Altocumulus is invariably composed of water droplets. This is evident from the fairly low transparency of the elements and the fact that the elements show sharp outlines when separate. When the temperature is very low, ice crystals may form. If the droplets then evaporate, the cloud becomes entirely an ice cloud and its elements cease to present sharp outlines. The formation of ice crystals may take place in all species of Altocumulus, and it occurs most frequently in Altocumulus castellanus and Altocumulus floccus.