Lightning is generated in cumulonimbus clouds (thunderheads), which have a negative electrical charge at the base and a positive charge at top. Scientists are not certain how these charges develop; they do know the charges are carried by water droplets and ice crystals. The negative charge at the cloud's base causes a "shadow" of positive charge on the earth below.
Conditions are then right to form an electrical circuit--this is what lightning is. An insulator--the air--holds up the connection, but eventually the negative charge within the cloud grows too great for the air to restrain it. An electrical impulse, called a leader, reaches downward from the cloud in steps, each step covering about 50 meters (150 feet). When the leader nears the ground, streamers arise to meet it, and the circuit is complete. A bright streak of electricity, the stroke of lightning, ascends along the same course the leader took. Several more strokes may follow this same path.
The whole sequence is "lightning fast." The leader travels at 220,000 kilometers (136,000 miles) per hour, the pauses between steps take 50 millionths of a second, the return stroke moves at over 100 million kilometers (62 million miles) per hour, and all subsequent strokes are so fast the eye sees a single flickering lightning bolt.
There are many variations. About half the time, lightning strikes between clouds or within a single cloud rather than reaching the earth. A rare but powerful type of lightning comes from a positively charged cloud that the wind has torn away from its negatively charged parent. Lightning can take other shapes as well: a ball, ribbon, sheet, or string of beads.
Benjamin Franklin grossly underestimated the force of lightning when he did his kite-and-key experiment. A current of 160,000 amperes has been recorded in one extremely powerful bolt. An average stroke can easily release 250 kilowatt-hours of energy, enough to operate a 100-watt light bulb continuously for more than three months. And at 30,000 degrees Celsius (54,032 degrees Fahrenheit), lightning is five or six times as hot as the surface of the sun. All of this energy is contained in a channel about the width of a human thumb.
Modern-day Ben Franklins at NCAR use more up-to-date techniques to investigate lightning. A specially equipped sailplane leased by NCAR has gathered data from inside electrical storms over Florida, New Mexico, and Colorado. Since the 1980s, a much clearer picture of lightning distribution has emerged from a nationwide network of detectors that tracks cloud-to-ground strikes. Data from this network often appear on television weathercasts in a map on which dots or x's indicate each flash. And starting in the mid-1990s, NASA's Optical Transient Detector began collected images of lightning flashes in clouds around the globe from an altitude of about 710 kilometers (446 miles) .
The highest point within the positively charged shadow under a thundercloud--a skyscraper, a tree in a meadow, a golfer--is the easiest place for the leader to reach. So avoid high places during storms.