Storm fronts are caused by the rising masses of warm air which expands at attitudes of upto 15 000 meters. At the centre, at about 6 000 meters, there is a powerful up draught, leading to the separation of charges. The presence of ice crystals and supercooled water droplets is closely associated with this formation of electric fields by separation of charges. Studies has demnstrated that hailstones bear negative charges and ice crystals bear positive charges. Being lighter, the ice crystals are carried by the up draught into the upper region of the cloud, whilst the hailstones fall into central regions of the cloud. So by this process, positive charges gets accumulated in the upper part and negative charges in the lower part.
On the other hand, close to the ground there are positive charges (ground level field strength). The field needede to trigger a lightning strike depends on the insulating capacity of air which lies between 0.5 and 10 kV/cm owing to presence of various particles. The packets of negative charges advance by stages in a length of about 50 m and with an average pause time of 50 μs. By progressive ionisation, this pre-discharging forms a channel capable of conducting, the thickness of channel is about few centimeters. So the lightning flash starts in the lower negatively charged regions of the cloud. The first part of the discharge process consists of a pilot leader stroke in which a pilot leader propagates from negative cloud center towards the earth, branching as it goes down (figure 1a). The pilot leader always moves in steps with a pause as explained above and lowers into the air a substantial part of the total charge trapped by it.
When the pilot leader reaches the earth, an upward positive streamer starts from the earth (figure 1b). This is immediately followed by a return stroke (main lightning stroke) which is equivalent to positive current flowing upwards along the highly ionized discharge channel formed by the pilot leader. At the instant of maximum current density, the temperature in the lightning channel rises to 30 000 K, which causes explosion in the air and is percieved as thunder. This temperature maintains the thermal ionisation and due to this secondary strikes may occur and after the first charge centre in the cloud is completely discahrged by the formation of a streamer between the first and second charge centre (figure 1e). The second charge centre is also discharged to the ground via the first charge centre and through a subsequent leader known as dart leader. When the dart leader makes contact with the earth, heavy return streamer proceeds upwards (figure 1f).
A different type of lightning discharge may occur in mountainous regions. For instance, a mountain peak may be surrounded by a thunder cloud or the bottom of the cloud may lie only slightly above the peak. Conducting objects such as human beings at the tip of the mountain may get heavily charged. Such discharges from these charged objects can develop into an upward growing leader stroke, a reverse of normal lightning discharge process.
About 90 % of the lightning strikes to earth are of the type explained above i.e. negative cloud/earth lightning. Other categories of discharges are: negative earth/cloud lightning, positive cloud/earth lightning and positive earth/cloud lightning. However, it may be noted that vast majority of discharges takes place within a cloud or between one cloud to another and only small discharges to earth are there.
The lightning discharge may be a single lightning current or a sequence of current discharges and in one instance twenty six discharges has been recorded. Based on which place a lightning strikes, there are different types of lightning strokes:
Types of Lightning Strokes:
Direct Stroke: As the lightning stroke progresses from the cloud towards the ground, the electrostatic field strengthy at the tip of the leader increases rapidly. When the tip of the leader nears the ground, the resulting field strength attains a critical value and a short upward streamer leads up from any sharp conducting object, resulting in direct lightning stroke.
Side Flash: When a lightning strikes, the lightning current is discharged through whatever conducting path it gets. As the lightning current discharges is very heavy (Some tens of kA), the potential difference between two objects may become high enough and side flash results. Two kids of side flashes are illustrated in fig below:
In the first case, a person is tuoching a water tap, when the house is stuck, the lightning current paseses through domestic wiring, and if resistance of wiring to ground is considered 5 Ω and lightning current is taken 20 kA, the potential to earth becomes 100 kV and can cause side flash.
In the second case, a person is standing under a dry timber structure with a corrugated iron roof. When the lightning strikes in the vicinity, the voltage of roof V2 becomes
V2 = V1 × C1 / (C1 + C2), and this value may be high enough to cause side flash.
Step Voltage Induced Stroke: This process is depicted in fig.
If the soil is of uniform constitution, the current, distribution in the earth is in a regular pattern, otherwise it is exteremely irregular. A person standing near the point where lightning has occurred is subjected to a potential difference between the legs, the magnitude of this step voltage is given by:
v = i × r / 2π × s / (d × (d + s)), where i is the lightning current and r is the soil resistivity.
Consequences of Lightning on Humans:
A typical lightning stroke has a central core of diameter of about a few centimeters. The core temperature reaches a temperature which may exceed 30 000 K. It generally decays to relatively low value within several tens of microseconds. It is this extremely short duration of heat which may save a person from extensive burning. Some lightning discharges have currents of several hundred amperes and may last over several tens of milliseconds, such discharges are caleed ‘hot lightning’ as they are responsible for settinf fire to inflammable materials and causing severe burns to humans and animals.
The highest resistance offered to current is offered by the skin, particularly when it is dry. Further as the cross section of the core is small, the heating effect is concentrated over a small area and because of these two resons burns are common in lightning accidents. Often the victim of lightning loses memory for a short while due to the malfunction of central nervous system. Second most frequent is the failure of the heart, sometimes which may be only temporary. Also rupture of ear drums and development of catract in the eye are some other consequences.
If the direct lightning strike passes from head and exits from feet, death is almost certain with excessive burning. In the case of side flash or step flash, there may be temporary paralysis of the hands or feet.
But, fortunately many times lightning accidents can be avoided by taking some simple precautions. Un upright person attract a lightning stroke, so it is safer to lie down. Carrying umbrella in thunderstorm days is dangerous, it is better to get wet, also swimming on thunderstorm day should be avoided. While walking in groups, a distance of 1 meter or so should be maintained. It is also advisable to keep away from large isolated tree, and shelter should be sought in an all metal vehicle like a motor car or building.