Short overview

Thunderclouds (Cumulonimbus, Cb) are the cradles of the most spectacular manifestations of atmospheric electricity. The dynamic processes that separate electric charges within these clouds create high-density regions of different charges. When the electric field of one of these charge centers exceeds a critical threshold, a current of charge is initiated, resulting in an electrical discharge.

Lightning can occur within the cloud itself (between oppositely charged centers), strike the ground, or happen between nearby clouds. There are so-called cloud-to-air lightning discharges, too, which occur between the cloud and the surrounding electrically charged screening layer. This charged envelope consists of free charged particles, always present to some extent in the atmosphere, which have been drawn close to the cloud by its electric field.

Types of lightning discharges: intra cloud, inter cloud cloud-to-ground, and cloud-to-air.

In lightning discharges, the electric charges carry high energy and can heat the discharge channel to such a degree that the air turns into hot plasma and glows bright white—this is the white flash of lightning.

A different group of atmospheric discharges begins in an electric field which is strong enough to initiate a weaker charge flow but is weak for producing a fully developed lightning discharge. In this case, ionization and excitation of the air occur in the discharge channel, but the process only reaches a so-called cold plasma state (where electrons are energetic, but the neutral air and ions remain relatively cool).

Such discharges mainly occur at the top of the cloud layer or above it at altitudes of several tens of kilometers. These can be referred to as transient luminous events (TLEs). The bluish corona discharge (St. Elmo’s fire) forming near the ground on pointed or sharp objects under strongly charged thunderclouds is essentially the same type of phenomenon.

The light produced in this process is less intense than lightning (though still clearly visible), and its color typically varies depending on the altitude (i.e., atmospheric pressure) and the type and energy of excited particles. At relatively low altitudes (up to 30–40 km), bluish-violet colors appear; higher up (up to 80–90 km), red hues are typical; while at altitudes close to 90–100 km, greenish colors may occur due to the excitation of atmospheric oxygen.

The types of known Transient Luminous Events (TLEs)

The shape of TLEs varies widely depending on their formation mechanisms and the properties of their source processes. All TLEs share the characteristics of lasting only a fraction of a second and being visible only at night.

A video showing several types of TLEs (Facebook) (Sound on!)