You need two shapes-
to the first You apply fill with fractal plasma,
stretch the plasma-rectangle to a samll amount of hight, a big amount of with, then apply transparency 100% from far to 0 % near.

to the second shape you also make fractal fill, stretch also the fractal rectangle handels smaller in hight but not so small as in shape 1
the fill shold be bigger then in shape 1
also apply transparency linear from 100% near to 0%far

shape 1 -the far - schold be more blue and lighter in colour to simulate the colour-distance-falling into blue and fog
shape 2 should be a little bit warmer and apply more colour to it

give on over the other and under the bridge.