While standing on a foggy railroad platform in Pittsburgh in late 1902, Willis Carrier had a revelation that would lead to the invention of the modern air conditioner. Carrier’s device evolved from candy-factory chiller to the heat- and humidity-busting personal cooling units now found in more than 80 percent of United States homes.

In July 1902, the 25-year-old engineer had finished designing the first modern air-conditioning system for a Brooklyn, NY, printing press that wanted to prevent its paper from warping. Unlike previous cooling systems, Carrier’s device regulated humidity in addition to temperature. But Carrier wasn’t satisfied with this first system; he felt that it needed more exact controls. On that cold, foggy platform a few months later, Carrier recognized that the lower the temperature, the less water the air could hold. He reasoned that he could raise or lower a room’s humidity by using a device that passed air through a hot- or cold-water sprayer.

This idea grew into a series of formulas for regulating air temperature and humidity-the basis of important calculations that still serve the air-conditioning industry. Carrier’s installations soon be-came a hit with candy factories, celluloid-film makers, and breweries. However, during World War I, Carrier’s parent company decided to cut back on his research funding. Undeterred, Carrier and six coworkers created an independent company, Carrier Engineering.

In 1922, Carrier unveiled another major breakthrough: the centrifugal refrigeration machine. Carrier’s new design pumped coolant through the machine  much more efficiently, allowing for larger, more stable machines. This made air conditioning practical for places like department stores and movie theaters.

In 1928 Carrier created a residential “Weathermaker” that heated, cooled, humidified, cleaned, and circulated air in homes, but the Great Depression put off its commercialization. World War II further delayed the arrival of residential air conditioners, as Carrier Engineering regeared its production lines to the war effort. The company’s systems were used to simulate freezing, high-altitude conditions for the testing of prototype planes, and Carrier chillers were taken from department stores and installed in war production plants.

Although Carrier died in 1950 before seeing his invention’s sweeping residential success, his company remains one of the world’s largest manufacturers of air-conditioning equipment for residential and commercial applications.