In clear weather there is a slow, steady, nearly uniform flow of current from the surface of the negatively charged earth and oceans into the positively charged atmosphere, about 50 km high. Where there are no clouds, between the earth and the 50 kilometer elevation, the electric field may reach values as high as 100 volts per meter. In thunderclouds the electric field may be much higher. Lightning is the means by which electrons are transported back to the earth to re-equilibrate the earth's charge balance.
Solar energy is the ultimate source for creating lightning.
Warmth from the sun's radiant energy is responsible for air convection, snow, rain, and hail within clouds. The electrical structure in thunderheads is complex. There are major differences between summer and winter thunderheads, and between those over Florida, New Mexico, South America and Japan. For the most common type of lightning, negative cloud to ground, there is a strong negative region in the lower section of the cloud. The base and top of the cloud may also have scattered positive regions, thus producing cloud to cloud lightning.