Take now an already prepared brush, picturing the sky, and the white color.
The brush may be downloaded on this address:
http://www.deviantart.com/deviation/34277964/?
&q=boost%3Apopular+in%3Aresources%2Fapplications%2Fpsbrushes+cloud

and insert more cloudiness on all the layers:

In the sky photo 
effects in adobe Photoshop cs

If that is necessary, we may change the layers' colors, using this command: Image – Adjustments – Hue/Saturation (ctrl+u).

In the sky photo 
effects in adobe Photoshop cs

Take now the Gradient Tool and paint the clouds on their lateral sides on a new layer. After that we should change the layer's Blending Mode on Soft Light and minimize the Opacity till 80%.

In the sky photo 
effects in adobe Photoshop cs

Give a certain nuance to the tree also:

In the sky photo 
effects in adobe Photoshop cs

Combine all the layers: Layer – Merge Visible (shift+ctrl+e) and insert several scratches on the entire picture's surface. Select for this operation the Dodge Tool of 1 px.

In the sky photo 
effects in adobe Photoshop cs