To understand why thunderstorms occur more often during the warm months requires some understanding of thunderstorm basics. Thunderstorms thrive under certain conditions. The high humidity, in conjunction with warm temperatures, creates massive amounts of warm, moist air rising into the atmosphere, where it can easily form a thunderstorm. The basic idea is that thunder clouds can become giant Van de Graaff generators and create huge charge separations within the cloud. Let's look at how it works.
Clouds contain millions and millions of water droplets and ice particles suspended in the air.
As the process of evaporation and condensation occurs, these droplets collide with other moisture that is condensing as it rises. The importance of these collisions is that electrons are knocked off of the rising moisture, creating a charge separation . The newly knocked-off electrons gather at the lower portion of the cloud, giving it a negative charge. The rising moisture that has lost an electron carries a positive charge to the top of the cloud. As the rising moisture encounters colder temperatures in the upper cloud regions and begins to freeze, the frozen portion becomes negatively charged and the unfrozen droplets become positively charged.