Start by opening an image that's showing some sky area.
Create a new layer and press Q to enable quick mask. From filter menu select render>>difference clouds.
Press Ctrl+F five times to get this result.
Press Q to disable quick masking.
Fill the selection with blue color.
From select menu click modify>>contract.
Contract the selection by 2 pixel.
Fill it with white color.
Press Ctrl+L to open level settings window. Apply settings as shown.
Press Ctrl+T and scale down vertically.
Select eraser tool and remove the unwanted areas.
Change the layer mode to screen.
Select the image layer and press Ctrl+M to open curve window. Apply curve settings as shown.
