Transcript Lecture 9

Meteo 3: Chapter 9
Thunderstorms
Read pp. 347-367,
371-372, 374-378
skipping MCCs,
precipitable water
 Time lapse
Chapter 8 CD
Animation of how
overshooting tops form
Terminology for T-Storm Coverage
 Coverage
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10-20%
30-50%
60-70%
80-100%
 Term
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Isolated
Scattered
Numerous
(no term used)
Thunderstorm Dangers
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Lightning
Hail
Damaging wind gusts
Tornadoes
Flooding rains
*Mediate exchange of electrical charge,
water, and energy between atmosphere &
surface*
Lightning
 Need charge separation in the atmosphere…accomplished
as precipitation particles collide
 Negative charge on bottom of cloud induces positive
charge at ground…electric potential difference leads to
discharge… cloud-to-ground lightning!
 Discharge can occur between any two oppositely charged
regions in/near a thunderstorm
 Most lightning is “in-cloud”
A little Video is more fun though, eh?
Thunder
 Lightning heats the atmosphere to about 50,000°F
 Air surrounding the bolt expands, creating a sound
wave traveling ~ 335 m/s
 You see lightning before you hear thunder because
light travels much faster than sound
 For every 5 seconds between lightning and thunder,
the lightning bolt struck 1 mile away
– If delay 30 s or less, seek shelter!
– Thunder cannot be heard > 12 mi from where it
originated…if lightning seen but no thunder, termed ‘heat
lightning’
Lightning Survey (Fact or Fiction)
 1. If your hair stands on end, you’re about to
be struck by lightning.
 Fact. If your hair stands on end, you could be in
an area where a charge is building before a
lightning strike.
 2. Lightning only happens where it’s raining.
 Fiction. Most deaths and injuries from lightning
occur as a storm is rapidly approaching or within a
half-hour after one has passed. Lightning can
strike as far as six miles ahead of or behind the
storm, even while the sky overhead is blue.
Lightning Survey (Fact or Fiction)
 3. What you wear (sneakers, rubber boots,
hairpins, cleats, etc.) can increase or decrease
your chances of being struck by lightning.
 Fiction. What studies have been done indicate
that what you wear doesn’t make much difference
either way.
 4. You’re safe from lightning inside a car
because the rubber tires act as insulators.
 Fact and Fiction. You are safe from lightning in a
car if it has a metal top and sides and if the doors
and windows are shut tight. Don’t touch the car
frame, steering wheel, ignition, gear shift, or radio.
The metal shell of the car conducts the electricity
away and that protects you, not the tires.
Lightning Survey (Fact or Fiction)
 5. Carrying an umbrella can be dangerous
when lightning is present.
 Fact. Anything that makes you taller increases
your risk.
 6. If lightning only hits the ground close by,
you’re safe.
 Fiction. When lightning hits the ground, the
current spreads along the surface to a depth of a
few inches. Any fence or pipe in its path will be
charged with energy for quite a distance. Any
person in its path can be injured.
Lightning Survey (Fact or Fiction)
 7. Lightning travels down telephone wires.
 Fact. If lightning strikes your home, the charge
travels through power lines, phone lines, and
plumbing until it reaches the ground. If you’re in
the bath or shower, touching an electric appliance,
or on a corded phone, you could be injured.
 8. People struck by lightning are electrified and
dangerous to touch.
 Fiction. Plus, when people are struck, their hearts
or breathing often stop. They must be given first
aid immediately.
“Bolt from the blue”
Lightning Safety
 54 people killed/year by lightning on average
 Seek shelter in a sturdy closed building w/ lightning rod on
top
– Lightning rod channels charge to ground
– Enclosed, insulating metal vehicles are good shelters
– If outside, avoid tall objects, elevated area, conductors
 Don’t talk on phone, take shower, or use electrical
appliances
 Take caution for 30 minutes after last audible thunder
 If outside and you feel tingling sensation or hair stands on
end, assume “upright fetal position”
Ingredients for thunderstorms
 Instability- remember to assess difference in temperature
between low levels and mid levels of atmosphere
– Warm and moist lower levels (solar heating)
– Cold air aloft (approaching upper-level trough)
 Trigger- fronts, differential heating of the ground, sloped
terrain
 Air-mass thunderstorms: result from differential
heating…brief, well-defined life cycle
 Synoptically-forced thunderstorms: form in response to
large-scale, organized patterns of wind flow (i.e.
fronts)…usually longer lived and severe
Sea Breeze- Florida’s big trigger
Rockies as a t-storm trigger
Lightning and forest fires
 Forests out west dry due to little rain and heat
 Thunderstorms have high bases due to low
dew points
– Falling raindrops evaporate
 But thunderstorms still produce lightning!
Air-mass thunderstorm life cycle
Stage 1: Cumulus
 Updraft dominates
(convectively driven rising
air column)
 Struggle to overcome
entrainment of dry air
Air-mass thunderstorm life cycle
Stage 2: Mature
 Cumulonimbus cloud
 Strong updrafts promote
precipitation formation
 Heavy rain/hail
 Falling
precipitation/evaporation
promotes downdraft (sinking
air column)
More Mature Stage Stuff
 Anvil appearance
…created as rising air
parcels encounter
stable tropopause
 Mammatus below anvil
Air-mass thunderstorm life cycle
Stage 3: Dissipating
 Downdraft overwhelms
updraft
 Precipitation diminishes
 Cloud evaporates from
bottom up
 But downdraft promotes gust
front formation, which can
generate new storms
Gust front on radar
What’s a severe thunderstorm?
 Wind gusts > 50 kts (57.5 mph)
 Hail with diameter > 0.75” (penny sized)
 Tornado
Flash Flooding
 Rapidly rising surges of
water; can occur with
little/no warning
 Average 107 fatalities/yr
 Moving water is powerful
 Mountainous
regions/deserts prone
 Forecasters look for upperlevel winds weak or parallel
to line of storms (training) &
moisture
 Big Thompson River
Canyon disaster
 Forecasters look for
upper-level winds weak or
parallel to line of storms
(training) & moisture
 “Turn around, don’t drown”
Hail
Hail
 Begins as small frozen raindrops
or graupel
 Updraft raises drops above
melting level, allowing
supercooled water to freeze on
them
 Develops layered, onion-like
appearance
 Falls when becomes too large for
updraft or enters downdraft
Hail damage- $1 billion in lost crops
Microbursts:Aviation Hazard
Microburst Characteristics
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Straight-line wind bursts
Diameter < 2.5 mi
Produced by evaporational cooling of air
Winds produced can exceed 100 mph