Introduction

The primary source of lightning is the cloud type termed cumulonimbus, commonly referred to as the thundercloud. The term “cumulonimbus cloud” is often used in the literature, although it is redundant. Strictly speaking, not every cumulonimbus produces lightning (e.g., Imyanitov et al. 1971), that is, a thundercloud could be more properly defined as a lightning-producing cumulonimbus. Sometimes the term “thunderstorm” is used as a synonym for thundercloud, although a thunderstorm is usually a system of thunderclouds rather than a single thundercloud. Lightning produced by thunderclouds formed over forest fires or contaminated by smoke is considered in Section 5.2. The electrical properties of clouds other than the cumulonimbus (primarily stratiform clouds) are reviewed in Section 3.3, and further information can be found, for example, in the books by Imyanitov et al. (1971) and by MacGorman and Rust (1998: Chapter 2 and Section 8.4). Lightning-like electrical discharges can also be generated in the ejected material above volcanoes, in sandstorms, and in nuclear explosions (Section 20.7). Clouds on planets other than Earth as potential sources of lightning are discussed in Chapter 16.