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

The beginning of the mature stage of a thunderstorm is characterized by the onset of rain. At this phase, the storm is fully developed, and precipitation starts to fall from the cloud base. This is a critical moment in the life cycle of a thunderstorm, as it indicates that the storm has enough moisture and energy to sustain itself.

During the mature stage, the thunderstorm reaches its peak intensity, exhibiting strong updrafts and downdrafts combined with heavy rainfall. The rain signifies the release of the energy that has built up within the storm, marking the transition from the development of the storm's structure to the active phase where various phenomena such as lightning, hail, and severe winds may occur.

The other options relate to various aspects of weather phenomena but do not specifically denote the beginning of the mature stage. Strong winds could occur at different times within a thunderstorm's lifecycle, and a decrease in temperature is not a reliable signal of maturity. The appearance of clear skies usually occurs after a storm has passed, rather than indicating the storm's intensity or stage. Thus, the initiation of rain effectively marks the shift into the mature stage of a thunderstorm.