Circumhorizontal arc over ON, CA
© Lori Dunn
Chatham, Ontario
This particular arc only lasted a few minutes before disappearing completely.

Southwestern Ontario was graced with a rare treat Wednesday afternoon when a circumhorizontal arc, or 'fire rainbow', lit up the sky.

Social media erupted with images of the rainbow, with many people asking for clarification on what they were seeing. The phenomenon - dubbed a "fire rainbow" by a Washington journalist in 2006 - is formally known as a circumhorizontal arc and while rare in Canada, it isn't unheard of.

The conditions have to be just right for the arcs to appear. The effect can occur when the sun travels through tiny ice crystals in the atmosphere, but the sun has to be high in the sky -- at least 58 degrees -- which is why they're most likely to occur in the afternoon.

Circumhorizontal arc over ON, CA
© Dave M.
London, Ontario
Circumhorizontal arcs most commonly form in cirrus clouds but only when they're aligned horizontally.

You're more likely to see this type of rainbow during the summer in North America.

This particular arc only lasted a few minutes before disappearing completely.