Coming back from work a while ago I looked up and saw this incredible sight. I had seen a sun halo before, but had never tried to take a photo of it, sometimes because I thought I could see it only through my sunglasses and other times because I had nothing to take a photo with. This time I tried catching the halo with my phone and it worked (see here) and then rushed home to take this mediocre photo with my camera.
Sun halos are caused by ice crystals in clouds, typically cirrus or cirrostratus clouds. This means that when you see a halo, it most probably is going to rain in the next 24 hours, as the clouds that carry ice crystals also forerun a frontal system.