Which weather phenomenon signals the beginning of the mature stage of a thunderstorm?

The beginning of the mature stage of a thunderstorm is signaled by precipitation beginning to fall. In this stage, updrafts that have developed in the earlier stages of the storm are now strong enough to allow water droplets to grow large enough to overcome the updrafts and begin to fall as rain. This is a key characteristic of the mature stage, as it indicates that the storm has reached a level of development where significant weather effects can occur, such as heavy rain, thunder, and lightning.

While other aspects like the appearance of an anvil top or loud thunder claps are associated with a mature thunderstorm, they do not specifically mark its onset. The anvil top forms as the storm reaches its peak height, and thunder is a result of the atmospheric dynamics occurring within the storm, but they occur after the precipitation has started. The maximum growth rate of the clouds is also a feature of the storm's development, but it occurs prior to rainfall, not at the onset of the mature stage.

Thus, recognizing that the onset of precipitation is the defining moment that transitions a thunderstorm from its developing phase to the mature stage is crucial for understanding thunderstorm lifecycle dynamics and anticipating weather phenomena.