Solar corona (upper left), with shades of green and purple colored cloud (centre). The term corona is here being used to refer to an optical phenomenon observed in the Earth's atmosphere. This sort of corona is caused by the diffraction of sunlight by water droplets or ice crystals in the air. Coronas are produced by high thin clouds, and are much smaller than other optical phenomena such as halos. This corona is also showing rainbow-like fringing effects, causing the colours over the clouds at centre. Photographed in Brittany, France.