Which Roman god was the god of nocturnal thunder and lightning?

Summanus was the Roman god of nocturnal thunder and lightning. Roman religious tradition contrasted his nighttime thunderbolts with Jupiter’s, which were associated chiefly with daylight.

Summanus was a relatively obscure deity, and even the poet Ovid remarked that his precise nature was unclear. Pliny the Elder, drawing on Etruscan religious traditions, reported that the Romans distinguished two principal kinds of thunder: Jupiter’s diurnal thunder and Summanus’s nocturnal thunder, which was considered rarer.

A temple to Summanus was dedicated near the Circus Maximus around 278 BCE, during the war against Pyrrhus. Earlier, a statue of the god had reportedly stood on the roof of Jupiter’s Capitoline temple. After lightning struck it and its head disappeared into the Tiber, Romans interpreted the event as a sign that Summanus required his own sanctuary.

His festival fell on June 20. Worshippers offered round cakes called summanalia, made with flour, milk, and honey, along with dark-colored animal sacrifices. Later writers sometimes identified Summanus with Pluto, but that association does not replace his distinctive Roman role as the deity of nighttime thunder.