We present the channel geometries and vertical profiles of tortuosity for five natural lightning flashes and seven triggered flashed photographed in Florida. The tortuosity variation with height was quantified using the tortuosity measurement method of Hill applied over successive 200 m vertical sections. Total channel lengths analyzed were between 1 km and 1.5 km. The triggered channels often show as significant reduction of tortuosity with height; the natural flashes show no apparent trend of tortuosity variation with height. A time-resolved record of one of the triggered flashes reveals an abrupt doubling in speed of the upward leader associated with a sharp reduction in channel tortuosity. This observation suggests that positive lightning leaders may manifest decreased tortuosity when driven by larger fields. ¿ American Geophysical Union 1988