What characterizes air mass thunderstorms in terms of development?

Air mass thunderstorms are primarily characterized by their development due to thermally-induced convection. This process occurs when the sun heats the Earth's surface, causing the air close to the ground to warm up. As the surface air becomes warmer, it becomes less dense and rises, which can lead to the formation of cumulonimbus clouds and, eventually, thunderstorms if conditions permit.

These thunderstorms tend to develop in regions where there is ample surface heating, such as during summer afternoons when solar radiation is strong. The presence of moisture in the air is also important, but the key characteristic that sets air mass thunderstorms apart is their reliance on the local heating of the ground rather than on other dynamic processes like cold air movement or frontal systems.

The other options all describe factors that do not typically apply to the development of air mass thunderstorms. For instance, while moisture is a contributor, air mass thunderstorms do not require continuous cold air movement, nor do they depend solely on frontal systems for their development. Instead, they thrive primarily on local thermal conditions.