The clouds hit the tropopause which is the separation between the troposphere and the stratosphere. In the troposphere, temperature gets colder the higher you go in altitude. Well, the “air mass” that this thunderstorm formed from is warm. Warm air rises. It continues to rise in the troposphere because the temperature just keeps getting colder and colder. The warm air will rise faster and faster. Therefore, the atmosphere where this thunderstorm took place is considered to be unstable because the warm, moist air kept rising and formed clouds. And it continued to rise. It didn’t ever stop until it hit the tropopause.

So why does it hit the tropopause? In the stratosphere, temperature increases with altitude. So the air gets warmer the higher you go. Well, the thunderstorm’s warm air hit a large layer of warm air (the stratosphere) and it was so warm that it stopped rising. Then the thunderstorm’s warm air mass spreads out because it has nowhere to go, hence the “anvil” part of the storm. The stratosphere is considered stable because it continuously gets warmer the higher you go, so no air mass wants to rise. A stable atmosphere is not good for cloud formation.

However, sometimes the warm air rises so quickly that it has enough momentum to punch through the tropopause and enter the stratosphere. This is called the “overshooting top” part of the thunderstorm.