* Cirrus clouds form in the troposphere. This is the lowest layer of Earth's atmosphere, where weather occurs.
* The exosphere is the outermost layer of Earth's atmosphere. It's incredibly thin and characterized by extremely low atmospheric pressure and density. There is simply not enough air in the exosphere to form clouds.
In summary: Cirrus clouds are found in the troposphere, while the exosphere is far too thin and devoid of significant amounts of gas to support cloud formation.
