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."

I cannot quite tell if meteorology has actually evolved into something scientific that can be prescriptive.

Perhaps as this color wheel demonstrates, meteorology is mostly about being DESCRIPTIVE.