Cloud-to-cloud lightning
Introduction
Cloud-to-cloud lightning (CC) is a type of lightning discharge that occurs between two or more separate cumulonimbus clouds. This type of lightning is less common than cloud-to-ground (CG) lightning, but it is significant in the overall lightning discharge process.
Characteristics
CC lightning is characterized by a series of rapid, high-current pulses. These pulses can reach peak currents of tens of kiloamperes and last for several tens of microseconds. The total duration of a CC lightning flash can range from less than a second to several seconds. The average length of a CC lightning channel is approximately 5 kilometers, but it can extend up to 15 kilometers or more.
Formation
The formation of CC lightning involves a complex process of charge accumulation and discharge within a thunderstorm. This process begins with the upward movement of warm, moist air in the form of updrafts. As these updrafts rise, they cool and condense to form cumulonimbus clouds. Within these clouds, ice particles and supercooled water droplets collide and become electrically charged. This leads to the separation of positive and negative charges within the cloud, with positive charges accumulating at the top of the cloud and negative charges at the bottom. When the electric field between two clouds or between different parts of the same cloud becomes sufficiently strong, a lightning discharge can occur.
Types of CC Lightning
There are several types of CC lightning, including intracloud (IC) lightning, intercloud (IC) lightning, and bead lightning.
Intracloud (IC) Lightning
Intracloud (IC) lightning is a type of CC lightning that occurs within a single cloud. It is the most common type of lightning, accounting for approximately 75-80% of all lightning discharges. IC lightning typically involves the discharge of negative charges from the lower part of the cloud to the upper part of the cloud.
Intercloud (IC) Lightning
Intercloud (IC) lightning is a type of CC lightning that occurs between two different clouds. This type of lightning is less common than IC lightning, but it can occur when the electric field between two clouds is sufficiently strong.
Bead Lightning
Bead lightning is a type of CC lightning that appears to break up into a string of short, bright segments. This type of lightning is relatively rare and is not fully understood. It is thought to occur when the lightning channel becomes highly fragmented, causing the light emitted by the lightning to appear as a series of 'beads'.
Effects
While CC lightning does not pose a direct threat to humans or structures on the ground, it can have significant effects on the atmosphere and the electrical systems of aircraft. CC lightning discharges can produce strong electromagnetic fields that can interfere with the operation of electronic equipment. They also contribute to the production of nitrogen oxides (NOx) in the atmosphere, which play a key role in the formation of ozone and other reactive nitrogen species.