I think that it is a sign from nature... to whom?
It could be address to anyone who takes the time to look up and appreciate every little bit of scenery that our wonderful planet is offering us, a sort of thankful greeting, if you will...
On the other hand, those who forgot how to look might need a wake up call now and then... What about those who don't know how to take the time, or who just cannot manage to see further than the tip of their nose, in their cases, it might even be a little bit of mocking from the sky.
So let us smile back to the sky shall we... ^_^
Love always
mudra