After their daughter was born, the linguist Guy Deutscher and his wife decided to try an experiment. The new parents taught little Alma all the colors of the rainbow and beyond, in the way most parents teach most kids those colors: they pointed to objects—an apple, a grape, a stretch of bright grass—and told their daughter the color of those objects. Red, purple, green.
The pair made one omission in this prismatic approach to their daughter's education, though: Deutscher and his wife didn't tell Alma the color of the sky. They wanted to see what would happen to a developing mind when that mind had not been informed about one of the oldest truisms we have: that the sky is blue.
One day, as father and daughter were taking a walk, Deutscher pointed to the sky and asked his daughter what color it was.
As Radiolab tells it: Alma was confused. To her, it seemed, the sky was not a thing; therefore, it could have no color. A little more time went by. The pair went for a walk. Deutscher asked again. This time, Alma answered that the sky is white. A little more time went by. It wasn't until much later that, when asked of the color of the sky, Alma responded with the answer most of us would consider to be obvious.
The sky is blue: this is a universal truth, even if it may also be, to a large extent, an arbitrary one. Which is in part why scientists have long sought to put numbers—metrics—to the sky's blueness. Long before little Alma Deutscher came along, there was a guy named Horace-Bénédict de Saussure. The naturalist and inventor—he created, among other devices, the horsehair hygrometer, a magnetometer, and an anemometer—was obsessed with the measurement of nature, and in particular with the color of sky. He wondered why, on some days, the sky seemed paler in its hue—white, as Alma might say. And why, on others, it seemed saturated with color.
So Saussure took advantage of the best color-perception aid he could imagine: his eyes. He created a device called a "cyanometer"—a kind of modified color wheel that contained every shade of blue that could be seen in the sky. He then assigned colors to those numbers, ranging from near-white (0) to near-black (52), with a host of blues in between. The idea was to hold up the device at a fixed distance from the observer's eye and then use it to determine the color of the sky at a particular moment.
Saussure had a hunch that the blueness of the sky had to do with the water vapor in the atmosphere. He wanted to test that hunch. In 1787, he embarked on an expedition to Mont Blanc (bringing with him 18 guides and a servant). At the mountain's summit, he measured the color of the sky—and documented a shade of 39 degrees blue, the deepest he'd seen.
While Saussure's work wouldn't explain why the sky is blue—that would be done by Lord Rayleigh and Gustav Mie, through their work with light scattering—it brought numbers to the colors that surround us. The naturalist and his MacGyvered little color wheel had figured out a way to quantify the sky. "It was," the chemist Andrea Sella writes, "the dawn of coordinated, quantitative meteorology."