Lightning occurs when electrostatic charges
build up in clouds and are then discharged,
similar to how you can get a shock from touching a
door knob after shuffling your feet on the
How a charge becomes charged is still an area
of active research, but we do know that the top
of the cloud becomes positively charged while the
bottom of the cloud becomes negatively charged.
Two processes that play a role in creating this
charge separation are the friction of colliding
droplets of water and ice in the cloud and
freezing. As the millions and millions of water
droplets and ice particles whirl about in the
cloud, they collide with rising moisture and in
the process knock off electrons (negatively
charged particles), leaving the rising water
droplets positively charged. The knocked off
electrons gather near the bottom of the cloud
producing a negative charge, while the positively
charged droplets rise toward the top of the cloud.
As water droplets rise, they encounter colder air
temperatures and start to freeze. The frozen
portion of the droplet tends to acquire a negative
charge while the unfrozen portion tends to acquire
a positive charge. Air currents can rip off the
liquid portion of the water droplet and carry it
to the top of the cloud while the frozen portion
falls toward the bottom of the cloud. Thus the
cloud becomes even more polarized (positive charge
at the top, negative charge at the bottom).
This separation of charges creates an electric
field that repels the negatively charged electrons
at the surface of the earth away from the
negatively charged bottom of the cloud (because
opposite charges attract and like charges repel
each other). This creates a net positive charge at
the surface of the Earth.
Once the electric field becomes strong enough,
it will strip electrons from the air molecules,
creating a soup of positively charged ions and
negatively charged electrons called a plasma.
Normally air is an insulator (that is, it does not
conduct electricity, think of the plastic material
on the outside of an electrical cord), but the
ionized air is a conductor of electricity (think
of the copper wire inside the electrical cord).
Once a conducting path of ionized air has formed
between the earth and the cloud, electrons will
flow rapidly along this path to neutralize the
charge build-up. This is the lightning strike.
This rapid discharge of electricity heats the
surrounding air to temperatures hotter than the
sun, producing the bright light we see. This heat
also causes the air air to expand violently,
creating a shockwave; this is thunder.
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