Cloud types:

Ci = cirrus (ice crystals with a wispy, thin appearance)

Cs = cirrostratus (ice crystals, a more continuous cloud deck)

Ac = altocumulus (ice and water, lower and darker than the cirrus clouds)

Cb = cumulonimbus (precipitating cloud with vertical development, so warmer with water droplets near the surface, that transitions to ice crystals near the much colder top of the cloud.)