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As storm clouds develop, the temperature at the top of the cloud becomes much cooler than that at the bottom. For reasons that scientists still do not understand, this temperature difference results in the accumulation of negatively charged particles near the base and positively charged particles near the top of the storm cloud. The negatively charged particles repel the electrons of atoms in nearby objects, such as the bases of other storm clouds or tall objects on the ground. Consequently, these nearby objects take on a positive charge. The difference in charge, or voltage, builds until an electric current starts to flow between the objects along a pathway of charged atoms in the air. The current flow heats up the air to such a degree that it glows, generating lightning. Initially, a bolt of lightning carrying a negative charge darts from one storm cloud to another or from a storm cloud to the ground, leaving the bottom of the cloud with a positive charge. In response, a second bolt ( reverse lightning ) shoots in the opposite direction (from the other storm cloud or the ground) as the mass of negative charges on it moves back to neutralize the positive charge on the bottom of the first cloud. The heat generated by the lightning causes the air to expand, in turn creating very large sound waves, or thunder.