Grampian Weather Services
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Lightning is a giant electrical spark and is caused by the separation of electrical charge in a thunderstorm. The mechanism of charge separation is still poorly understood but there now seems little doubt that ice particle collisions are involved and the conditions favourable to ice particle collisions are strong updrafts which invigorate a thunderstorm's ice factory. Charge separation by particle collisions occurs until the voltage difference in the cloud reaches 10 to 100 mega (million) volts and the local electric field reaches several hundred kilovolts per meter. The latter field is the approximate dielectric strength of the thundercloud medium and its attainment is required to initiate lionization, the initial stage of the large spark that is lightning.
Cloud to ground lightning
Cloud-to-ground lightning is the most spectacular lightning. It is the lightning bolt we see reaching from the sky to the ground forming a bright, usually forked bolt of light. The ground is the most conductive object that will release the most energy. This form usually occurs from the base of a negatively charged cloud. Sometimes there is a rarer form that happens from the the positive charge in the top of the cloud to the negative charge on the ground known as a positive flash. Only about 20 percent of all lightning reaches the ground. Over the entire Earth there are over 100 strikes every second.
Cloud-to-ground lightning is the most damaging and dangerous form of lightning. The great majority of ground flashes are negative, but the most powerful and most dangerous are the positive ground flashes that can also produce sprites in the mesosphere. Positive flashes occur in the decaying phases of large thunderstorms and in the very active stages of severe storms. Most cloud-to-ground lightning strikes come from the negatively charged bottom of the cloud travelling to the positively charged ground below.
Some meteorologists believe that positive lightning strikes indicate storms that are more likely to spin out tornadoes.
Intra cloud lightning
This most common type of lightning strikes between positive and negative areas in the same cloud. The bolt is not usually visible, but rather appears like a broad flash in the sky.
Intra cloud lightning is far more prevalent than cloud-to-ground lightning, and the mean peak currents in intra cloud lightning tend to be smaller. Intra cloud flashes that leave cloud base and approach ground, but do not reach ground, are called air discharges. Intra cloud lightning that leaves cloud top and heads upwards are sometimes referred to as cloud-to-ionosphere flashes.
In the active stages of severe storms, intra cloud flashes can outnumber ground flashes by 10-100 to one.
Inter cloud lightning
Intra cloud flashes that bridge two thunderstorms are called intercloud flashes. This less common lightning strike occurs between oppositely charged areas of separate clouds. Known as intercloud lightning, the strike passes through clear air and provides a stunning bolt of light. Intercloud lightning poses a particular hazard to airplanes in flight because it passes through the clear air between clouds.
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