Its hard to know the best approach without seeing the actual image but my inclination would be something like this
1. Create a new empty layer on top of the stack for the sky
2. Create a selection of the sky area, possibly a just a long rectangle using the rectangular marquee
3. Add a gradient in the layer within the selection, have the gradient vertical and light at the bottom
4. Change the Blend Mode of the sky layer to Multiply
Awesome David. Thank you for your help. Very nice and very simple.