At http://phys.org/print375969776.html … the subject title is, 'what causes lightning?' – and it begins by saying it is well known that lightning is an electric current, a powerful burst of charge that flows within clouds and between a cloud and the ground. However, scientists don't fully understand how the initial spark forms – and still don't. What the researchers found is that lightning sparks not upwards (neccessarily) but downwards. How the clouds get the energy to spark such a powerful burst of electricity remains elusive.