Lightning is a large spark produced by an abrupt discontinuous discharge of electricity though the air, resulting most often from the creation and separation of electric charge in cumulonimbus clouds (dense and vertically developed clouds). When electric fields generated by the charge buildups become too strong — typically 3-4 kV/cm at the altitude of the negative charge region of the cloud — electrical breakdown of the air occurs and charge is exchanged within the cloud or to the ground.
Lightning, form of visible electric discharge between rain clouds or between a rain cloud and the earth is seen in the form of a brilliant arc, sometimes several kilometers long, stretching between the discharge points. The discharge also sets up a sound wave that is heard as thunder.
How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. The various hypotheses that explain how the polarization occurs may be divided into two categories: those that require ice and those that do not. Most meteorologists believe, however, that ice is a necessary factor, because lightning is not usually observed until ice has formed in the upper layers of thunderclouds.
Experiments have shown that when dilute solutions of water are frozen the ice gains a negative charge but the water retains a positive charge. If, after freezing has started, rising air tears small droplets of water away from the frozen particles, the droplets are concentrated in the upper part of the cloud and the larger ice particles fall toward the base.
On the other hand, experiments have also shown that large, swiftly falling drops of water become negatively charged whereas small, slowly falling drops become positively charged. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor. When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionised along a narrow path and a lightning flash results. Many meteorologists believe that this is how a negative charge is carried to the ground and the total negative charge of the surface of the earth is maintained.
A new theory, suggesting that the electrical polarization in a thundercloud may cause precipitation rather than be a consequence of it, postulates that the electrical potential existing between the ionosphere— the highest layer of the atmosphere— and the earth initiates the polarization in a thundercloud.
According to this theory, the upward flow of warm air through a thundercloud carries with it positively charged particles. These accumulate at the top of the cloud and attract negative charges from the ionosphere.
Source : The Hindu