A thunder glissando is a fast glissando on the lowest strings of the harp.
Thunder glissando is a fast and loud glissando played by the left hand on the lowest strings of the harp. The strings are played forcefully so that they rattle against one another creating a metallic thunder-like sound.
A thunder glissando is notated by writing the range of the glissando as an interval with an arrow that shows the direction of the glissando. The noise symbol shows that rattling between the strings is wanted.
A thunder glissando is played on the lowest wire strings, normally on the lowest octave. The low wire strings have a low enough tension that it is possible to make them rattle against each other.