During times when there is turbulence in the atmosphere, different clouds may become positively charged or negatively charged. When a cloud is positively charged, the earth below it will become negatively charged. And when a cloud is negatively charged, the earth below it will become positively charged. When the difference in potential (i.e. difference in voltage) of the cloud and the ground becomes great enough, an arc will form between the two and allow the electricity to travel from one to the other disipating that potential. Or if a negative cloud is close enough to a positive cloud, the arc will jump from one cloud to the other and not go to the ground. In effect, it is still "grounding" the arc by traveling to the negatively charged area.
At least that's the way I remember it. That's my story and I'm sticking to it. /forums/images/graemlins/grin.gif