In ancient Roman religion, Summanus (Latin: Summānus) was the god of nocturnal thunder, as counterposed to Jupiter, the god of diurnal (daylight) thunder. His precise nature was unclear. Every June 20, the day before the summer solstice, round cakes called summanalia, made of flour, milk and honey and shaped as wheels, were offered to him as a token of propitiation: the wheel might be a solar symbol. Summanus also received a sacrifice of two black oxen or wethers. Dark victims were typically offered to chthonic deities.
Summanus’ domain and element is air, his symbols are the solar wheel, lightning bolt. Varro (De Ling. Lat. v. 74) describes the god as of Sabine origin; but the ancients themselves on this as on many other points connected with their earliest religion, were in great uncertainty both in regard to the nature and the origin of Summanus ; and some connecting the name with sub and manes regarded him as a deity of the lower world, an opinion which is totally at variance with the attributes given him by most writers, and there is ample reason for regarding him as the Jupiter of night.
According to Martianus Capella, Summanus is another name for Pluto as the "highest" (summus) of the Manes. This identification is taken up by later writers such as Camões ("If in Summanus' gloomy realm / Severest punishment you now endure …") and Milton, in a simile to describe Satan visiting Rome: "Just so Summanus, wrapped in a smoking whirlwind of blue flame, falls upon people and cities".