What two conditions are necessary for cumulonimbus cloud formation?

Cumulonimbus clouds are formed through specific atmospheric conditions, primarily requiring a lifting action and unstable, moist air. The lifting action, which can be generated by various mechanisms such as convection, orographic lift, or frontal lifting, causes air to rise. As this air ascends, it expands and cools due to the decrease in atmospheric pressure at higher altitudes.

In conjunction with this lifting, the air must be unstable and moist. Unstable air is characterized by a lapse rate where the temperature decreases rapidly with altitude, allowing for the rising air to continue to ascend as it remains warmer (and thus less dense) than its surroundings. Moisture is critical because it contributes to the formation of cloud droplets as the air cools and condenses, leading to the growth and development of the cumulonimbus cloud. These clouds are often associated with severe weather phenomena, such as thunderstorms, due to the significant energy release as the water vapor condenses.

The other options do not accurately describe the conditions necessary for cumulonimbus formation. For instance, stable air typically inhibits vertical movement and cloud formation, while dry, warm air lacks the moisture content needed for significant cloud development.