Cirostratus is a subtle, understated cloud that can easily go unnoticed. A delicate layer of ice crystals, often spread over vast areas of the sky, the cloud can appear as no more than a light, milky whitening of the blue. It can sometimes be hard to distinguish Cirrostratus from the lower layer cloud, known as Altostratus. But not when its ice crystals are producing an optical effect like this 22-degree halo. Since it is composed of droplets, rather than ice crystals, the lower Altostratus never produces halo phenomena.