The previous section describes what is called "negative lightning" because it is the transfer of negative charge from the cloud to the ground. However, not all lightning forms in the negatively charged region under the thunderstorm base.
Some lightning originates in the cirrus anvil or upper parts, near the top of the thunderstorm, where a high positive charge resides. Lightning that forms in this region follows the same scenario as previously described, but the descending stepped leader will carry a positive charge while its subsequent ground streamers will have a negative charge.
These bolts are known as "positive lightning" because there is a net transfer of positive charge from the cloud to the ground.
Positive lightning makes up less than 5% of all strikes. However, despite a significantly lower rate of occurrence, positive lightning is particularly dangerous for several reasons.
Since it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, electric fields associated with positive Cloud-to-Ground (CG) strikes are typically much stronger than those associated with negative strikes. The flash duration is also longer, with peak charge and potential up to ten times greater than negative CG strikes, as much as 300,000 amperes and one billion volts!
Some positive strikes can occur within the parent thunderstorm and strike the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or from the thunderstorm's anvil and can occur MORE THAN 25 MILES AWAY from precipitation. If you hear thunder, you are close enough to be struck by lightning regardless of where the Doppler radar says the storm is located.
Positive flashes are also believed to be responsible for a large percentage of forest fires and power line damage. Thus, positive lightning is much more lethal and causes greater damage than negative lightning.
Some interesting properties of positive lightning:
- Positive lightning can be the dominant type of cloud-to-ground during the winter months and in the dissipating stage of a thunderstorm.
- Positive lightning has been identified as a major source for electrical discharges that happen above the thunderstorm, like sprites and elves. Sprites are brief but powerful discharges of electricity that occur 25-45 miles (40-75 km) in altitude. Elves can occur as high as 60-250 miles (100-400 km) above their parent storm and are a brief halo. Both phenomena are often red.
- Positive lightning is usually composed of one stroke (negative lightning typically consists of two or more strokes).
Finally, there is bipolar lightning, which actually changes its polarity (positive becoming negative or vice versa). It is no less dangerous than any other type of lightning but shows that we live on a complex planet with many aspects we do not fully understand.