Some years ago we were visiting Point Reyes light house near San Francisco and we noticed there was a brown and grey cloud floating over the lighthouse in an otherwise clear blue sky.

Curious about the haze we entered the visitor center and asked them what it was. They said "What cloud?" and insisted the sky was clear. Since then we have come across a number of other people that have insisted their skies were clear when definitively they were not.

We were in Hanoi for three weeks and this was about as "clear" as it got. And while we never asked anyone about it specifically, no one seemed to think anything was unusual about this omni-white sky...