A Letter from the Sky
In 1932, physicist Ukichiro Nakaya arrived at Hokkaido University. With little equipment for his intended nuclear physics research, he turned his attention to the region's most abundant resource: snow. Inspired by the American farmer Wilson Bentley's 1931 book of snowflake photomicrographs, Nakaya began an exhaustive effort to document and classify natural snow crystals. He took over 3,000 photographs, creating a classification system that described the flakes in relation to the weather conditions.
This documentation led to a fundamental question: what physical conditions create these varied shapes? To answer this, Nakaya knew he had to grow crystals in a controlled setting. He famously called snowflakes "letters sent from heaven," and he was determined to learn how to read them. Inside a specially constructed cold laboratory, which opened in 1935, he began his attempts to grow snow. After many frustrating trials, a breakthrough occurred on March 12, 1936. Nakaya successfully grew the world's first artificial snow crystal on the tip of a single rabbit fur hair.
The Nakaya Diagram
That first success unlocked a new field of atmospheric physics. By precisely controlling the temperature and water vapor supersaturation inside his apparatus, Nakaya could grow specific types of crystals on demand. His experiments produced the "Nakaya Diagram," a crystal morphology chart that is still a foundational reference in meteorology today.
The diagram maps crystal habits to environmental conditions with remarkable precision. It shows that thin, plate-like crystals form near -2°C (28°F). Slender columns and needles dominate around -6°C (21°F). The large stellar dendrites—the classic snowflake shape—grow most readily at a chilly -15°C (5°F). The diagram also demonstrates that higher humidity levels lead to more branched structures, while lower humidity produces simpler prisms. This work transformed snowflake observation from a curiosity into a tool; by examining the shape of a fallen snowflake, one could infer the temperature and humidity conditions of the clouds where it formed. The research legacy continues at the Institute of Low Temperature Science, founded at Hokkaido University in 1941 as a direct result of Nakaya's work.