How do scientists reconstruct the climate of the past? They often turn to ice cores or growth rings from trees or deep-sea corals. But a new study gleans a wealth of weather intel from a largely untapped source: old documents.

Researchers from Spain scoured manuscripts from 9th- and 10th-century Baghdad, in modern-day Iraq, for references to the weather. Baghdad, where the Tigris and Euphrates Rivers meet, was at that time the new and bustling capital of the vast Islamic Empire, which stretched from India to the Atlantic Ocean. Much was written about the city and why it was chosen as the capital, including its population size, agricultural potential and climate.

In the 10 analyzed texts, most of which give exhaustive political histories of the region, the researchers found 55 meteorological citations, many of which were referring to the same event. The study points out that although the social and religious content of the documents is probably biased, the historians weren’t likely to fabricate an off-hand mention of a drought, hail storm or solar eclipse.

The researchers were shocked by the number of references to cold periods in this notoriously hot and dry region. They identified 14 chilly periods in all: five in winter, two in spring, one in summer and two that denoted cold weather for a whole year. Some of the descriptions specified snowfalls, ice and frozen rivers.

For instance, an entry from December 23, 908, noted when “four fingers of snow accumulated on the roofs,” and another, on November 25, 1007, that the snow reached somewhere between 30 and 50 inches. One particularly odd event was in July 920, when it was too cold for people to sleep on their roofs, as they did on most summer nights. This temperature drop could have been caused by a volcanic eruption the previous year, the researchers speculate.

In any case, it seems safe to say that the weather of that Islamic Golden Age was much more variable than it is today. The only time that snow has hit Baghdad in modern memory was on January 11, 2008, melting as soon as it hit the ground.

Images from Domínguez- Castro et al., ”How useful could Arabic documentary sources be for reconstructing past climate?” appearing in Weather, published by Wiley.