A fire rainbow, via deviantART user: ~daslasher1 For more pictures from ~daslasher1, click here.
I’m sure we all have seen rainbows after the rain and rosy-hued clouds during the sunset. The rainbows and the sunsets can make beautiful sceneries, but if the two phenomena could happen at once, wouldn’t it be twice as beautiful?
A 22° halo, via wikimedia commons user: Andrzej Barabasz
Indeed, such phenomenon does occur. It is better known to the public as the “fire rainbow,” than its actual name, the “circumhorizontal (or circumhorizon) arc.” Although it is called a fire rainbow, it is neither a rainbow nor is related to fire. It is really a halo that is made by ice crystals in the cloud refracting the sunlight. The circumhorizontal arcs are unlike common halos (22° halos). Whereas 22° halos are seen fairly close to the sun, circumhorizontal arcs form much further, closer and parallel to the horizon. In addition, to observe, the circumhorizontal arc requires specific angles of the sun and a specific kind of cloud.
cirrus cloud, via wikimedia commons user: Saperaud
Firstly, there need to be cirrus clouds. Cirrus clouds form more than 6000m high, and contain ice crystals in which sunlight is refracted. The arc is brighter if the cloud is thicker, since more crystals will refract more sunlight. Secondly, the sun must be higher than 58°. Such height of the sun is possible only between latitudes 55°N and 55°S. It is impossible to see the circumhorizon arc outside of these latitudes, because the sun will never rise high enough. Fortunately, the latitude of Vancouver is 49°N. This link (click here) shows a graph of months in a year when circumhorizontal arcs can be formed. In Seattle, the arcs may be observed from mid-April through mid-August. Since Vancouver is just north of Seattle, there won’t be much difference.
So, during the summer when the sun is at its highest and if there are some cirrus clouds in the sky, be sure to look around! You may be able to witness a beautiful fire rainbow as these seen in New Brunswick, Seattle (and another), and Toronto.
