Extreme Instability:
Thunderstorms, Lightning
Thunderstorm
Convective storm with lightning
3 stages
1. Cumulus phase
Parcel of air
warmer than
surroundings
rises as updraft
draws in air
(40 mph)
Condensation releases
heat
Air mass expands
density decreases
Speed of updrafts
increases
9000
6000
M
meters
3000
Ice
crystals
water
At first, strong updrafts prevent rain
When cumulonimbus tower is high enough
Bergeron process begins and rain falls.
2. mature phase
downpours and lightning
Precipitation causes downdrafts of cold
air, which spread out below cloud
Updrafts and downdrafts
12000 m
ice
-26°C
9000
6000
snow
0˚C
water
3000
Heavy rain at surface
3. dissipating
supply of moisture is spent,
latent heat decreases
downdrafts overpower updrafts
storm dies….
ANIMATION
Lightning
+++++++++
Ionosphere (30 – 40 km) is +
Earth’s surface is
-
induces in cloud droplets:
- top and + bottom
--------------
Charges separate within cloud
As big and small particles collide :
heavier (hail) becomes lighter (ice crystals) become +
heavier fall ; lighter carried up by updrafts
Result: + top of cloud ; - bottom of cloud
t
-
- Cloud bottom repels - earth’s surface
induces + on ground under cloud.
+
+
potential builds up;
mature phase: resistance of air breaks down
lightning flows to neutralize charge
stroke
Stepped leader
Return stroke
Dart leader
Return stroke follows path of least
resistance; e.g., tall structures
• ANIMATION
Lightning safety:
1. seek shelter
2. avoid conductors : pipes, stoves, wires
(metal car or truck is safe,
back of a pickup is NOT safe;
cloth-top convertible is NOT safe)
Avoid:
Tall,isolated objects
Bicycles
High areas
Bodies of water
Mowers or tractors
Individual trees (groves are better)
Drop to knees, bend forward to make
body small, do NOT lie flat.
Lightning can strike same spot twice.
Warning signs:
Hair stand up
Underground metal pipe
Victims:
Carry no charge
Need: mouth to mouth
Treatment for burns, shock
struck by lightning,
causing burn scars, a
fractured skull and a gold
chain and crucifix to melt
around his neck.
thunder
Heating and expanding of air in wake of
lightning
……sends out sound waves
(It takes thunder 5 seconds to travel 1 mile)
Hail
Hard balls of ice, at least 5 mm across;
made of layers of ice;
Hailstones :
caught in updraft
begin to fall
causes surface melting
caught again in updraft
re-ascend
outer part freezes; this continues
• ANIMATION