Transcript Lightning

The Thunderstorm: Ultimate Instability
 A convective rain or snow shower accompanied
by thunder
– What causes thunder?
– Lightning (spell it correctly): a giant spark between
two regions of opposite electrical charge. 50,000
deg F, hundreds of thousands of amps
– Air is a poor electrical conductor (an insulator),
therefore a huge charge difference builds up either
in cloud (80% of all lightning) or between the cloud
and the ground (20%) (cloud to ground)
Lightning I
 Formation: Precipitation Theory
– Collision of ice crystals and
hailstones leaves each with opposite charge,
lighter ice crystals transport positive charge
upward while heavier hail concentrates
negative charge at lower levels
– Probably more complicated than this, may need
supercooled water in cloud in addition to ice,
but it’s likely that ice is
required to produce lightning
– Hurricanes = little lightning
– Sprites
Lightning II
 Thunder: a warning that lightning is near,
explosive expansion of superheated air
– Computing distance to the lightning
 Speed of light: see lightning when it happens
 Speed of sound: thunder is delayed 5 sec / mile
– All lightning produces thunder
 The rumble
 “Heat lightning”
Lightning Safety
Photo Courtesy of Dave Crowley
 Don’t
– Swim, boat
– Stand in an open field
– Stand under an especially tall or isolated tree
– Use a wall phone
– Use the shower
 Do
– Go inside
– Get into a nonconvertible car
– Get to lower elevation
– Crouch on balls of feet
– Perform CPR if someone
is struck
Thunderstorm Triggers
 Start with an environment in unstable
equilibrium
 Diurnal heating primes the atmosphere
– Localized thunderstorm triggers
 Sea breeze (Florida, Gulf Coast)
 Elevated convection (eastern Rockies)
– Large scale thunderstorm trigger
(discussed later)
 Convergence into low pressure
 Fronts (all types, but cold preferred)
Air Mass Thunderstorms
 Typical lifespan 30 minutes to an hour
 Life Cycle
– Cumulus Stage
 Instability and adequate moisture required
 Building cumulus clouds dominated by updrafts
Air Mass Thunderstorms
 Life Cycle
– Mature Stage
 Top of cloud glaciates (freezes)
– Precipitation through Bergeron-Findeisen Process
– Lightning
 Updraft continues and downdraft develops from falling
precipitation
Air Mass Thunderstorms
 Life Cycle
– Dissipating Stage
 Downdraft dominates
 Updraft ceases, convection collapses
 Dry air entrainment
Thunderstorm Dangers
 Flash flooding: more rain falls in a short time
than ground can absorb
 Hail: “The White Plague”
 Microbursts: unusually strong downdrafts
 Lightning, of course
 Thunderstorm Complexes
– Squall Lines
– Mesoscale Convective Systems
Flash Floods
 A big U.S. killer (~100 people/yr)
 DO NOT DRIVE or walk on flooded streets
 Ingredients for flash flooding
– Weak upper-level winds
– Orography
– Training
Hail
 Ingredients for hail
– Strong updrafts to support growing chunks of ice
– Below-freezing air relatively close to ground
 Formation
– Ice crystals become coated with supercooled
water blown upward within updraft
– Hailstones grow as long as updraft can offset
gravity
– Fall to earth when updraft weakens or hail is
thrown out of updraft
The Microburst
 Huge threat to aviation
– Rapid loss of lift
– Occurs close to ground
 Damaging “straight-line” winds
Thunderstorm Complexes
 Mesoscale Convective Systems
– Long-lived bundle of thunderstorms
– Greatest threat is flooding
– Beneficial to U.S. crops (usually)
 Squall Lines
– Organized band of thunderstorms, usually well
ahead of a cold front
– Greatest threat is damaging winds
Key Figures
 9.5, 9.11, 9.14, 9.16, 9.19, 9.21, 9.32, 9.53