A librarian at this 10th century monastery leads a visitor beneath the vaulted ceilings of the archive past the skulls of two former abbots. He pushes aside medieval ledgers of indulgences and absolutions, pulls out one of 13 bound diaries inscribed from 1671 to 1704 and starts to read about the weather.

“January 11 was so frightfully cold that all of the communion wine froze,” says an entry from 1684 by Brother Josef Dietrich, governor and “weatherman” of the once-powerful Einsiedeln Monastery. “Since I’ve been an ordained priest, the sacrament has never frozen in the chalice.” “But on Janauary 13 it got even worse and one could say it has never been so cold in human memory,” he adds.

Diaries of day-to-day weather details from the age before 19th-century standardised thermometers are proving of great value to scientists who study today’s climate. Historical accounts were once largely ignored, as they were thought to be fraught with inaccuracy. But the booming interest in climate change has transformed the study of ancient weather records,” says Christian Pfister, a climate historian.