A bolt of lightning usually begins when an invisible negatively charged stepped leader stroke is sent out from the cloud. As it does so, a positively charged streamer is usually sent out from the positively charged ground or cloud. When the leader and streamer meet, the electric current greatly increases. The region of high current propagates back up the streamer into the cloud. This "return stroke" is the most luminous part of the strike, and is the part that is really visible. Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect. Thunder is caused when the discharge rapidly superheats the air around the strike, causing a shock wave to be sent out.












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Positive lightning makes up less than 5% of all lightning. It occurs when the stepped leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike several miles or kilometers distant from the clouds. During a positive lighting strike, huge quantities of ELF and VLF radio waves are generated.

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