Lightning is a powerful electrical discharge that occurs in mature thunderstorms. Lightning most commonly occurs within a cloud, either from a cloud to the surrounding air, or from a cloud to another cloud. Only about 20% of all lightning strikes occur between a cloud and the ground, which is what most people think of as classic lightning. Lightning strokes can heat the air through which they travel to an amazing 30,000 degrees Celsius (54,000F). This extreme heating causes the air to explosively expand, creating a shockwave that travels outwards through the air as the booming sound that we call "thunder." Lightning is still not fully understood aspect of thunderstorms, but much has been learned through observations with modern technology.
Lightning forms through the electrification of clouds, and the most popular theory is that this comes about through graupel and hail falling through the region of the cloud containing supercooled water droplets and ice crystals. As liquid droplets collide with hailstones and freeze on contact, the latent heat released keeps the surface of the hailstones warmer than the surfaces of the surrounding ice crystals. When the hailstones collide with the colder ice crystals, there is a net transfer of positive ions from the warmer hailstones to the colder ice crystals. The hailstones therefore acquire a negative charge, and the ice crystals become positively charged. The lighter ice crystals get carried upwards into the cold part of the thunderstorm by strong updrafts, causing the upper part of the cloud to become positively charged. The heavier hailstones fall toward the bottom of the cloud, causing the middle part of the cloud to become negatively charged. The lower part of the cloud is generally of negative and mixed charge, with some positive charged areas where precipitation is leaving the cloud.
Image courtesy: JASON Science
Because opposite charges attract each other, the negatively charged bottom of the cloud causes positive charges to build in the ground directly beneath it. These positive charges will follow the cloud wherever it goes. The largest buildup of positive charges will occur in protruding objects such as houses, trees, and poles, which reach up toward the sky. The difference in charges causes an electric potential to build between the cloud and the ground, but in dry air, a flow current does not occur because air is a good electrical insulator. However, as the electrical potential gradient gradually builds, it will eventually become large enough (on the order of a million volts per meter) to overcome the insulating properties of the air, causing a current to flow, and lightning occurs. Cloud-to-ground lightning begins within a cloud when a sufficiently strong localized electric potential gradient causes a discharge of electrons to surge towards the cloud base and then towards the ground. This discharge proceeds in a series of steps, each about 50-100m long, with pauses of about 50 millionths of a second between each step. This is called a "stepped leader," and is usually quite faint, sometimes too faint to see with the naked eye.
As the tip of the stepped leader nears the ground, the electric potential gradient increases sharply, and a current of positive charge starts upward from the ground to meet it. Once they meet, a surge of electrons flows to the ground, and then a much more intense "return stroke" races upward towards the cloud along the same path traced by the stepped leader. This return stroke is what causes the bright flashes that we see as lightning,
and the transfer from ground to cloud
is so fast that it is imperceptible to the human eye, and thus we see
it as just a continuous flash of light. Lightning can occur only once,
but more often there are multiple strokes that occur in the same
channels that the air has already been ionized, and thus several
strikes can occur in under a second. Sometimes these multiple strikes
are perceptible to the eye, giving the appearance of flickering to the
observer.Lightning animation depicting intercloud lightning in Toulouse, France. This sequence occurred during a duration of 0.32 seconds.
Image courtesy: Sebastien D'ARCO
Lightning striking east of Vail, Arizona in 1988. Photo courtesy: Jeff Smith