Cumulonimbus clouds indicate unstable atmospheric conditions. What stops the upward growth and causes the top of a cumulonimbus cloud to flatten out into an anvil?

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The Answer of Cumulonimbus clouds indicate unstable atmospheric conditions. What stops the upward growth and causes the top of a cumulonimbus cloud to flatten out into an anvil?

Cumulonimbus Clouds Indicate Unstable Atmospheric Conditions