You can produce very accurate renderings of daylight scenarios for the current camera using the mental ray physical sun and sky shaders. To use these shaders, simply click the Create button in the Environment section of the Render Settings window and Maya automatically creates a network of required nodes.
The shaders mia_physicalsky and mia_physicalsun should be used together along with a directional light and the mia_exposure_simple shader. Maya automatically connects all the necessary attributes from mia_physicalsky, mia physicalsun, mia_exposure_simple and directionalLight nodes and connects them to all existing renderable cameras.
See Adding sun and sky to your scene for more information.
