The formation of lightning is still something of a mystery. Even the perceived motion of lightning is false. The light that seems to come down from the sky actually jumps up from the earth’s surface. Lightning bolts are triggered when a negatively charged cloud base induces a positive charge in the ground. Negative particles, small streaming sparks known as stepped leaders, begin to flow downward, creating a conductive channel an inch or two wide. At the same time, similar streamers are moving upward from the ground, especially high places like treetops and tall structures. When the two streamers meet, they form a channel and a subsequent lightning bolt. The bright flash of light happens when the electrical charge excites air molecules in its path, forcing them to release light.

Lightning travels up to 60,000 miles per hour, cutting a jagged path through the air as much as 10 miles long! A lightning flash is brighter than ten million 100-watt lightbulbs, and contains billions of watts - as much power as in all the electricity plants in the United States.

Lightning releases enormous amounts of energy, but also comes in a wide variety of shapes and colors. There are three major types:

• intracloud lightning - The most common type, intracloud lightning occurs when lightning moves between oppositely charged points inside one cloud.
• cloud-to-ground lightning - This is the most dangerous form but the kind we know most about. Like the name indicates, it moves between clouds and the ground.
• intercloud lightning - This occurs when lightning jumps from cloud to cloud.

In addition to these types of lightning flashes, there are several other ways to describe lightning. Most are just names that describe when bolts appear or how they look.

• heat lightning - This happens when the weather is very hot.
• summer lightning - This is used to describe lightning that appears in the summer.
• sheet lightning - This comes in flat waves.
• ribbon lightning - Multiple-stroke flashes take place when several bolts travel the same channel at split-second intervals. A heavy wind during such a flash can blow the conductive channel several feet sideways between light flashes, producing “ribbon lightning,” in which parallel glowing lines flare, each tracing a different path of the conductive channel.
• silent lightning - This type of lightning is not accompanied by thunder because it is so far away from the viewer.
• ball lightning - A bright round spark that appears to float mid-air.
• bead lightning - Bead lightning is much more rare, and appears as a series of glowing dashes. Scientists still do not know what force could cut lightning into pieces, leading some to suspect that the alternating segments and gaps might be an optical illusion in themselves.
• elves - A phenomenon discovered in 1995, these are bright short lightning flashes that appear above the clouds, at the edge of space. They appear for only a thousandth of a second. Though their color is unknown, scientists think they are green.
• jets - These are fast-moving sprays of blue light that explode upward from storm cloud tops to a height of about twenty miles.
• sprite - A red light flash that appears as much as sixty miles above thunderstorms.

Interestingly, lightning will also sometimes tunnel into the ground! The bolt’s intense heat fuses sand particles together, creating a tube-shaped crust known as fulgurite, after the Latin word for lightning. They can reach lengths longer than ten feet.

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Forces of Nature: ThinkQuest 2000 (Team #C003603)

http://library.thinkquest.org/C003603/english/severestorms/lightning.shtml