Lightning
Review: Different types of precipitation
Depends on atmospheric temperature and winds
Snow
Rain
Sleet
Freezing rain
Graupel/hail
Lightning: Introduction
• Lightning is a discharge of
electricity, a giant spark
• 80% of lightning occurs when
clouds discharge electricity. This is
referred to as cloud-to-cloud
lightning; occurring when voltage
gradient overcomes the electrical
resistance of the air.
• 20% of lightning occurs when
electrical discharge travels
between the base of the cloud and
the surface. This is referred to as
cloud-to-ground lightning.
Duration of a lightning flash
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The averaged duration is 0.00003 second
Video: Lightning in super slow
motion
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https://www.youtube.com/watch?v=hFM05
Ja3fbw
Processes of Lightning Formation
1. Charge separation. Charge layers in the
cloud are formed by the transfer of positive
ions from warmer graupel to colder ice crystal
when they collide with each other.
2. Stepped leader. When the negative charge
near the bottom of the cloud is large enough to
overcome the air's resistance, a stepped
leader forms.
3. Return stroke. A region of positive ions
move from the ground toward this charge,
which then forms a return stroke into the cloud.
4. Dart leader. Not all of the first stroke
neutralizes the negatively charged ions and
results in another leader in 1/10 of a second
Lightning: Runaway discharges
• High-speed moving electrons radiate light as lightning.
• Electrons approach the speed of light, gaining energy. Fast
moving electrons create others by colliding with atoms. This
can result in an avalanche called runaway electrons.
• When a large number of runaway electrons accumulate in a
small volume, the energy is released in a so-called
runaway breakdown.
Shao et al.
2011
Thunder
• Charge differences between the thunderstorm and ground can
cause lightning strokes of 30,000°C, and this rapid heating of air
will creates an explosive shock wave called thunder.
• It takes about 3 seconds for thunder to travel 1 kilometer (5 sec
per mile). A lag in lightning strike and thunder occurs due to
sound traveling slower than light.
• When thunder is farther away, the echoing of sound waves off of
objects (like buildings and hills) causes thunder to sound
rumbling.
Refraction of thunder
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Sound waves move faster in
warm air than they do in cool air.
Typically, the air temperature
decreases with height. When
this occurs, thunder will normally
have an audible range up to 10
miles (16 km).
However, when the air
temperature increases with
height, called an inversion,
sound waves are refracted (bent
back toward the earth) as they
move due to their faster motion
in the warmer air effectively
amplifying the thunder.
Global distribution of lightning strikes
Significant difference between continent and ocean
Annual mean number of days with
thunder
U.S Distribution of lightning time
Video: All Alone in the Night –
Time-lapse footage of the Earth as seen
from the International Space Station
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http://www.youtube.com/watch?v=FG0fTKAqZ5g
Climate impacts of lightning
(1) Nitrogen cycle
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The enormous energy of lightning breaks nitrogen molecules and
enables their atoms to combine with oxygen in the air forming
nitrogen oxides.
These dissolve in rain, forming nitrates, that are carried to the
earth.
Contributes some 5– 8% of the total nitrogen fixed.
Was probably the only source of fixed nitrogen for the earliest forms
of life.
Climate impacts of lightning
(2) Ozone production
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Ozone was named after the Greek
verb ozein (ὄζειν, "to smell"), from
the peculiar odor in lightning storms.
Nitrogen oxides produced by
lightning can react with others in the
presence of sunlight to produce
ozone.
Since most lightning occurs inside a
storm, the added ozone tends to
show up several miles high rather
than near the earth's surface, so it
doesn't add significantly to ozone
pollution at ground level.
Climate impacts of lightning
(3) Damages
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Annual mean fatalities 69 for U.S.,
600 world-wide.
Hundreds of people are
permanently injured
Annual mean property loss $38
million for U.S.
Lightning is the leading cause of
wildfires (25,000 each year)
Especially caused by lightning in
dry thunderstorms (virga)
Lightning Safety
Practice the following when lightning is present:
• Always take cover in a building.
• Do not stand under a tree or other tall object that might
serve as a lightning rod.
• Avoid standing on mountain summits, ridges, rooftops,
or other high areas.
• Avoid caves.
• Avoid open water (pool, lake, or hot tub).
Summary
1. Two types of lightning (cloud-to-cloud 80%, cloud-toground 20%)
2. 4 steps of lightning development.
3. How fast does thunder travel?
4. Climate impacts of lightning: nitrogen cycle, ozone,
wildfire
5. Lightning safety