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What creates Positive Lightning?
"Positive lightning" is a simple process of completing an electrical circuit from the cloud to the ground and back to the cloud. As negative lightning strikes the ground, the ground becomes negatively charged over a wide area. As the storm moves over this area and past it, the H+ in the upper reaches of the cloud feels this potential difference from the ground and sends out a positive streamer which, for all intents, looks like normal negative lightning. The ground sends up a negative streamer and once these two join, electrons from the ground flow upwards to the H+ in the top of the cloud. The ground then becomes neutral once again. The ground will hold the negative charge of dozens of negative strikes, building up a huge potential, before positive lightning will occur. This is why positive lightning is so powerful.
Positive lightning, also known colloquially as "bolts from the blue", makes up less than 5% of all lightning.[30] It occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike tens of kilometres/miles from the clouds.[31] The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/feet the strike must travel. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.[32]
As a result of their greater power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[33]
One type of positive lightning is Anvil to Ground, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke of lightning issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (and often ahead of) the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and if one occurs in a largely clear sky, it is known colloquially as a "Bolt from the blue."[34]
Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707.[35] Subsequently, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the chances of a similar occurrence.
Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.[36]
An average bolt of positive lightning carries a current of up to 300 kA (kiloamperes) (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300 GJ (gigajoules) (a billion joules).
Extracted from Wikipedia.org
