Transcript 1027 Lecture, Tornadoes

Lightning
strike
details
Lightning strike is very fast and very complex:
1. Cloud to ground stepped leader moves along a branching path in
20 to 100 meter steps at 10 to 20 km/sec.
2. Ground to cloud return stroke covers the final 100 meter spark
gap to a high spot (tree, tower, golf club).
3. Subsequent (3-40) strokes called dart leaders give lightning its
forked appearance, completing the strike.
Lightning formation
Necessary condition: Separation of charge.
Cb is a static electricity generator.
Charge separation caused by collisions of water and ice.
Electrons (-) and ions (+ -) transported to different levels via
updrafts and downdrafts.
Air is a good insulator.
Cloud can reach 100 million volts, 100,000 amps.
Car battery is 12 volts, 8 amps.
House current is 120 volts, 20 amps.
Sufficient condition: Voltage difference > insulating
ability of the air.
Thunder
Thunder = sound wave
Long tube of air, 6 km x 15 cm diameter heated almost
instantaneously to 12-30,000 K expands air violently.
Shock wave = thunder (both audible and inaudible).
Speed of light = 300,000 km / sec
Speed of sound ~ 0.33 km / sec
3 second delay = 3 km; 5 second delay = 1 mile
Lightning fatalities
About 60 fatalities per year in the US, 360 severe injuries
Benefits of lightning
May have played a role in the pre-biotic formation of amino acids.
Fixes nitrogen, creates natural fertilizer.
Ignites forest fires -- forest ecology.
May have been the source of fire for early humans.
E-M fields of lightning used to study the atmosphere.
Global lightning strikes represent a global “thermometer.”
Lighting strikes on other planets (Jupiter) allow remote study.
Tornado
A small, funnel shaped, rapidly rotating
vertical column of air.
Generated in severe thunderstorms
(supercells) in a rotating mesocyclone.
Extremely low central pressure with
winds of up to 300 mph and updrafts of
100 mph.
Made visible by dust and debris sucked
into the vortex.
Typically 100-600 meters in diameter.
Travel at about 30-40 mph, usually in a NE
direction.
May travel in groups, alternately skipping
and touching down.
Above the ground they are called funnel
clouds.
Waterspouts (tornadoes
over water) are visible
due to condensation
of water vapor.
Tornado
Formation
Supercell, Mesocyclone and Tornado
Before thunderstorms
develop, wind shear, a
change in wind direction
and an increase in wind
speed with height,
creates an invisible,
horizontal vortex in the
lower atmosphere.
Thunderstorm updrafts tilt
the vortex from
horizontal to vertical.
An area of rotation, 2-6
miles wide, now
extends through much
of the storm. Most
strong and violent
tornadoes form within
this area of
strong rotation.
Enhanced Fujita Damage Scale for Tornadoes
Scale
EF0
EF1
EF2
EF3
EF4
EF5
Winds
65-85
86-110
111-135
136-165
166-200
> 200
Damage and frequency
Light, tree branches broken, 38.9%
Moderate, trees snapped, 35.6%
Considerable, trees uprooted, 19.2%
Severe, cars overturned, 4.9%
Devastating, homes destroyed, 1.1%
Incredible, steel structures
highly damaged, 0.1%
Tornado hazards
39 Fatalities in 2005, 22 so far in 2009
Vast majority from F3 tornado hitting mobile homes
700 killed, 11,000 homeless in Ohio, 18 March 1925.
335 killed, $600 million damage in 148 tornado outbreak from
Alabama to Ohio, April 1974.
Aviation hazards from
thunderstorms
Impaired visibility
Turbulence
Downbursts
Wind shear
Icing conditions
Rapid pressure changes (can cause altimeter errors)
Rain on runways (can cause hydroplaning)
Flash floods: 130 US fatalities per year