What atmospheric condition is usually associated with the formation of thunderstorms?

The formation of thunderstorms is typically linked to increased humidity and instability in the atmosphere. High humidity provides the necessary moisture to fuel convection currents, allowing warm air to rise. This rising air cools as it ascends, leading to condensation and cloud formation. Instability, which occurs when warm air at the surface is overlain by cooler air aloft, causes the warm air to continue rising rapidly. This process creates strong updrafts, which are essential for the development of thunderstorms.

In contrast, a stable atmosphere does not support the rising motion of air needed for thunderstorm development, while clear and calm conditions indicate a lack of significant weather activity. High pressure systems usually lead to stable conditions, further inhibiting the formation of thunderstorms. Hence, the presence of both increased humidity and instability is crucial for the development of thunderstorms, making this the correct option.