Some materials easily give up electrons and others are eager to take them. When givers and takers come in contact the flow of charged particles creates an electrical current. Objects that give up electrons become positively charged and those that receive electrons acquire a negative charge.
There are a number of ways electrons can escape from one object and accumulate on another. Sometimes just touching is enough to free electrons from the donor surface. At other times a forceful collision or vigorous rubbing is necessary. Applying pressure or heating can also free electrons from some materials. When opposite charges are separated from one another it is called charge separation.
MORE KINDS OF LIGHTNING
The negative and positive lightning described above is cloud-to-ground lightning (CG).CG lightning is numbers 5 and 6 in the diagram above. If lightning starts on a tall tower or skyscraper and goes up it is called ground-to-cloud (GC) lightning. GC lightning can be negative and positive.
For more on GC lightning watch this 2 minute and 11 second video.
The electric field inside a thunderstorm is much greater than the cloud-to-ground electric field so most lightning occurs within thunderstorms. This type of lightning is called intra-cloud (IC) lightning (just above #1 in the diagram).
If a bold of lightning shoots out of a thunderstorm into the surrounding air it is called cloud-to-air (CA) lightning. Lightning can also travel between clouds and is called cloud-to-cloud (CC) lightning. CC lightning is rarely observed.
This 48-second video below shows CA and IC lightning.
Anvil Crawlers are horizontal forking bolts that cross the sky. They progress slowly enough that you can see them "crawl". They are well above the ground often seen near the underside of the thunderstorm anvil. Look at the video below.
Bead lightning is really just the decaying stage of a lightning strike. When parts of the lightning strike are becoming less luminous it often looks like a string of beads. This animation from a high speed move made by E. Philip Krider, Institute of Atmospheric Physics, U. Arizona.
Ribbon lightning occurs when the lightning channel is being blown downwind fast enough that there is a visible gap between individual bolts. The first image is of a bolt striking the top of the Empire State Building. The right half is an enlargement. The second image is from Chardon, OH taken in 1898 by W.H. Osborne.
Sheet lightning is just regular lightning in the distance. Its light is reflected off of a high overcast and much of the sky seems to light up in a sheet.
HOW GRAUPEL FORMS
Left above: Supercooled (now frozen) drops accreted on ice needles. Right above: Graupel, supercooled drops accreted and original particle obscured. Courtesy: USDA.
The next image shows what happens when graupel falling through a thunderstorm collides with an ice crystal.
1. ELAPSED TIME = ZERO. As the negative charge in lower part of a thunderstorm increases positive charges from the "ground shadow" are pulled up through trees, telephone poles, buildings and towers.
2. Invisible negative charges descend in jagged steps towards the positive charges below as the "stepped leader".
3. As the negative charges get closer to the ground invisible channels of positive charges called the "streamers" surge upwards. This is another example of induction.
4. ELAPSED TIME = 0.005 SECONDS. When the stepped leader meets the streamer, a couple hundred feet or so above the ground,the insulating capability of the air has broken down and a dimly visible surge of negative charge races down the channel.
5. At the same time a brilliant return stroke of positive charge bursts up the channel.
6. ELAPSED TIME = 0.005001 SECONDS. The colossal current heats the air to as much as 30,000°C (53,540°F). The air around the channel rapidly expands and - KABOOM! - a shock wave we hear as thunder.
The image below was taken in Tucson, AZ and reported in 2012 at the 22nd Annual International Lightning Detection Conference. The paper is located here: