Final answer:
Convection and phase changes, such as water vapor condensing into clouds, are phenomena that provide the initial lifting force for air mass thunderstorms, leading to the formation of thunderheads and potentially severe weather.
Step-by-step explanation:
Phenomena that can provide the initial lifting leading to air mass thunderstorms include convection accompanied by a phase change. When water evaporates from the ocean, it removes heat from the surface. As the water vapor rises and condenses into clouds, heat is released into the atmosphere, aiding in the creation of thunderheads, substantial cumulus clouds that may ascend up to 20 km into the stratosphere. This release of energy during condensation causes the air to expand and rise into cooler areas, prompting further condensation. Such a continuous cycle is an example of positive feedback, which can escalate and reinforce the process, resulting in severe thunderstorms, lightning, and hail.