Clouds are classified as low, middle, or high, based on the altitude of the cloud base. Additionally, a cloud may be classified as one with vertical development. These are the "four families" of clouds.
Cloud names are also based on a cloud shape and behavior.
Stratus describes clouds that have a layered appearance. Stratus clouds form in more stable air. For example, if moist, stable air were flowing upslope, stratus clouds would form. Cumulus clouds are billow and are more associated with less stable air.
If a cloud produces rain, "nimbo" is used to indicate this. Nimbo means rain bearing.
Low
Low clouds are clouds with bases lower than 6,500 feet above the ground. Typically, low clouds are stratus, nimbostratus, or stratocumulus.
Middle (alto)
Altostratus or altocumulus clouds exist in the middle altitude ranges, having bases between 6,500 and 20,000 AGL.
High (cirro)
Clouds with bases above 20,000 feet AGL are typically cirrus, cirrostratus, or cirrocumulus. Cirrus clouds are wispy and made up of ice crystals. If they have a layered appearance, they are cirrostratus, while cirrus with a billowy appearance are cirrocumulus.
Clouds with Extensive Vertical Development
When a cumulus cloud continues to develop upward, it is referred to as a towering cumulus. Once that towering cumulus matures into a thunderstorm and produces rain, it is referred to as cumulonimbus. Violent turbulence often exists inside the hazardous cumulonimbus cloud. Towering cumulus clouds also indicate the presence of this convective turbulence. Clouds with considerable vertical development and associated turbulence can be expected whenever moist, unstable air is forced upward by some lifting force. Other Clouds and Descriptors
Castellanus - clouds having a common base with separate vertical development; castle-like
Lenticularus - clouds with an almond or lens shape, formed over mountains in strong winds. These clouds may appear stationary, but often contain winds of 50 knots or more. Standing lenticular clouds sometimes mark the crests of mountain waves. Mountain wave turbulence can be quite severe, and should be anticipated by pilots when winds of 40 knots or more blow across a mountain or ridge and the air is stable.
Fracto - describes ragged or broken clouds