Photo of snowy Sussex woodland trail Photo of snowy Sussex woodland trail

22 Degree Halo and Contrail Shadow

My thanks to Alan Gibson, urbane forager and regular contributor to this blog, for another great image.

In this picture we are looking at two high cloud phenomena, neither of which is super-rare, but I don’t recall ever having seen them together before.

The first is the 22 Degree Halo. The moon or in this case the sun’s light is refracted by ice crystals in upper clouds, typically cirrus or cirrostratus. The halo can presage a deterioration in weather, especially if followed by thicker clouds like altostratus. There is a traditional Native American saying that the sun goes into its tepee to avoid the rain.

The second is a ‘contrail shadow‘, where the long thin cloud formed by an aircraft’s vapour trail casts a shadow on the clouds below it. Look closely and you’ll spot there are actually two shadows, a fainter one just above the darker one, caused by two layers of cloud. A sudden increase in contrails indicates increasing moisture in the upper levels.

On their own the halo and contrail don’t indicate bad weather, but they do indicate increased moisture high up and this is one sign of an advancing warm front and the associated bad weather. They are both signs to be vigilant for the next clouds and any wind shifts.

If the wind backs and clouds thicken and/or lower the probability of imminent bad weather shoots up.

You might also enjoy:

Alan’s excellent pictures of a related, but different phenomenon, sun dogs.

My page on Weather Lore and the truth or otherwise in these sayings.

A Brief Introduction to Cloud Clues.