There are more thunderstorms in summer than at other times of the year. This is mainly because thunderstorms need moisture and warm, fast-rising air to develop, so certain types of humid summer weather are ideal for them (if you think of the weather before a summer storm, it is usually warm and humid). Basically, if you have a lot of damp in the air and then the temperature rises quickly, you are likely to get a storm.

The water droplets in clouds collide with other moisture in the air as they condense and evaporate. The collision causes a charge separation, with negative electrons collecting at the bottom of the cloud and positive at the top. As the separation becomes more intense, the electric field created by it grows stronger. The field can eventually form a 'path' to conduct electricity to the earth's surface, which creates lightning.