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