http://www.youtube.com/watch?v=2iJtysax3nE&NR=1

http://www.youtube.com/watch?v=6ikvJrf_XYE&NR=1

 

 

 

 

The most common forms that you see from storms.  They are either quick or can pulse as the connection within the lightning shaft takes place.

Whilst it's not quite understood how lightning actually occurs here's the bare basics of the process.  That the cumulonimbus cloud becomes supercharged by positive and negative particles, add to that instability, strong updrafts, water droplets and ice crystals colliding with each other throughout the process create latent heat.  We still don't know exactly why it happens, but time lapse photography/video of lightning channels has shown us that even before we see the flash the leaders are darting forward, backward, disappearing, reappearing, up and down until a connection is made.

Step leaders are usually not visible to the naked eye because the whole process takes a fraction of a second - but the end result is the main connection from one of these leaders to a point on the ground and that's when you see the strike or the brightly illuminated channel. 

 You can see in this photo above I took of lightning that on the left hand side there are faint 'leader' strokes visible.  The two far right leaders found a compatible earth grounding source and the connection was complete.

 

 Ground to cloud lightning

 

 

 Rocket lightning

 

 

 

 

Transient Luminous Events

 

The rarest and astonishing lightning types.  extremely hard to see with the naked eye unless you have an image enhancer on the end of your lens or video camera.  You need to be in total darkness with a perfect view of a lightning active storm, if you see them or capture them on video contact your local weather service and send it to them with a report - they would be very interested in it as not much is known about them.

Large thunderstorms are capable of producing other kinds of electrical phenomena called transient luminous events (TLE's). The most common TLE's include red sprites, blue jets, and elves.

Red Sprites can appear directly above an active thunderstorm as a large but weak flash. They usually happen at the same time as powerful positive CG lightning strokes. They can extend up to 60 miles from the cloud top. Sprites are mostly red and usually last no more than a few seconds, and their shapes are described as resembling jellyfish, carrots, or columns. Because sprites are not very bright, they can only be seen at night. They are rarely seen with the human eye, so they are most often imaged with highly sensitive cameras.

Blue jets emerge from the top of the thundercloud, but are not directly associated with cloud-to-ground lighting. They extend up in narrow cones fanning out and disappearing at heights of 25-35 miles. Blue jets last a fraction of a second and have been witnessed by pilots.

Elves are rapidly expanding disk-shaped regions of glowing that can be up to 300 miles across. They last less than a thousandth of a second, and occur above areas of active cloud to ground lightning. Scientists believe elves result when an energetic electromagnetic pulse extends up into the ionosphere. Elves were discovered in 1992 by a low-light video camera on the Space Shuttle.

 

As a thunderstorm moves over the Earth's surface, an equal but opposite charge is induced in the Earth below, and the induced ground charge follows the movement of the cloud.

An initial bipolar discharge, or path of  ionized air, starts from a negatively charged mixed water and ice region in the storm. The discharge ionized channels are called leaders. The negative charged leaders, called a "stepped leader", proceed generally downward in a number of quick jumps, each up to 50 meters long. Along the way, the stepped leader may branch into a number of paths as it continues to descend. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground.

This initial phase involves a relatively small current (tens or hundreds of amperes), and the leader is almost invisible compared to the subsequent lightning channel.When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. The electric field is highest on trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a postive streamer) can develop from these points.

 

This was first theorized by Heinz Kasemir. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible. When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the lightning discharge.

 

Aircraft struck by lightning courtesy of Zen Kawasaki
Department of Electrical Engineering
Faculty of Engineering, Osaka University
Yamada-Oaka 2-1, Suita, Osaka)

 

When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.

The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke