New surprising research shows that lightning seems to consist of electrons moving towards the earth from the ionosphere (several hundreds of kilometres above the surface of our planet). see  her

We don't know how it is possible that electrons to reach the lower atmosphere from that altitude.

A cloud system (extended mass) will have a weak force of gravity which increases in the direction of the systems centre, this helps keep the mass together.

This means that in the centre of a low pressure area we should be able to measure a stronger force of gravity.

A low pressure area is rotating. This property combined with an increased gravity in the centre of such a low pressure area, means that a low pressure area is rotating deformed space. As explained in previous chapters, space follows the motion of matter.

Such a rotating could maybe pull down electrons from the ionosphere. Heavy clouds have also very weak gravitational fields and therefore they are likely able also to catch such electrons. The result is that heavy clouds become electrically charged and lightning becomes inevitable. To fully understand this chapter please make sure you have read the chapter 'magnetism & electricity'