It refers to the optical phenomena produced by the sun light. The light is diffracted by small water droplets or ice crystals. This type of optics are mostly occurs in the middle cloud or the high cloud, and occasionally the low cloud. This kind of optic phenomena presents a ring shape with the light source at the center, and the radius of the halo is about five degrees from the viewing angle. It is called Solar Corona. If moon as a light source, it is called Lunar Corona. During the diffraction process, the red light has a longer wavelength, and the color deviation angle is large; the purple light has a shorter wavelength, and deviation angle is small. The smaller the droplet, the better diffraction. The diameter of the cloud droplets is about 10μm to 40μm.