Final answer:
Downdrafts dominate the dissipating stage of an air mass thunderstorm, marking the weakening of the storm and leading to its eventual end as precipitation ceases.
Step-by-step explanation:
Downdrafts totally dominate the dissipating stage of an air mass thunderstorm. During the dissipating stage, the inflow of warm, moist air that feeds the storm is overwhelmed by the outflow of cooler, drier air.
This results in the weakening of the storm, as the downdraft cuts off the updraft that is needed to sustain the thunderstorm. It is the final stage of the thunderstorm life cycle, following the cumulus and mature stages. As the thunderstorm dissipates, the cloud tops begin to evaporate, and precipitation weakens until it eventually ceases.