In the 19th and early 20th centuries, millions of weather observations were carefully made in the logbooks of ships sailing through largely uncharted waters. Written in pen and ink, the logs recorded barometric pressure, air temperature, ice conditions and other variables. Today, volunteers from a project called Old Weather are transcribing these observations, which are fed into a huge dataset at the National Oceanic and Atmospheric Administration. This "weather time machine," as NOAA puts it, can estimate what the weather was for every day back to 1836, improving our understanding of extreme weather events and the impacts of climate change.

Why we need more data

We know a lot about weather conditions today, thanks to technologies such as satellites that deliver high-resolution imagery. Before satellites, forecasters relied on weather observations gathered with much less sophisticated instruments located around the globe. Here, one-degree squares show observations collected in January 1979.