[Ben Ficklin]

"why is there always beautiful blue sky"

Look closer. Look at the right hand side of the photo.

Closer to the horizon, the blue is darker/dirtier and as the elevation rises, the blue gets cleaner and a deeper shade of blue.

That's smog.

[Parmy]

There has always been smog. That is why the Smokey Mountain are called smokey. And, that was before there was industrialization.