form of lightning of longer duration than more typical lightning that appears as a string of luminous segments instead of a continuous channel. It occurs infrequently but has been observed many times. Its causes are unknown, but among the theories proposed are the following: portions of the lightning channel are slanted toward or away from the observer and thus seem brighter or dimmer; parts of the channel are obscured by rain or clouds; and sections of the channel with large radii cool more slowly than do those with small radii.