Lightning and Tornadoes
Thunderstorms: brief review
• There
are two basic types of thunderstorm cells: ordinary cells and
supercells.
• Ordinary
cell thunderstorms tend to form where warm, humid air
rises in a conditionally unstable atmosphere and where vertical wind
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shear is weak. They are usually short-lived and go through their life
cycle of growth (cumulus stage), maturity (mature stage) and decay
(dissipating stage) in less than an hour. They rarely produce severe
weather.
As wind shear increases (and the winds aloft become stronger)
thunderstorms are more likely to become severe and produce strong
surface winds, large hail, heavy rain, and even tornadoes.
The cells that comprise a multicell thunderstorm can be ordinary or
supercell. A squall line is a long line of multicell thunderstorms that
may form along a frontal boundary or out ahead of it. A Mesoscale
Convective Complex is a large circular cluster of multicell
thunderstorms.
Flooding: typically in the spring, when the snow is melting and the
rivers are full. Flash floods: floods that rise rapidly with little or no
advance warning.
Lightning
Compare to:
Plasma ball in the
lobby outside
•
What is it? An electrical discharge – a giant spark!
Electricity 101
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• Same charge particles repel each other
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• have no net charge.
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Particles may carry positive or negative charge.
Opposite charge particles attract each other
Electrons are negatively charged. They are very mobile.
Nuclei are positively charged.
Molecules are made up of nuclei and electrons. They
Ions are molecules which have gained or lost electrons:
they can have a negative or a positive net charge.
The motion of the charges results in electrical current.
+
+
-
+
Charging a Thundercloud
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Raindrops, snow crystals and hail
stones collide inside the cloud.
During these collisions they may
exchange electrons and ions.
The exact mechanism is not well
understood, but the bottom line is:
♦ Larger particles become
negatively charged.
♦ Smaller particles become
positively charged.
Larger particles settle down to the
bottom of the cloud.
Smaller particles are lifted to the
top of the cloud by strong updrafts.
Note precipitation area: void of
large particles -> positively charged.
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Type of Discharges
Cloud-to-ground
♦ 90% of the time: negative cloud to positive ground
♦ 10% of the time: positive cloud to negative ground
Cloud-to-cloud: between oppositely charged regions of
different clouds.
Cloud-to-atmosphere: self-explanatory
Base-to-top: discharge within the same cloud
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If all the charges are in the cloud why
does the lightning strike the ground?
The negative charges at the bottom of
the cloud repel the electrons on the
ground beneath the cloud. The ground
just below the cloud is then positively
charged (notice the tree). The ground
away from the cloud will be negatively
charged with excess electrons (not
shown) which came from (used to be in)
the tree
Once the accumulated charges become
large enough some electrons will “jump”
to the ground. A current starts to flow.
The molecules in the atmosphere below
the cloud are bombarded with flying
electrons. They become excited and
then emit light (we see a spark).
Stages of Cloud-to-Ground Lightning.
• The
lightning is initiated with a flow of electrons from the base of the
cloud towards the ground (stepped leader)
• When
the leader gets close to the ground a flow of positive charges
surges upwards.
• When
they meet a strong current (return stroke) transfers a charge
between the cloud and the ground.
• The
process can be repeated several times in the same channel (dart
leader, return stroke…)
1
2
3
Lightning Varieties
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• Ribbon lightning: when a strong wind shifts the
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• precipitation
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Forked lightning: when the dart leader deviates from
the original path of the stepped leader
ionized channel between successive return strokes
Bead lightning: when the ionized channel breaks up
Ball lightning: a floating luminous sphere
Sheet lightning: when the flash is inside or obscured
by a cloud
Dry lightning: when there is lightning but no
Heat lightning: distant lightning which is not heard.
Since it happens very far, its short-wavelength light
is scattered and it appears orange (recall setting sun).
Different Types of Lightning
Ribbon lightning
Forked lightning
Bead lightning
Heat lightning
Different Types of Lightning
Cloud-to-ground
Sheet
Cloud discharge
Ball
Other lightning phenomena
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• Fulgurite: fused sand particles as a result of the strong
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St. Elmo’s fire. Corona discharge at the top of antennas
or ship masts.
heating at the location of the lightning stroke.
Cloud-to-upper atmosphere lightning
♦ Elves, red sprites, blue jets
Thunder
•
• It results from the fast expansion of the gas that is
•
We see the light emitted from the air molecules as they
become excited from the electrical discharge.
The thunder is the sound that accompanies the lightning.
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heated to high temperatures from the electrical discharge.
Sound propagates much slower than light
V(sound)
=
330 m/s
V(light in vacuum) = 300,000,000 m/s
That is why we hear the thunder long after the lightning
How far is the lightning? Count the seconds between the
flash and the thunder. Each second is worth 330m~1000ft.
What if there is no thunder? Sometimes the sound wave is
reflected or absorbed while traveling through the air and
may not reach us
Sonic boom: just a jet going supersonic.
In case of a thunderstorm:
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If outdoors:
♦ Seek shelter inside, preferably inside a
building with a lightning rod
♦ If not, stay inside the car
♦ Do not stay under trees.
♦ If hiking above the treeline, descend
immediately.
♦ It is NOT safe to be in a tent, small picnic
shelters, near heavy machinery.
If indoors:
♦ Stay away from water, plumbing, doors and
windows
♦ Do not use land line telephones.
♦ Turn off, unplug, and stay away from
appliances, computers, power tools, TVs.
♦ Consider purchasing a heavy duty surge
protector
♦ Bring pets inside - especially dogs chained to
trees
Lightning safety awareness week
♦ June 24-30, 2007.
US Map of Lightning Distribution
Lightning distribution worldwide
Tornadoes: Some Statistics
• Tornado
is a rapidly rotating column of air around an intense low
pressure center that touches the ground.
•• The
rotation is typically cyclonic (counterclockwise) in the NH.
In the US there are about 1000 tornadoes a year.
•• On
average there are about 100 tornado casualties a year.
Most occur in the tornado alley that runs through the central
plains
• Tornado
season: March-July
♦ Spring in the south;
♦ Summer in the north.
• Conditions:
♦ Unstable atmosphere: warm humid air below, cool dry air aloft
♦ Strong wind shear.
• Most
likely time of the day: late afternoon (the atmosphere is
most unstable)
•• Typical
diameter: 100-600 m, extreme cases ~ 4 km
Typical wind speed: 40-300 mi/h
• Typical duration: a few minutes to several hours.
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Tornado Occurrences in the US
Number of tornadoes within a 25 year period
♦ Upper figure: total per state
♦ Lower figure: annual rate per state per 10,000 sq. miles
Fujita Scale for Tornadoes
Severe Weather and Doppler Radar
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• measures the intensity of the
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• Doppler effect: the change of the
A Doppler Radar uses radio waves
To measure precipitation it
reflected radio waves.
To measure wind speed it
measures the frequency of the
reflected radio waves
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wave frequency as the object
moves – the siren of an ambulance
has a higher pitch when it is
approaching.
Cannot measure the speed of
parallel winds: need 2 radars
Doppler radar can reveal a
Tornado Vortex Signature (TVS).
Not a real tornado:
Funnel cloud
Wall cloud
Landspout
Dust-tube tornado
Waterspout
(see active figure)
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Most famous tornado of all times
It’s a twister! It’s a twister!