What weather phenomenon indicates the mature stage of a thunderstorm?

The mature stage of a thunderstorm is characterized by the presence of a variety of weather phenomena, one of which is the appearance of an anvil top. During this stage, the storm has developed sufficiently in intensity and vertical growth that it reaches the tropopause, leading to the anvil shape commonly associated with mature thunderstorms.

At this point, the thunderstorm exhibits significant instability, strong updrafts, and can produce severe weather, such as heavy rain, hail, lightning, and even tornadoes. The start of rain typically signals the onset of the mature stage, but the defining characteristic is primarily the formation of the anvil top, which signifies the storm's height and strength.

Consequently, while rain beginning to fall can indicate development, the appearance of the anvil top is a more definitive sign of the thunderstorm reaching its mature phase, as it highlights the full development of the storm structure.