This is the most colourful of the haloes and may be seen several times a year. It looks a bit like an upside down rainbow high above the sun.
It is caused by sunlight refracted through the edges of plate shaped crystals. In effect each crystal is acting as a 90 degree prism which gives the arc such pure colours.
Apart from having the right shape the base of the crystals must also be horizontal, which is why this arc is not always seen