1. NATS 101 Lecture 28 Lightning

2. Review: Thunderstorms
A cumulonimbus with lightning and thunder!
Deep layer of conditionally unstable air is necessary to produce a thunderstorm.
Several types of thunderstorms.
Single Cell, Multicell, Squall Line, Mesoscale Convective Complexes, Supercells
Pose major hazards to public and economy.
Lightning, Hail, Microburst Winds, Flash Flooding, Tornadoes

3. Lightning Basics What is lightning?
An electric discharge, or spark, that occurs in thunderstorms (usually)
80% occurs within clouds
20% occurs between cloud and ground
Lightning is ubiquitous, with more than 6,000 ground strikes per minute from 40,000 thunderstorms per day worldwide

4. Lightning Videos Examples were shown of In-Cloud (IC) Lightning
Cloud-to-Cloud (CC) Lightning
Forked Lightning
IC Lightning Video from MetEd/UCAR
MCC Lightning from Space Shuttle
Lightning from Space Video from NASA

5. Chuck Doswell's Lightning Pictures-Very Nice!
Examples were shown of
Cloud-to-Ground (CG) Lightning
In-Cloud (IC) Lightning
Cloud-to-Cloud (CC) Lightning
Forked Lightning
Chuck Doswell's Lightning Pictures-Very Nice!
Excellent photography tips can be found at Chuck Doswell’s web site. He’s good!

6. Cloud-to-Ground Lightning

7. In-Cloud Lightning

8. Forked Lightning

9. Charge Separation
Lightning requires the separation of different charges into different regions of a cloud.
How does charge separation in clouds occur?
We don’t know for certain, but we observe this:
Lightning only occurs in cold clouds with supercooled droplets and temps below 5oF.
Thus, the ice crystal processes responsible for precipitation in cold clouds likely plays an critical role in charge separation.

10. Charge Separation: One Theory
Hailstones are covered by a layer of liquid water.
The thin layer of liquid is positively charged.
When hailstones and ice crystals collide, some of liquid molecules stick to the ice crystals.
Along with the mass transfer, positive ions transfer from the hailstones to the ice crystals.
The heavier, negative hail falls to cloud bottom.
The lighter, positive ice crystals drift to cloud top.
Produces negative lower, positive upper cloud.

11. + – Charge Separation E E points toward positive polarity
Polarization
Top of cloud top has a positive charge.
Lower and middle of cloud has a negative charge.
Charge separation in cloud maintains the earth’s fair weather electric field denoted by the arrow E
E points toward positive polarity E –
Williams, The Weather Book

12. Fair Weather Electric Field
An electric potential exists between the ionosphere (positive) and surface (negative)
Potential varies between 200, ,000 Volts
Average current is 2x10-12 Amps/m2
Power is  10-6 W/m2
www://thunder.msfc.nasa.gov/primer

13. Lightning Stroke Cloud-Ground Sequence
1) Downward stepped leader. Stepped leader is invisible.
2) Upward return stroke.
3) Downward dart leaders.
4) Upward return strokes.
Dart leaders-return strokes: up to 25 cycles, 3-4 usually.
Ground strikes are usually negative, that is electrons flow from cloud to ground.
Williams, The Weather Book

14. Types of Discharges
www://thunder.msfc.nasa.gov/

15. Lightning Safety
Williams, The Weather Book

16. Thunder What Causes Thunder?
Lightning rapidly heats air to more than 30,000oC.
The intense heating causes the air to expand rapidly.
The expanding air cools, then contracts rapidly.
The expansion-contraction generates sound waves.
Williams, The Weather Book

17. How Far Away Is It? We see lightning instantly.
But sound travels 1,000 ft every second. If you hear thunder 10 seconds after seeing lightning, the bolt is 2 miles (~10,000 ft) away.
We hear thunder from closest part of flash first, farthest part last. This causes the rumble sound.
Williams, The Weather Book

18. Why Thunder Rumbles?
Assume that you are one mile away from a a one mile long bolt.
You hear thunder from the lower part of flash in 5 seconds, from the upper part of flash 7 seconds.
1.4 miles
7 seconds
1 mile
1 mile
5 seconds

19. National Lightning Detection Network
thunder.msfc.nasa.gov/primer

20. Gallup
Flagstaff
Phoenix
Tucson

21. Phoenix
Tucson

22. Global Lightning Distribution from Satellite, Take 2

23. Let’s Play “Who Gets Toasted”
What is the probability in Tucson of a Cloud-to-Ground lightning stroke hitting within a certain Radius R of you in an “average” year?
Guesses? No peeking!

24. Courtesy Prof. E.P. Krider Dept. of Atmospheric Sciences

25. CG Lightning over Tucson (2000-2002)
65,000 flashes in 80 km  80 km over 3 years
~3.3/km2 per year
Much higher during monsoon ~12/km2 per year
Courtesy Prof. E.P. Krider Dept. of Atmospheric Sciences

28. Summary: Key Points Lightning - electric discharge in thunderstorms
80% within clouds, 20% cloud to ground
Lightning is ubiquitous, with more than 6,000 cloud-to-ground strikes per minute from more than 40,000 thunderstorms per day worldwide.
Lightning requires the separation of different charges into different regions of cloud.
Charge separation maintains the earth’s fair weather electric field.

29. Assignment for Next Lecture
Topic - Tornadoes
Reading - Ahrens, p
Problems , 10.26, 10.29