The opacity of lightning

Lightning channels studied are shown to be optically thin to two N II multiplets by comparing the relative intensities of multiplet members with the predictions of theory and with previous laboratory measurements. From this result it is shown that the lightning channels studied are optically thin to all measured N II and O II line radiation and to the high-temperature continuum. The absorption coefficient for N II 4630.5 A is calculated. From this calculation and the requirement of optical thinness, the effective diameter within which the radiating N II atoms are contained is determined to be less than a few millimeters. The opacity of the lightning channel to neutral atom radiation and to the low-temperature continuum is considered. The results of a paper by Zhivlyuk and Mandel'shtam in which the lightning channel is assumed to be optically thick to three N II lines and one O II line are discussed.