Upward positive leaders (UPLs) that develop from the towers in Rapid City, SD usually exhibit low luminosity during their initial development. For UPLs that do not initially branch, they tend to exhibit pulsing/stepping within the first 500 m of their growth. The luminosity pulses originate at or near the tip of the leader with a luminosity front that travels down the leader toward the tower tip. As the leader grows in length and brightens, the pulse frequency decreases and becomes more irregular. The leader also then exhibit a more continuous development unlikely negative leaders which continue to clearly step during their propagation. These observations are similar to those reported by Idone, 1992 and most recently by Biagi et al., 2011. Below is high-speed video of an UPL’s initial development filmed at 54,000 images per second.
Below is another high-speed video of an UPL filmed at 100,000 ips. There are two bright luminosity pulses that travel from the tip of the leader down to the tower tip during its development.
UPLs that branch shortly after initiation tend to branch widely and remain weakly luminous. They do not exhibit pulsing/stepping like the bright non-branched leaders, however, they are very difficult to see at the higher recording speeds due to their weak luminosity.
Idone, V. P., 1992: The luminous development of Florida triggered lightning. Res. Lett. Atmos. Electr., 122, 23–28.
Biagi, C. J., M. A. Uman, J. D. Hill, and D. M. Jordan, 2011: Observations of the initial, upward-propagating, positive leader steps in a rocket-and-wire triggered lightning discharge. Geophys. Res. Lett., 38, L24809, doi:10.1029/2011GL049944.