High Altitude clouds fall into 2 categories,
- Those with a low base and vertical development are Cumulonimbus and Towering Cumulus. At the base these are water clouds, so the ceilometer only “sees” a few hundred feet into the cloud.
- Those with a high base, including Cirrus, Cirrostratus and Cirrocumulus and Altostratus
The Cirrus cloud family are composed of Ice Crystals, and are very often “optically thin”and they have low backscatter coefficients, so are difficult to detect with ceilometers, because the laser pulse energy is limited to eye safe levels.
Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again
Altostratus cloud with a water phase may have a strong backscatter signal and can be picked up as in the case below
Ref 1 : Wikipedia : Alto Stratus entry.