There is various ways to go about adding weather to your shots, but the trick is to make it look great.  Even by adding some user defined clouds or fog can really add to the shot.  To do this, pause the game hit ALT, select world then weather.  On the next screen select the radio button for user defined weather and hit customize weather.  You will then be presented with the following screen….



On this screen select advanced weather and then you can play around by adding layers of clouds and different types of clouds.  You can also adjust the heights of the layers to create some cool effects.  The screen looks like this….





Try to use the user defined weather as stated above, follow the instructions but then choose the visibility tab along the top and play around with the settings, this should help you produce the desired effect