Erica's Answer:
This is due in part to the humid summer air. Humid air is less dense than dry air, which makes it rise. Rising air eventually cools and condenses into clouds. But of course, it's not just humid in the summer- it's also hot! Hot air is less dense than cold air, so as hot air tries to rise, it also cools and forms clouds. The hot air also pushes cold air out of the way, causing it to sink and get warmer. This cycle repeats itself many, many times during the summer. That rising and sinking air is the key to thunderstorm instability!- Erica
