The Mammatus Clouds Optical Illusion isnt so much an optical illusion but its is a very rare site to see. These photos were taken by Joran Olsen of Hastings, Nebraska. He took them in Hartwell park in his home tow.
Mammatus Clouds are formed when air is over saturated with water droplets that begin to freeze or cool then sink. Around the clouds lies sharp gradients in temperature, moisture and wind momentum, that combined to strongly influence the clouds formations. They are very rare to see but defiantly a beautiful site.