A Divided Sky

It was late afternoon and we were driving along the open space of Candelaria bypass road when we noticed something unusual about the sky. The left side of the sky was dark, while the right side is bright and they are clearly divided in the middle. We decided to park beside the road to take a picture.

We do not have our DSLR, but we have our gadgets. I gave my iPhone 5 to my wife beside me, and she took a few pictures. I scanned the sky and noticed that the sun was setting on our right. I have no idea what we are seeing. It was as if something big was blocking the rays of the sun and casting a shadow on the left side of the sky. It could be the horizon covering part of the sun, but why do we not see this every day? We moved on since it is about to get dark.

Here are a few more re-sized, but unedited pictures.  I tried to search the web for something similar, but I cannot find any. 

/royc