The highest altitude clouds are composed of tiny ice crystals. These clouds are diffuse enough to be partly transparent and so light that they are easily blown into wispy shapes by air turbulence.
Between 2,000 and 6,000 metres (6560 and 19685 feet), clouds are mainly composed of water droplets, which absorb more light so they appear more opaque, and the attraction between droplets tends to keep clouds grouped together into rounder shapes.
Advancing weather fronts can create bands of cloud, like waves tolling onto a beach. Below 2,000 metres (6,560 feet), the droplets clump together into larger drops that absorb enough light to appear dark grey and the clouds are affected by convention currents rising off the ground, flattening the underside.