The CII is a satellite based product combining different satellite based instability indices (such as LI, KI and TT), atmospheric moisture (PW) and height above sea level into a probability map for the occurrence of convection. IN this way the three needed components for convection are all used: instability, moisture and orgraphic lift.
Just as one would use the early morning upper air soundings, one should have a look at this map early in the day to see the probability for the developement of convection later in the day. The probabilities are indicated in the different colours.
Satellite based instability indices need clear skies to determine a vertical
profile of temperature and moisture through the atmosphere, thus they cannot
be calculated in clouy areas. CLoudy areas are indicated in grey. 
Three-hourly time average maps are used to get as many clear sky gridboxes as
possible and also to get an average of the instability indices to avoid
extreme values which don't last long.