Cumulonimbus clouds are the tallest clouds in the sky, reaching well into the troposphere. Although the bottom of a cumulonimbus cloud contains water, the top of the cloud reaches so high that it contains ice crystals.
Cumulonimbus clouds are the clouds that produce rain on Earth. They form from cumulus clouds, and if the weather is right, they grow into large, heavy clouds capable of producing a large thunderstorm.
The troposphere is the lowest layer of the atmosphere, beginning on the ground and reaching to about 7 miles upwards. The air in the troposphere is warmer and thicker toward the ground and colder and thinner higher up. The border between the troposphere and the next layer, the stratosphere, is called the tropopause.