MET 125 Physical Meteorology
Atmospheric Electricity, Part 1
Lightning
Henry Bartholomew (M.S.)
San Jose State University
1
What is electricity?
 It is energy carried by charged particles
 Flow of charge is called current
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
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
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)
Thunderstorm
http://www.youtube.com/watch?v=2Ey4KSnoReo
Three Types of Lightning Strikes
 Cloud-to-Ground (CG)
 Cloud-to-Cloud (CC)
 Intra-Cloud (IC)
Lightning Schematic
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
Cloud-to-Cloud Lightning
 Electrical Discharge between two oppositely
charged regions of different clouds
 Responsible for the majority of lightning strikes
Cloud-to-Cloud Lightning
Intra-Cloud Lightning
 Electrical Discharge between two oppositely
charged regions within the SAME cloud
 Makes up a small percentage of lightning
strikes
Intra-Cloud Lightning
Charge Separation in Clouds
 Still a subject of debate as to WHY charge
separation develops within a cloud
 There are two plausible mechanisms
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
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
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
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
Return Stroke
 Electrical discharge that carriers positive
charge UP into cloud
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 30-40 milliseconds,
and thus appear to us as one lightning strike
Lightning Flash Sequence
Steps of Lightning: Animation
Lightning Sequence Video
http://www.youtube.com/watch?v=6MUYsIjTKvk
Lightning in Slow Motion
http://www.youtube.com/watch?v=W9xzU0xjlhE
More Types of Lightning
 Positive Lightning
 Heat Lightning
 Red Sprites and Blue Jets
Positive Lightning
 This is a variation of CG lightning
 Electrical discharge between positively charged
upper part of cloud and negatively charged
ground
Positive Lightning
Positive Lightning
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
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
Red Sprites and Blue Jets
Lightning Frequency
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!