What typically marks the transition from the cumulus phase to the mature phase of a thunderstorm?

The transition from the cumulus phase to the mature phase of a thunderstorm is primarily marked by the initiation of precipitation. During the cumulus stage, warm moist air rises, leading to the development of cumulus clouds. As these clouds grow taller, they eventually reach a critical point where the water droplets within begin to combine and grow heavy enough to fall as precipitation. This is when the storm matures and significant weather phenomena, such as heavy rainfall and possibly severe weather, begin to occur.

While heavy lightning and the development of downdrafts are also characteristic of the mature phase, they typically occur after precipitation has started. The formation of clear air is not associated with the transition between these phases and instead indicates an absence of precipitation or storm activity. Thus, the initiation of precipitation is the defining moment that signals the storm's maturity and its potential for further severe weather development.