A god Hadad who was a prominent deity in ancient Syria is identical with Adad, and in view of this it is plausible to assume - for which there is also other evidence - that the name Adad represents an importation into Assyria from Aramaic districts.
The process of assimilation did not proceed so far in Babylonia and Assyria, but Shamash and Adad became in combination the gods of oracles and of divination in general.
Andrae, Der Anu Adad Tempel (1909).
Evidence seems to favour the view that Ramman was the name current in Babylonia, whereas Adad was more common in Assyria.
The older incantations, associated with Ea, were re-edited so as to give to Marduk the supreme power over demons, witches and sorcerers; the hymns and lamentations composed for the cult of Bel, Shamash and of Adad were transformed into paeans and appeals to Marduk, while the ancient myths arising in the various religious and political centres underwent a similar process of adaptation to changed conditions, and as a consequence their original meaning was obscured by the endeavour to assign all mighty deeds and acts, originally symbolical of the change of seasons or of occurrences in nature, to the patron deity of Babylon - the supreme head of the entire Babylonian pantheon.
Whether the will of the gods is determined through the inspection of the liver of the sacrificial animal, through observing the action of oil bubbles in a basin of water or through the observation of the movements of the heavenly bodies, it is Shamash and Adad who, in the ritual connected with divination, are invariably invoked.
In this process of accommodating ancient prerogatives to new conditions, it was inevitable that attributes belonging specifically to the one or the other of these gods should have been transferred to Marduk, who thus from being, originally, a solar deity becomes an eclectic power, taking on the traits of Bel, Ea, Shamash, Nergal, Adad and even Sin (the moon-god)- a kind of composite residuum of all the chief gods.