Final answer:
Thunderstorms are caused by warm, moist air that is less dense than the surrounding cooler air, which rises and cools to form cumulus clouds and potentially thunderstorms.
Step-by-step explanation:
Thunderstorms occur where warm, moist air has absorbed enough heat and moisture to be significantly less dense than the surrounding air. Warm air, because its molecules are moving more rapidly, is less dense than cool air. This causes the warmer, less dense air to rise through a process known as convection.
As it rises and reaches higher, cooler altitudes, the water vapor present in this warm air mass can condense, releasing latent heat and creating the towering cumulus clouds associated with thunderstorms, a process that often leads to positive feedback, further driving the storm's development.