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.