Researchers have uncovered and digitized nearly two million 18^th and 19^th century weather observations from across Canada that offer new insights into how the country’s climate has changed over time.

The international project draws on handwritten records dating from 1768 to 1884. The data include temperature, precipitation, wind and detailed descriptions of such events as storms and floods.

“The paper is a data study, so in that sense we were looking for, documenting and transcribing meteorological observations from overseas archives, given Canada’s colonial history,” said Victoria Slonosky, an afiiliated researcher in the Department of Geography and lead author of a recent study in Scientific Data.

To build the dataset, researchers obtained fragile documents digitized by archives, transcribed the observations into a database and converted the measurements into modern units. Slonosky also helped establish Open Data Rescue, a not-for-profit organization that supports the search and transcription of Canadian weather records, along with related efforts such as McGill’s DRAW (Data Rescue: Archives and Weather) project.

Researchers drew on military and medical weather logs held in the United Kingdom’s Meteorological Office archives and records at the U.S. National Archives, including observations by Hudson’s Bay Company employees and missionaries.

One surprise was the level of detail: Observers often recorded conditions two or three times a day and noted far more than temperature.

“As well as thermometer observations, there are wind, cloud, rain, snow, atmospheric humidity and atmospheric pressure observations, along with descriptions of the weather and sky,” Slonosky said.

These accounts include descriptions of floods, fires, storms and aurora, allowing researchers to identify past extreme events.

Until now, much of Canada’s early weather record was poorly understood, especially pertaining to northern and western regions.

“There was a ‘fog of ignorance’ … for this area,” Slonosky said, noting that limited data has made it harder to understand how storms form and evolve over time.

The newly digitized records suggest the climate in this period, near the end of the Little Ice Age, was highly variable.

“We knew it was generally colder than today, but discovering such a wealth of information also helps us see that the weather was more variable than in the 20th century generally,” she said.

Researchers say the findings will help improve understanding of extreme weather events that affect people now and may also help put today’s climate headlines in context.

“We see the word ‘unprecedented’ a lot … so we’d like to find out if some of these things really are unprecedented, or if we just have short records and shorter memories,” Slonosky said.

Transcribing historical Canadian weather data by Victoria Slonosky et al. was published in Nature’s Scientific Data. It was funded by Environment and Climate Change Canada.