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.

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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.