Do you want to know how it's done? Learn how to use Photoshop the easy way!
If you want to make some nice clouds with a blue sky, or if you want to create a volcanic looking clouded sky, Photoshop makes this task easy.
Create a new document. Select a shade of blue for the foreground color tile, and white for the background color tile.
From the file menu select Filter/Render/Clouds. And quite simply that is all there is to it. Use this for any background layers where you need some sky.
Add a little grass and you have the beginning of a fine picture of a cool spring day.
Try playing around with different colors and see what happens. If you use orange and black, you get something with a bit of an evil flavor.