What is lightning:

Lightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 220,000 km/h (140,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground. There are some 16 million lightning storms in the world every year.

What is Lightening Arrester:

A lightning arrester is a metal rod mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning. If lightning strikes the building it will preferentially strike the rod, and be conducted harmlessly to ground through the wire, instead of passing through the building, where it could start a fire or cause electrocution. A lightning rod is a single component in a lightning protection system. In addition to rods placed at regular intervals on the highest portions of a structure, a lightning protection system typically includes a rooftop network of conductors, multiple conductive paths from the roof to the ground, bonding connections to metallic objects within the structure and a grounding network.

Bolts of lightning:

To understand how lightning conductors work, you must first understand how lightning is formed. You will not be able to distinguish between the different stages in the formation of a bolt of lightning simply by observing lightning.

Yet the following phenomena occur in most bolts of lightning:

A luminous track - a downward leader-is formed at a point in a cloud and extends out from it about 50 m (55 yards), moving at 50,000 km (31,070 miles) per second.

A second downward leader is formed at the same point in the cloud, and follows the same trajectory as the first one at a similar speed, extends out about 50 m more, and then it disappears too.

The process continues in the same way until the tip of the last tracer comes to within a few dozen yards of the ground, and in some cases within a few yards of it. As soon as the tip of a downward leader gets close to the ground a link between the cloud and the earth's electrostatic discharge (know as the corona effect) is formed. Then what happens is that an upward streamer from the ground to the cloud is formed - this is called sparkover or grounding.

Note: The "strength" of grounding is inversely proportional to the distance from the storm cloud. This is why tall building are more likely to be damaged by lightning.

The connection between the two phenomena - the downward leader and the upward streamer - causes the main discharge (lightning strike), which may be followed by a series of secondary discharges along the channel that was ionised by the main discharge of a single tracer (the pilot discharge).