You know it’s cold when…

… There are sundogs outside!

So what is a sundog? It’s this:

That beautiful circular rainbow around the sun is known as sundog and is caused by ice particles floating in the air, reflecting light (same process as a rainbow, just much colder). The ice crystals act like a prism, bending the light that passes through them and giving the appearance of a rainbow.

These don’t just appear because of the cold, but are usually visible when the sun is low on the horizon on very cold days.