If the air is moist enough, it will reach its saturation point as it rises, and some of the water vapor will condense, forming a cloud. There are usually large spaces of blue sky between cumulus clouds. Explain why this is so. As the convection cell rises, the surrounding air is forced down to take the place of the warm, rising air. Clouds cannot form in the presence of subsiding air.
Most thunderstorms have flat tops because they have risen all the way to the tropopause.
Above the tropopause is the statosphere, which is an extremely stable atmospheric layer. Because the stratosphere is so stable, the thunderstorm cloud cannot penetrate it and instead spreads out horizontally.