It raises many eyebrows when it comes to the subject of how lightning stroke occurs and what magic happens in the sky that blurs our vision in the form of light, makes our ears deaf in the form of sound and burns everything it encounters on earth. Perhaps lightning mechanism is the most evident and elaborated example of an electrostatic discharge. Such kind of discharges have always been the center of attention for mankind because the anger of a stroke can never go unnoticed, it can interrupt us midday or can wake us up at the night increasing our heartbeats.
From electrical point of view materials have been divided into three major classes known as conductors ,semi conductors and insulators. The air surrounding us everywhere inhibits current flow and thus is known as a good insulator. However each insulator has a certain strength to resist flow of charges. This dielectric strength is 30000 volts for one centimeter of dry air and 17000 volts for humid air. Lightning occurs on a rainy day so air is heavily humid and it merely needs 17000 volts to behave as a conductor of electricity. Clouds layers are contaminated with tons of oppositely charged particles thus creating a massive potential difference far beyond the one needed to make a conductive path through surrounding air and approaching to the ground. Clouds are some hundreds of kilometers from earth so how electrostatic discharge affects things residing at ground, this is what I am going to discuss in detail.
According to the most authentic research based on charge formation in clouds, the upper side of a floating cloud contains positive and lower one contains negative charges and this charge separation is the fundamental reason of electric field development and this charge formation is caused by either upward moving water droplets that collide with the ice particles and water droplets present in clouds, or due to the conversion of water droplets into ice as they gain higher altitude. The frozen part of the cluster of rising drops becomes negatively charged and the outer portion acquires the positive charge. The frozen portion of the droplets with their negative charge tends to gravitate towards the lower side of the clouds, thus making the clouds further charged. As mentioned earlier that lower side of a cloud contains negative charge so as the electric field stretches downward through air, it repels the negative charge off the material residing on earth thus inducing a positive charge on surface.
The lightning stroke travels down to earth in the form of a step ladder. Most of the charge is neutralized by an intra cloud discharge process in which positive charge of one cluster finds its way to be discharged by the negative charge of the other cloud. In case of a cloud to ground stroke the air particles start behaving like a conductor because of thousands of volts present in the form of charge. As the air gets ionized the charge follows its way to ground in zigzag manner and the visible zigzag light pattern traversing its path downwards is because of the fundamental phenomena of physics which states that whenever an electron goes from a higher energy state ( bound in radius around nucleus ) to a lower energy state ( getting free in case of charge formation on cloud) it emits visible light.
As the ladder proceeds, the negatively charged cloud induces the opposite charge on tall buildings and mountain tops by repelling electrons (like charges repel each other) thus creating an equivalent amount of positive charge. when the complete air gap loses its dielectric strength or up to a certain distance above ground, the positive charge induced on the stuff on ground travels upward by an approximate speed of 50000 miles per second to meet the ladder in the air. This phenomena creates a fully ionized and conductive air channel and hence a cloud having a potential of thousands of volts discharges itself by letting thousands of amperes of current to the earth and this massive current causes all the damage that happens on earth by a lightning stroke.
Scientists have worked long to understand the phenomena of lightning but are still unable to use this energy for human well being because this energy can’t be controlled due to its outrageous nature. All what science could do is to propose methods to avoid damages to buildings and people living inside. For this purpose lightning arresters are used which are normally rod shaped. The working mechanism behind using such rods is to provide incoming current and heavy static charge an alternative path. These rods are connected to the earth which is a huge reservoir of charge. When stroke occurs, the rod prevents damage by providing a conductive path to the earth.