Why do thunderstorms form on hot days?

Thunderstorms depend on a condition called instability. This occurs when, if you take a parcel of air and lift it, it will remain warmer than its surroundings. Instability is greatest when the air in the lowest few thousand feet of the atmosphere is hot and humid. If this hot, humid air is lifted, it will cool, but remain warmer then the surrounding air, and so will continue to rise on its own. The moisture in the cooling air condenses, forming rain and clouds and releasing the energy that powers the storm.