The predicted hight of the base of the cloud above the ground level. 

VFR Minimums require a cloud base of at least 1500' above ground level. this means that you should be avoiding any areas that are not green or grey. 

You can also calculate the cloud base using the following formula:
1. Calculate the ‘spread,’ where you’re taking the current temperature and subtracting the dew point.
2. Divide the spread by 2.5, then multiply by 1000.

These cloud types can be found from ground level to about 6,500ft above it. They include Stratus, Stratocumulus, and Nimbostratus clouds.

Stratocumulus: low level layered cloud, series of rounded rolls, generally white. Precipitation: drizzle.
Stratocumulus: low level layered cloud, series of rounded rolls, generally white. Precipitation: drizzle.
Cumulus: low level, individual cells, vertical rolls or towers, flat base. Precipitation: showers or snow.

These are found between 6,500ft to 20,000ft above the land surface. Altostratus and Altocumulus are the cloud types found here.

Altocumulus: middle level layered cloud, rippled elements, generally white with some shading. Precipitation: May produce light showers.

Altostratus: middle level grey sheet, thinner layer allows sun to appear as through ground glass. Precipitation: rain

Nimbostratus: thicker, darker and lower based sheet.
Precipitation: heavier intensity rain

A cloud that extends from the lower levels to a high upper level has significant vertical developemt. They form by thermal convection or frontal lifting, sustained by the powerful convectional current that holds and pushes the moisture in the clouds further upward. An example of a vertical cloud is the Cumulonimbus cloud.