Why thunderstorms happen mostly in summer?

Thunderclouds form when warm air goes up in the atmosphere. Such a flow can only occur if cold air enters a warm air mass.

In summer, this happens quite often: the wind brings cold air to the region where the weather is hot and thunderclouds are born there.

In winter though temperatures are often negative and even with the appearance of an upward flow of air, thunderclouds cannot form, since there is no liquid water in the atmosphere (as we all know, water freezes at zero degrees). And instead of a thunderstorm with heavy rain and strong thunder, we get a soft and fluffy snowfall.