Old Chinese chronicles are full of weather entries: "no rain in the third month," "the river overflowed its banks." But these are words, not numbers. Until now, turning them into precise precipitation data was nearly impossible.
The new approach works like a translator from the language of chronicles to the language of climatology. It was trained on millions of records: it learns that an event like "drought" typically corresponds to a certain level of precipitation, and it reconstructs a rainfall map across southeastern China. The result is a continuous weather record spanning 543 years — from 1368 to 1911.
The map also reveals seasonal shifts linked to El Niño — the warm Pacific current that alters weather worldwide. Short modern records don't capture such slow rhythms. Here they emerge like a pattern on old fabric.
These data will benefit not only climatologists. Historians can examine how droughts and floods affected harvests, uprisings, and the fall of dynasties. Old words about weather have finally spoken in the language of numbers.
In Chinese chronicles, weather records were kept with day-level precision: for example, "rain fell for three consecutive days in the fourth month." This allows even short downpours to be reconstructed.