What key factor influences the occurrence of thunderstorms?

The occurrence of thunderstorms is primarily influenced by instability in the atmosphere. Instability refers to a condition where warm air rises and cools, leading to the development of cumulonimbus clouds, which are essential for thunderstorms. When the atmosphere is unstable, the vertical motion of air is enhanced; warm, moist air can rise quickly, leading to the development of strong updrafts and eventually thunderstorms.

In contrast, stability in the atmosphere tends to suppress vertical motion, making it less likely for thunderstorms to develop. While humidity levels do play a role—since moist air is necessary for growth and development—it's the instability that directly fosters the rising currents of air that lead to thunderstorms. Wind speed may influence the structure and movement of a storm once it forms, but it is not a primary factor in initiating the thunderstorm itself.

Thus, instability is the critical factor in the development of thunderstorms, as it allows the conditions necessary for rapid vertical development and convective activity.