Introduction

Thunder can be defined as the acoustic emission associated with a lightning discharge. It appears that all impulsive processes in both cloud-to-ground and cloud flashes, including M-component-type processes (Section 4.9), produce thunder. The significant part of the thunder spectrum extends from a few hertz or less to a few kilohertz. It is the general view that audible thunder (above 20 Hz or so) is a series of degenerated shock waves produced by the gas dynamic expansion of various portions of the rapidly heated lightning channel, while infrasonic thunder (approximately 20 Hz and below) is associated with the sudden contraction of a relatively large volume of the thundercloud when lightning rapidly removes the charge from that volume.

Observations

There have been remarkably few measurements of the properties of thunder in the two decades or more since the detailed experiments of the groups from Rice University, Texas (e.g., Few et al. 1967; Few 1968, 1969a; Teer 1972, 1973; Few and Teer 1974) and from the New Mexico Institute of Mining and Technology (e.g., Holmes et al. 1971a; McCrory 1971). The most significant observations, including recent measurements of the acoustic signatures of rocket-triggered lightning (Depasse 1994), are reviewed below in this section.