1. Extreme Instability: Thunderstorms, Lightning

2. Thunderstorm
Convective storm with lightning
3 stages

3. 1. Cumulus phase Air mass expands Speed of updrafts increases
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
Ice
crystals
6000 M
meters
water
3000

4. At first, strong updrafts prevent rain
When cumulonimbus tower is high enough Bergeron process begins and rain falls.

5. 2. mature phase downpours and lightning
Precipitation causes downdrafts of cold air, which spread out below cloud
Updrafts and downdrafts

6. 12000 m
ice
-26°C
9000
snow
0˚C
6000
water
3000
Heavy rain at surface

7. 3. dissipating supply of moisture is spent, latent heat decreases
downdrafts overpower updrafts
storm dies….

8. ANIMATION

9. Lightning Ionosphere (30 – 40 km) is + Earth’s surface is -
Ionosphere (30 – 40 km) is +
Earth’s surface is -
induces in cloud droplets:
- top and + bottom

10. 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 -

11. 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 + - +

12. stroke
Stepped leader
Return stroke
Dart leader

13. Return stroke follows path of least resistance; e.g., tall structures

15. ANIMATION

16. 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)

18. Avoid: Tall,isolated objects Bicycles High areas Bodies of water
Mowers or tractors
Individual trees (groves are better)

19. Drop to knees, bend forward to make body small, do NOT lie flat.
Lightning can strike same spot twice.

20. Warning signs:
Hair stand up

21. Underground metal pipe

24. Victims: Carry no charge Need: mouth to mouth
Treatment for burns, shock

25. struck by lightning, causing burn scars, a fractured skull and a gold chain and crucifix to melt around his neck.

29. thunder Heating and expanding of air in wake of lightning
……sends out sound waves
(It takes thunder 5 seconds to travel 1 mile)

30. 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

31. ANIMATION