MET 102 Pacific
Climates and Cultures
Lecture 17: Thunderstorms in
the Pacific
Discussion Questions – Ahrens 2015
• What are the three basic ingredients Thunderstorms need to form? Do the Pacific
Islands satisfy these requirements?
1. Moist surface Air
2. a conditionally unstable atmosphere
3. a mechanism or “trigger” that forces the air to rise
They definitely satisfy the moist surface air and, in some locations, the “trigger” to force air to rise
(mountains). Those islands near the ITCZ will see more lightning then those in South Pacific.
• What are supercells and how are they related to tornadoes? In general, do we see
supercells in the Pacific Islands?
• Supercell thunderstorms are intense thunderstorms with a single rotating updraft. The updraft
and the downdraft in a supercell are nearly in balance, so that the storm may exist for many
hours. Supercells are capable of producing severe weather, including strong damaging tornadoes.
• We are unlikely to see supercells in the Pacific do to the lack of ideal thunderstorm forming
conditions.
Discussion Questions – Ami 1860
• In what context is lightning and storminess mentioned? What do you think
this signifies?
• The birth of the Hawaiian Prince
Kalanikauikeaoli, the third in the line
of the Kahekili
• Storms are powerful natural events
and it is not surprising that it would
be associated with the birth of a
member of the ruling class. The
lightning flashes above the royal flag
and the thunder (clarion trumpet) of
the storm likely signify power.
Discussion Questions – Thompson 1988
• Who is Hiiaka? Who sent her on a dangerous
journey? What dangers did she face?
• Hiiaka is the Goddess of lightning, sister and equal to Pele
• Pele sent her mission to deliver a message to the far side
of Hawaiian island, a mission with many dangers
• She faces and subsequently defeats with her lightning:
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Forest Demon and his agents (trees, birds, etc)
Lizard Women at a stream crossing
Man-Eater Shark and a break in the cliff trail
Mo-o from a cave (mythical lizards) she defeats with the help of
her brother and sisters to call forth a great storm with rain
On the southeastern seacoast of the island Hawaii, a large stone which was
formerly so balanced that it could be easily moved. One of the severe earthquake
shocks of the last century overthrew the stone and it now lies a great black mass of
lava rock near the seashore.
Thunderstorm Introduction
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A storm that generates lightning and thunder.
Are characterized by strong “up” and “down” motions
Frequently produces gusty winds, heavy rain and hail.
They can form on their own or they can form in
conjunction with mid-latitude cyclone on the edge of a
Cold-Front
They can form in conjunction with hurricanes
Locations in the continental US
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Florida has the most thunderstorms (100s per year)
Commonly occur in the foothills of the Rocky Mountains
Associated with Tornado Alley in the middle of the country
Associated with drylines and air with different humidities
Thunderstorm Formation Introduction
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Form when warm, humid air rises in a conditionally
unstable or unstable environment
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There are two categories:
1. Air Mass Thunderstorms – formed by unequal heating
of the Earth’s surface within a maritime tropical (mT) air
mass.
2. Severe Thunderstorms – formed by unequal heating &
lifting of warm air along a front or mountain.
Air Mass Thunderstorms
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Happens inside an air mass (usually mT)
Usually in spring and summer
Usually in mid-afternoon
Not associated with a front
Due to local differences in temperature
Air Mass Thunderstorm Life Cycle
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Like mid-latitude cyclones T-Storms have
a “Life Cycle”
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Stages of Development
– Stage 1: Cumulus Stage
– Stage 2: Mature Stage
– Stage 3: Dissipating Stage
Air Mass Thunderstorm Life Cycle
Stage 1
Stage 2
Stage 3
Stage 1: The Cumulus Stage
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Rising air only
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Makes a cloud
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Cloud gets bigger and bigger
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Moisture is being added to higher and
higher altitudes
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Needs a continuous supply of
moisture to survive
Stage 2: The Mature Stage
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Precipitation forms
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Rain begins to fall
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Associated with the downdraft (air going
down)
Most active stage
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Mainly by the Bergeron Process
Gusty winds, lightning, heavy rain, hail
Updrafts and downdrafts exist
side by side
Stage 2: The Mature Stage
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Downdrafts form for two reasons
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Entrainment – mixing of dry air with cloud air
at the edge of the cloud
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Causes the cloud drops to evaporate
Energy from evaporation comes from the temperature
of the air parcel
By evaporating drops, you use up the energy and end
up cooling the air parcel
Temperature drops, parcel cools
Cooler air sinks to the surface
Drag – Air is dragged downwards as
precipitation falls.
Stage 3: Dissipating Stage
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Cooling effect of falling precipitation
and influx of colder air up top mark it’s
END!
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Downdraft cuts off updraft
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Cloud stops growing
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You’re left with weakly descending air
parcels
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The cloud basically KILLS itself
Severe Thunderstorms
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Heavy downpours, flooding, gusty straight-line
winds, large hail, lightning and tornadoes!
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NOT Common on the Hawaiian and Pacific
Islands.
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To be classified as Severe:
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Must have winds in excess of 93 km/h or 58 mph
or hail 0.75 inches
or a confirmed tornado
Severe Thunderstorm
Severe Thunderstorm
Overshooting Top
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Persist for many hours
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Anvil
Vertical wind sheer causes precipitation to fall in
downdraft, allowing the updraft to retain strength.
Sometimes the updraft is so strong you get
overshooting tops and anvils!
Cold air of the downdrafts spread out
making:
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“mini cold front”
Called a GUST FRONT
Called an Outflow Boundary
Can form a Roll Cloud
Supercell Thunderstorm
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Causes dangerous weather
Large, very powerful, up to 20 km in height
Last many hours
20-50 km in diameter
Vertical wind profile may cause the updraft to ROTATE!
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MESOCYCLONE – usually spawns Tornadoes!
Need a lot of Latent Heat
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Requires moist troposphere
An inversion a couple of km above the surface (caps moisture)
Unstable air can break through the inversion by “eroding” it
Supercell Diagram
Lightning and Thunder
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Lightning is ulia in Hawaiian
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Thunder is hekili in Hawaiian
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A storm is classified as a
thunderstorm only after
thunder is heard!!
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Because thunder is produced
by lightning, lightning must also
be present!
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Lightning is a discharge of
electrical energy.
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Essentially a giant “spark”
between regions of positive (+)
and negative (-) charge.
Lightning
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May occur:
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Between cells in the same storm
• inter-cloud lightning
– Within a cloud
• intra-cloud lightning
– Cloud to air
– Cloud to ground (CG)
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Lightning forms when a charge separation occurs in a cloud
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The earth is trying to equalize the electrical difference
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Negative charges want to flow to the ground.
Lightning Formation – Charge Separation
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We don’t exactly know why it happens
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One theory:
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Hail stones tend to have a warmer surface
than ice crystals
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When warm hail collides with colder ice,
electrons transfer from ice to hail
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Hail (-) is bigger and heavier and settles
toward the bottom of the cloud
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Smaller (+) ice crystals are lofted to the top.
Lightning Formation
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Excess electrons cause the
air to ionize
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Rip molecules apart (N2 or O2
become N+, O -…)
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Air is normally very
INSULATING
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Ionized air is very conducting
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like a metal wire
Lightning Formation
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Ionized air forms tubes of ~50 m in
length (150 ft) and 10 cm in diameter
(4 in)
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Called a LEADER
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A bunch of leaders that are
connected is called a STEPLEADER.
NOTE: There still hasn’t
been a flash yet!!!
Lightning Formation
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Each electron contains LOTS of energy
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When it reaches the surface the energy
is RELEASED
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This energy is RELEASED as HEAT!!
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Electrons are drained at the surface first
so the FLASH starts at the ground!!!
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Can have several (3-4) in rapid
succession (1/10 of a second apart).
(lead by a DART LEADER)
Lightning Formation
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AGAIN: Electrons are drained at the surface first so
the FLASH starts at the ground!!!
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This is why lightning is said to GO UP, rather than down
The movement is STILL from CLOUD to GROUND
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But the “heating” happens at the surface FIRST.
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The electrons closest to Earth are “discharged” first
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Heat that is released causes air around a step-leader
to reach 30,000°C (54,000°F)
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The warmer, the shorter the shorter the wavelength
30,000 K
8,000 K
Lightning Occurrence on the Mainland
Global Lightning Occurrence
Real-time Lightning Websites
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World Wide Lightning Location Network
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http://wwlln.net/
http://wwlln.net/new/map/
Blitzortung.org - Oceana
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http://www.blitzortung.org/Webpages/index.php?l
ang=en&page_0=20
Thunder
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When air is heated quickly a shockwave
forms  causes thunder
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Similar to planes crossing the sound barrier,
firecrackers and gun shots
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Sound travels at 330 m/s or 1000ft/s
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If thunder takes 3 seconds to happen after
observing lightning then the storm is:
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3 s * 1000 ft/s = 3000 ft away (6/10 of a mile
away).