1. MET 125 Physical Meteorology
Atmospheric Electricity, Part 1
Lightning
Henry Bartholomew (M.S.)
San Jose State University

2. What is electricity? It is energy carried by charged particles
Flow of charge is called current

3. Thunder Sound heard as a result of lightning
Lightning is an electrical discharge
Peak temperature of lightning bolt is around 30,000 K (about 55,000°F)!
Due to this intense heating of the lightning “channel,” air spreads out, and sound travels faster than it would in cooler surrounding air
Outward moving pulse causes shock wave

4. Sound of Thunder
When lightning is nearby, thunder often sounds like clap
Farther away, it may sound more like a rumble
Can be caused by sound originating from different locations of stroke, and highlighted when sound wave reflects off obstacles, such as buildings and mountains

5. Determining the distance from lightning
You can determine your distance from lightning by counting the number of seconds between when you see the flash and hear the thunder
The speed of sound is approximately 1 mile per 5 seconds
Distance = Time*Speed
Thus, multiply time (in seconds) by speed of sound (1 mile/5 seconds) to get distance from lightning (in miles)

6. Thunderstorm

8. Three Types of Lightning Strikes
Cloud-to-Ground (CG)
Cloud-to-Cloud (CC)
Intra-Cloud (IC)

9. Lightning Schematic

10. Cloud-to-Ground Lightning
Electrical Discharge between negatively charged base of cloud and positively charged ground or object (usually)
Responsible for about 20-25% of global lightning strikes

11. Cloud-to-Cloud Lightning
Electrical Discharge between two oppositely charged regions of different clouds
Responsible for the majority of lightning strikes

12. Cloud-to-Cloud Lightning

13. Intra-Cloud Lightning
Electrical Discharge between two oppositely charged regions within the SAME cloud
Makes up a small percentage of lightning strikes

14. Intra-Cloud Lightning

15. Charge Separation in Clouds
Still a subject of debate as to WHY charge separation develops within a cloud
There are two plausible mechanisms

16. Mechanism 1
After evaporation from surface, rising moisture can condense and form clusters of droplets, which collide with falling supercooled droplets and ice crystals
The collisions may tear off electrons from rising droplets, which then collect near base of cloud

17. Mechanism 2
As clusters of rising droplets travel higher, temperature drops, and freezing can occur
Inner portion of cluster may freeze first and become negatively charged, while outer portion may stay as liquid and develop positive charge
Air currents can tear away positively charged surroundings and carry them to top of cloud

18. Steps of Lightning 1. Stepped Leader extends down from cloud
2. Upward Leader extends upward from objects on ground to meet Stepped Leader
3. Return Stroke occurs
4. Additional Strokes take place

19. Stepped and Upward Leader
A stepped leader is a negatively charged channel of air that extends downward from cloud
Positively charged channel (upward leader, or streamer) then extends upward from object or ground
These leaders occur too fast and too close to time of main discharge to appear visible to naked eye
When these two channels meet, return stroke occurs

20. Return Stroke
Electrical discharge that carriers positive charge UP into cloud

21. Additional Strokes
A typical CG lightning strike is made of 3 or more strokes (can be as many as 30)
After initial “return stroke,” additional strokes can occur using discharge channel taken by previous stroke
Strokes are separated by milliseconds, and thus appear to us as one lightning strike

22. Lightning Flash Sequence

23. Steps of Lightning: Animation

24. Lightning Sequence Video

25. Lightning in Slow Motion

26. More Types of Lightning
Positive Lightning
Heat Lightning
Red Sprites and Blue Jets

27. Positive Lightning This is a variation of CG lightning
Electrical discharge between positively charged upper part of cloud and negatively charged ground

28. Positive Lightning

29. Positive Lightning

30. Heat Lightning
On clear night, lightning can be seen from hundreds of miles away
Too far away to hear thunder
Called “Heat Lightning” because of its occurrence during hot summer nights over Eastern USA

31. Red Sprites and Blue Jets
Electrical discharges that occur above thunderstorms
Faint and fast moving
Red Sprites occur in mesosphere, and are result of positive CG lightning
Blue Jets occur in stratosphere

32. Red Sprites and Blue Jets

33. Lightning Frequency

34. Observations
Greater frequency of lightning flashes over land than ocean
The region with the highest amount of flashes per time is equatorial Africa
Can notice the effect of ocean currents!