Cumulonimbus clouds range in size from a few kilometers to more than a dozen kilometers, and are more than 10 kilometers high. How are such large cumuliform clouds formed? First, let's look at how clouds are formed (see related articles "What are clouds made of and how are they formed​?) Clouds are created by an updraft, a flow of air that rises into the sky. One of the reasons for updrafts is that the air near the ground is warmed. When a mass of air heats up, its volume increases and it becomes lighter than the surrounding air, causing it to rise.

Because of the low atmospheric pressure above, the rising air mass expands further. Since air has the property of decreasing in temperature as its volume increases, the temperature of the air mass decreases as it rises. The air then reaches the maximum amount of water vapor that it can contain (saturated water vapor content), and a bubble of water or a block of ice is formed. These float in the air above and become clouds.

There are three conditions that facilitate the formation and development of cumulonimbus clouds: [1] unstable atmospheric conditions, [2] warm, moist air, and [3] energy to lift the air high.

［The "unstable atmospheric conditions" described in [1] refers to a state in which there is a layer of cold air above and a layer of warm air on the ground, and the temperature difference between the two is large enough to cause strong updrafts to occur easily. ［The warm, moist air in (2) is the "source" of clouds. ［The "lifting energy" in [3] refers to the heat called "condensation heat" that is generated when water vapor becomes water when clouds form. This heat causes the cloud to expand, creating a buoyancy force that lifts the cloud higher.

Thus, when there is strong cold air in the sky, when the air near the ground is warmed by strong sunlight, or when there is warm moist air, clouds grow vertically under their own power and form cumulonimbus clouds that reach more than 10 km above the ground.