount to millions of volts, and eventually the electrical resistance in the air
breaks down and a flash begins. Lightning, then, is an electrical discharge
between positive and negative regions of a thunderstorm.
lightning flash is composed of a series of strokes with an average of about
four. The length and duration of each lightning stroke vary, but typically
average about 30 microseconds. (The average peak power per stroke is about 1012
is generated along the length of the lightning channel as the atmosphere
is heated by the electrical discharge to the order of 20,000 degrees C
(3 times the temperature of the surface of the sun). This compresses the
surrounding clear air producing a shock wave, which then decays to an
acoustic wave as it propagates away from the lightning channel.
the flash and resulting thunder occur at essentially the same time, light
travels at 186,000 miles in a second, almost a million times the speed of sound.
Sound travels at the relatively snail pace of one-fifth of a mile in the same
time. Thus the flash, if not obscured by clouds, is seen before the thunder is
heard. By counting the seconds between the flash and the thunder and dividing by
5, an estimate of the distance to the strike (in miles) can be made.
CLOUDS AND RAIN
When moisture-laden warm air is heated, it begins to rise.
As these currents or bubbles of warm moist air rise higher in the atmosphere,
both the surrounding air pressure and temperature decrease. The air bubbles
expand, causing cooling of the moisture which eventually condenses to form
clouds. As the cloud cools further, more moisture condenses and the water
droplets making up the cloud grow and merge until some become so large and heavy
that the air currents within the cloud can no longer support them. These water
droplets begin to fall as rain.
Air currents in cumulonimbus clouds can be very violent.
Even when lightning is not produced, pellets of ice may grow by the accumulation
of liquid droplets. When the updrafts are very strong, the growing ice pellets
can be suspended for long periods, allowing them to grow larger. Eventually some
may become too large for a given updraft and begin to fall as hail. Diameters
are typically 5 to 10 mm, although a l40 mm hailstone has been recorded.
THE MOST COMMON
TYPES OF LIGHTNING
lightning is the most damaging and dangerous form of lightning. Although
not the most common type, it is the one which is best understood. Most flashes
originate near the lower-negative charge center and deliver negative charge to
Earth. However, an appreciable minority of flashes carry positive charge to
Earth. These positive flashes often occur during the dissipating stage of a
thunderstorm's life. Positive flashes are also more common as a percentage of
total ground strikes during the winter months.
lightning is the most common type of discharge.
This occurs between oppositely charged centers within the same cloud. Usually
the process takes place within the cloud and looks from the outside of the cloud
like a diffuse brightening which flickers. However, the flash may exit the
boundary of the cloud and a bright channel, similar to a cloud-to-ground flash,
can be visible for many miles.
ratio of cloud-to-ground and intra-cloud lightning can vary significantly from
storm to storm. Storms with the greatest vertical development may produce
intra-cloud lightning almost exclusively. Some suggest that the variations are
latitude-dependent, with a greater percentage of cloud-to-ground strikes
occurring at higher latitudes. Others suggest that cloud-top height is a more
important variable than latitude.
of why a discharge stays within a cloud or comes to ground are not understood.
Perhaps a flash propagates toward the Earth when the electric field gradient in
the lower regions of the cloud is stronger in the downward direction.
upon cloud height above ground and changes in electric field strength between
cloud and Earth, the discharge stays within the cloud or makes direct contact
with the Earth. If the field strength is highest in the lower regions of the
cloud a downward flash may occur from cloud to Earth.
lightning, as the name implies, occurs between charge centers in two
different clouds with the discharge bridging a gap of clear air between them.
OTHER TYPES OF
There are numerous names and descriptions of various types
and forms of lightning. Some identify subcategories, and others may arise from
optical illusions, appearances, or myths. Some popular terms include: ball
lightning, heat lightning, bead lightning, sheet lightning, silent lightning,
black lightning, ribbon lightning, colored lightning, tubular lightning,
meandering lightning, cloud-to-air lightning, stratospheric lightning, red
sprites, blue jets, and elves.
LIGHTNING DISCHARGE PROCESSES
With the initial breakdown of the air in a region of
strong electric fields, a streamer may begin to propagate downward toward the
Earth. It moves in discrete steps of about 50 meters each and is called a
stepped leader. As it grows, it creates an ionized path depositing charge along
the channel, and as the stepped leader nears the Earth, a large potential
difference is generated between the end of the leader and the Earth. Typically,
a streamer is launched from the Earth and intercepts the descending stepped
leader just before it reaches the ground. Once a connecting path is achieved, a
return stroke flies up the already ionized path at close to the speed of light.
This return stroke releases tremendous energy, bright light and thunder.
Occasionally, where 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 characterized by upward pointing branches.
|The lower part of a thundercloud is usually
negatively charged. The upward area is usually positively charged.
Lightning from the negatively charged area of the cloud generally
carries a negative charge to Earth and is called a negative flash. A
discharge from a positively-charged area to Earth produces a positive
initial breakdown and propagation are similar for intra-cloud lightning, but the
discharge generally occurs between regions of opposite charge. Without the
benefit of air conducting Earth, intra-cloud lightning does not produce a
return-stroke-like feature. Rather, it is characterized by slower propagating
"recoil streamers" and "K" changes. Nevertheless, tremendous
energy, bright light, and thunder are still produced by intra-cloud lightning.