‘This is a circumhorizon arc,’ said Bobbie Turner (Member 34,187). ‘It is only visible when the sunlight is refracted by the ice crystals in the Cirrus cloud at just the right angle.’ She is absolutely right. The optical effect, which Bobbie spotted over Smithfield Township, Pennsylvania, US, can appear only when the cloud consists of ice crystals in the shape of clear, hexagonal plates that are oriented horizontally, like autumn leaves. The Sun must also be very high in the sky, at an angle of more than 58 degrees above the horizon. It only climbs that high at a latitude like Pennsylvania’s during the summer months (April to August), which is why the circumhorizon arc is sometimes called the summertime halo. ‘We pulled the car over to look at this spectacular sight,’ said Bobbie. ‘Soon, other drivers did the same behind us.’