Rain requirements vary tremendously. 

The camera moves planned on this commercial required depth as well as coverage of a wide area.

Cherry pickers were used to mount large rotating ‘nozzles’ high above the action - each one fed from high pressure pumps with their own water tankers.  This filled the background, while lower fixed nozzles (including one mounted on the camera crane) provided coverage for specific foreground areas.

The result...a traffic jam caught in a torrential downpour.