This simple tutorial demonstrates how to create a cloud effect using a combination of the Volume Shader with a Maya Cloud 3d texture. It is possible to achieve a wide range of cloudy skies using this setup. 

The scene can be downloaded here.

 

The step size has a fixed size, therefore, the larger the container, the more steps will be taken and the longer it will take to render. Also, beware that if the step size is too small your render times will increase.

Step Size in Arnold Volume Attributes of sphere