Powerpoint about Lightning

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Lightning
Facts about lightning
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80% of lightning is from cloud-to-cloud.
Only 20% strikes the ground.
Primarily in mature cumulonimbus clouds.
More frequent at higher altitudes.
Lightning always produces thunder-however
you can’t always here it.
• Thunder arrives after 5 seconds for each mile.
Why is there
lightning?
• As we all know positive and
negative charges attract one
another while like charges repel .
• In a storm cloud most of the
positive charges are pushed
towards the top of the cloud,
while the negative charges are at
the bottom.
•This is especially true when
positive charges are sticking up
from the ground.
•These positive charges want to
jump up from the ground to the
negative cloud base, but they
can’t due to the fact that the air is
an electrical insulator.
Stages of a Lightning Strike
1.
Stepped Leader
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The strong attraction between the cloud
and the ground produces a large
electrical field.
Near the cloud, the air molecules can
break apart into positive and negative
ions, allowing electricity to flow more
freely.
These regions of “ionization” finger down
from the clouds in a branching structure
in rapid jumps, or steps.
This is called the “stepped leader,” and
has the structure of an inverted tree.
You can’t see the lightning yet…it’s
invisible.
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Stages of a Lightning Strike
1.
Stepped Leader
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As the stepped leader gets closer to the
ground, it draws the positive charge even
stronger to its location.
If you are the positive charge then you
may feel your hair standing on end.
Eventually, a positive ionization leader
leaves the ground and meets the
stepped leader a few hundred feet above
the ground.
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Stages of a Lightning Strike
2.
Return Stroke
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When the ionization channel reaches
the ground, it has “broken the dam”
and charge rushes between the cloud
and the ground.
This is the lightning stroke you see.
The electric flow heats the air to 30,000
K, and you see a bright white or blue
glow.
The heat expands the air. This 100-fold
rapid expansion and subsequent
collapse creates the shock wave called
thunder.
Note that the whole branched
structure lights up, not just the direct
path from the ground to the cloud.
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Stages of a Lightning Strike
3.
Dart Leader
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After the stroke has finished, charge within
the cloud may redistribute, putting negative
charge back near the old stroke.
This negative charge finds the old channel,
and rapidly creates a new leader, now
called a “dart leader.”
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Stages of a Lightning Strike
4.
Multiple Return Stroke
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When the dart leader hits the ground,
you get a new return stroke, usually less
bright.
This time, only the direct path lights up,
since the other paths dissipated and
weren’t reestablished in the dart leader.
Many strokes may occur in one “strike”
within the first second.
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