We will first define an air parcel. We are going to be using this term a lot. An air parcel is an imaginary volume of air. It can be at any altitude, any temperature, any pressure. Keep this concept in mind.
The fundamental physical phenomenon behind thunderstorms is convection. This is a process in which less dense fluid (be it a liquid or a gas) rises within a more dense fluid.
Convection is driven by buoyancy. Buoyancy is an upward force experienced by an object immersed in a fluid. The force is equal to the weight of the fluid displaced by the object. If the object is denser than the fluid, it sinks – the object is negatively buoyant. If it is lighter it rises up and floats on the surface – it is positively buoyant.
Let us see some examples.