Clouds are not only described by their shape (cumuliform or stratiform) but also by the altitude the cloud starts at. When it comes to cloud heights, the atmosphere is broken up into three layers – low, middle and high.
Clouds with a base between sea level and roughly 6,500 feet fall into the low level category.
Clouds with a base between roughly 6,500 feet and 20,000 feet fall into the middle level category.
The prefix ‘alto‘ is used to describe middle level clouds.
If the cloud base is higher than about 20,000 feet the cloud falls into the high level category.
The prefix ‘cirro‘ is used to descripe high level clouds.