Managing levels in two different parts of the image?

[Blighty]

As always, there is more than one method.

1. Use Curves
See (1) below. Play around with curves to suit.

2. Use Levels
See (2) below. Play around with the sliders.

3. Make a selection. Only the area within the selection is altered.
3a. Make a selection on the sky. See (3) below. Use curves or levels on the sky. 
3b. Invert the selection. The lower area is now selected. Again, use curves or levels.

4. Use luminosity mask
Not illustrated. Requires a plug-in for easy use