  Thunderstorms produce strong electric fields over regions on the order of
kilometer. The corresponding electric potential differences are on the order of
100 MV. Secondary cosmic rays reaching these regions may be significantly
accelerated and even amplified in relativistic runaway avalanche processes.
These phenomena lead to enhancements of the high-energy background radiation
observed by detectors on the ground and on board aircraft. Moreover, intense
submillisecond gamma-ray bursts named terrestrial gamma-ray flashes (TGFs)
produced in thunderstorms are detected from low Earth orbit satellites. When
passing through the atmosphere, these gamma-rays are recognized to produce
secondary relativistic electrons and positrons rapidly trapped in the
geomagnetic field and injected into the near-Earth space environment. In the
present work, we attempt to give an overview of the current state of research
on high-energy phenomena associated with thunderstorms.
