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Lightning


Lightning

Lightning occurs when a region of cloud attains an excess electrical charge, either positive or negative, that is powerful enough to break down the resistance of the surrounding air. This process is typically initiated by a preliminary breakdown within the cloud between its high top region of positive charge, large central region of negative charge and its smaller lower region of positive charge.

The different charges in the cloud are created when water droplets are supercooled within it to freezing temperatures and then collide with ice crystals. This process causes a slight positive charge to be transferred to the smaller ice crystal particles and a negative one to the larger ice-water mixture, with the former rising to the top on updrafts and the latter falling to the bottom under the effect of gravity. The consequence of this is gradual separation of charge between the upper and lower parts of the cloud.
This polarisation of charges forms a channel of partially ionised air – ionised air is that in which neutral atoms and molecules are converted to electrically charged ones – through which an initial lightning stroke (referred to as a ‘stepped leader’) propagates down through towards the ground. As the stepped leader reaches the Earth, an upwards connecting discharge of the opposing polarity meets it and completes the connection, generating a return stroke that due to the channel now being the path of least resistance, returns up through it to the cloud at one-third the speed of light and creating a large flash in the sky.

This leader-return stroke sequence down and up the ionised channel through the air commonly occurs three or four times per lightning strike, faster than the human eye is capable of perceiving. Furthermore, due to the massive potential difference between the charge areas – often extending from an incredible ten to 100 million volts – the return stroke can hold currents up to 30,000 amperes and reach heights of 30,000°C (54,000°F). Typically the leader stroke reaches the ground in just ten milliseconds and the return stroke reaches the instigating cloud in 100 microseconds.
Lightning, however, does not just occur between clouds (typically cumulonimbus or stratiform) and the ground, but also between separate clouds and even intra-cloud. In fact, 75 per cent of all lightning strikes worldwide are cloud-to-cloud or intra-cloud, with discharge channels forming between areas of positive and negative charges between and within them. In addition, much lightning occurs many miles above the Earth in its upper atmosphere (see ‘Atmospheric lightning’ boxout), ranging from types that emanate from the top of clouds, to those that span hundreds of miles in width.
Despite the high frequency of lightning strikes and their large amount of contained energy, current efforts by the scientifi c community to harvest its power have been fruitless. This is mainly caused by the inability of modern technology to receive and store such a large quantity of energy in such a short period of time, as each strike discharges in mere milliseconds.
Other issues preventing lightning’s use as an energy source include its sporadic nature – which while perfectly capable of striking the same place twice, rarely does – and the difficulties involved in converting high-voltage electrical power delivered by a strike into low-voltage power that can be stored and used commercially.