There isn't a specific stage of a thunderstorm that is considered the "birth" in the way we might think of a living thing being born. However, there are a couple of ways to think about the beginning of a thunderstorm:

* Cumulus Stage: This is the initial stage where warm, moist air rises and forms towering cumulus clouds. This is often considered the "birth" of a thunderstorm, as it's where the storm's potential is developing.

* Mature Stage: While not technically "birth," this is the point where the thunderstorm is fully formed and active. It's characterized by heavy rain, lightning, thunder, and strong winds. This stage is crucial for the thunderstorm's lifespan, as it's where the primary processes of rain, lightning, and wind development occur.

It's important to remember that thunderstorms are complex meteorological events, and their development is continuous. There isn't a single, definitive "birth" moment, but rather a series of stages where the storm builds and strengthens.