Teton Thunderstorm

I went to this location in Grand Teton National Park to photograph sunset. The sunset was somewhat underwhelming, but shortly after sunset a thunderstorm started approaching. I stayed and took photos of the thunderstorm for as long as I safely could, and this was my favorite.
Why some lightning is purple is not totally understood, although it is believed that is is dependent upon the presence of certain gases and particles in the atmosphere at the time of the strike.