Summanus was the Roman god of nocturnal thunder, often contrasted with Jupiter, who governed thunder and lightning by day. His name and precise identity were already somewhat obscure to ancient Roman writers, including Ovid.
Roman religious thought distinguished between different kinds of thunder. Jupiter represented the familiar public and celestial power of daytime storms, while Summanus embodied the rarer, more ominous thunder heard at night. Pliny the Elder preserved this distinction, attributing nocturnal thunder to Summanus and diurnal thunder to Jupiter.
A temple dedicated to Summanus stood near the Circus Maximus. Ancient tradition dates its dedication to about 278 BCE, during the Pyrrhic War. His festival fell on June 20, when Romans offered round cakes called summanalia, made with flour, milk, and honey. Dark animals were also associated with his sacrifices.
Summanus should not simply be treated as another name for Jupiter. He represented a distinct nighttime aspect of divine thunder, although later writers sometimes connected him with Pluto or the underworld deities. That uncertainty is part of what makes his surviving cult unusual.