In order to make our cloud brush, we need to isolate the cloud image from the sky background. We do this by using the Select>Color Range command. Once you have selected the color range command, place the eye dropper within the blue region of the sky. Fuzziness should be set to 200 and the you should select the invert option as shown below.
Once you hit OK, the cloud will be selected. You will see the marching ant pattern around the cloud as shown in the picture below. Now we need to copy the cloud onto its own layer.