The iridescent colors that appear in pileus are diffracted sunlight.
How a rainbow cloud is created
The formation of a pileus cloud capping a common cumulus cloud indicates that the lower cloud is expanding upward.
These unique clouds contain warm air and water droplets or ice crystals. When the air boils it convects upwards.
Layers of air above the cloud are also pushed upwards as the cumulus rises.
Rising air expands and adiabatically cools. This means it can cool in a curve similar to the arch of a rainbow or a circle.
Pileus clouds don’t appear for long periods of time. The main cloud beneath them is rising through convection to absorb them.
The appearance of this cloud system means there is a high probability that a storm might develop.
