In terms of positive and negative charges what causes lightning?

Electrostatic Discharge during the equalization of the separated positively and negatively charged electrostatic charges in the storm clouds.

Explanation:

In nature, lightning occurs during the discharge of the large charges contained in the separated positively and negatively particles in the atmosphere in the process equalizing (neutralizing) of the charges, leading to the release of instantaneous energy in the form of heat, light and sound

During a storm, as the clouds mixes by the wind with different densities, they rub each other forming electrostatic charges. The lighter positively charged particles rise to the top clouds while the heavier negatively charged particles move to the bottom. The charges continue to add up to a point where their magnitudes is large enough such that a big spark or lightening occurs between the positive and negative charges during the process of re-equalization of the charges to revert to neutrality.