Summanus was the Roman god of nocturnal thunder, contrasted with Jupiter’s daytime thunder and lightning.
Roman religious tradition divided thunderbolts by time of day. Jupiter controlled diurnal thunder, while Summanus governed storms and lightning at night. Ancient writers did not agree on Summanus’s precise identity or origin: Pliny associated him with Etruscan thunder lore, while Varro connected him with gods honored by the Sabine king Titus Tatius.
A temple to Summanus was dedicated around 278 BC, during the Pyrrhic War. It stood near the western end of the Circus Maximus, probably on the Aventine slope. The cult may have been established after a statue of the god on the roof of Jupiter Capitolinus’s temple was struck by lightning.
Summanus received offerings on June 20, including wheel-shaped cakes called summanalia made from flour, milk, and honey. Dark animals were also sacrificed, reflecting associations with nighttime and chthonic powers. Some later writers identified Summanus with Pluto, but that is a secondary interpretation, not the standard answer to the thunder question. Ovid even remarked that the god’s nature was mysterious.