The thick, puffy clouds that occur at middle altitudes are called cumulus clouds.

Here's a little more about them:

* Appearance: They have a cotton-like appearance, often described as "puffy" or "fluffy."

* Altitude: They typically form between 6,500 and 20,000 feet (2,000 and 6,100 meters).

* Formation: They form when warm, moist air rises and cools, leading to condensation and cloud formation.

Let me know if you have any other questions!