Published: 2007-09-19 10:23:24
Updated: 2007-09-19 10:23:24
Posted September 19, 2007
By John Rooney
MIKE MOSS SAYS: John, Most of the newer lightning display graphics used by on-air meteorologists discriminate between strikes having negative polarity (transferring negative charge to the surface) and those with positive polarity, which are notably less common at about 5-15% of all strikes. On the HD Doppler Network display often shown on WRAL, negative strikes are color-coded with white lines and positive ones are yellow, with the amplitude, or peak intensity of each strike proportional to the vertical length of the plotted lines.
Regarding the significance of this information, positive flashes usually originate higher in clouds and may occasionally strike up to ten miles (and very rarely even farther) from the storm cloud. In addition, while negative flashes transfer about 5 coulombs of charge, a positive flash may transfer 300, at a current of 350 kiloamperes versus 30 or so for a negative flash, and with a continuing current duration around 5-10 times as long.
All lightning strikes have the potential to injure, kill or cause damage. However, the much larger currents, charge transfers and current durations associated with positive flashes make them more lethal on average in direct or near-direct strikes to humans, and likewise more likely to damage power transmission facilities and to start fires.