Which of the following is NOT an ingredient for thunderstorm formation?

Thunderstorms require specific atmospheric conditions for their formation, and one of the critical elements is unstable air. Unstable air allows for rising air currents, which are essential for the development of thunderstorms. High relative humidity contributes to the moisture in the atmosphere, which is necessary for cloud and precipitation formation. A strong lifting agent, such as a front or thermal, can initiate the upward movement of this warm, moist air, leading to thunderstorm development.

In contrast, stable air inhibits vertical motion and tends to suppress the formation of thunderstorms. When air is stable, vertical air currents are minimized, making it unlikely for convective processes to occur that would lead to cloud and storm development. Therefore, stable air is not an ingredient necessary for thunderstorm formation, making it the correct choice for this question.