CG (cloud to ground) lightning can occur with both positive and negative polarity. The polarity refers to the polarity of the charge in the region that originated the lightning leaders. An average bolt of
negative lightning carries an
electric current of 30,000
amperes (30 kA), and transfers 15
coulombs of
electric charge and 500
megajoules of
energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs.
[32]
Unlike the far more common "negative" lightning, positive lightning originates from the positively charged top of the clouds (generally
anvil clouds) rather than the lower portion of the storm. Leaders form in the anvil of the cumulonimbus and may travel horizontally for several miles before veering towards the ground. A positive lightning bolt can strike anywhere within several miles of the anvil of the thunderstorm, often in areas experiencing clear or only slightly cloudy skies; they are also known as "bolts from the blue" for this reason.
Positive lightning typically makes up less than 5% of all lightning strikes.
[33] Because of the much greater distance to ground, the positively-charged region can develop considerably larger levels of charge levels and voltages than the negative charge regions in the lower part of the cloud. Positive lightning bolts are considerably hotter and longer than negative lightning. They can develop six to ten times the amount of charge and voltage of a negative bolt and the discharge current may last ten times longer.
[34] A bolt of positive lightning may carry an electric current of 300 kA and the potential at the top of the cloud may exceed a billion volts ? about 10 times that of negative lightning.
[35] During a positive lightning strike, huge quantities of
extremely low frequency (ELF) and
very low frequency (VLF)
radio waves are generated.
[36]
As a result of their greater power, as well as lack of warning, positive lightning strikes are considerably more dangerous.
