The highest of the stratiform clouds is cirrostratus. It occurs as thin layers at heights of over 5km, and is formed from ice crystals. During the day it can give the sky a milky white appearance. The cloud is not thick enough to obscure the sun or the moon, both of which can produce halos.
Cirrostratus is often the first indication of an approaching warm front. In this photograph Cirrostratus is the palest and highest cloud, but there is also a clearer, thin band of grayer altostratus below it, and lower still there some small Cumulus humilis.
Quite often we become aware of the presence of high cirrostratus cloud when a day which started out very clear and sunny with a bright blue sky, gradually begins to fade, losing its sharpness and perhaps giving a slightly hazy appearance to the sky.
Cirrostratus and Cumulus humilis at Aberffraw, Anglesey, Wales
July 2002
© Paul Swinhoe
Cirrostratus is probably the least photogenic cloud there is, but it can produce some spectacular optical effects when the sun shines through it. The cloud is made entirely from ice crystals and given the right conditions it can produce a 22o halo, when the sun, or the moon, shines through it. The straight bands of cloud are contrails.
This image has been substantially modified using Photoshop to ensure the halo is clearly visible. If you look closely, you may be able to see a hint of the colours of a rainbow, reddish near the sun through to more violet on the outside, but they are always very washed out. The sky inside the halo is usually a shade darker that that outside. Very often, the halo is very faint and difficult to see. The 22o halo is the most common of a range of optical phenomena.
Cirrostratus with 22o halo, Harriseahead, Staffordshire, 24th April 2018
© Lionel Burch