Liquid water molecules need more than sub-freezing temperatures to freeze. They need something to freeze on—something like cloud condensation nuclei but with a surface that mimics the hexagonal structure of ice itself. Suitable nuclei, called ice nuclei, do exist but are rare—perhaps one in a million particles among airborne particles in the atmosphere. These nuclei are more abundant in air -5F (-15C) or lower. Generally speaking these temperatures are found above 10,000 feet (3 km). At such heights, liquid water droplets in sub-freezing air—droplets known as “supercooled”—have two choices: They can wait it out until just the right ice nuclei comes along or they wait a little longer and freeze on their own.
The sources of ice nuclei and their distribution in the atmosphere are still not well known. Scientific research suggests that the maritime clouds forming over the Pacific Ocean and heading inland may contain icing nuclei formed from phytoplankton, bacteria, and other organisms present in the ocean and transferred into the atmosphere through sea spray. Phytoplankton emits a sulfur compound, dimethyl sulfide, to form aerosols that serve as cloud condensation nuclei and, at sub-freezing temperatures, ice nuclei. None of the water-cycle map I have seen show anything but water in the upswooping arrows leading from the ocean’s surface into the clouds. Nor do they show what is taken up into the clouds from over land.
One the most common and most effective ice nuclei in the atmosphere is a mineral called kaolinite. I had never heard of kaolinite but my husband, who has a master’s degree in geology and remembers everything he learned from 6th grade onward, knew all about it.
“It’s known for its whiteness, purity, and fineness,” he said. “It’s a silicate mined it all over the world, mostly for industrial uses. It’s what’s in porcelain clay, in the glossy coating on paper, rubber, paint, deodorants, make-up—all sorts of things. I’m pretty sure it’s the kao in the original Kaopectate.”
“So people eat ice nuclei to stop diarrhea?” I asked. “I don’t want to think about that.”
But I did want to think about porcelain. I had been handling porcelain clay for years in my weekly ceramics class without knowing I was stealing ice nuclei from the clouds. The many types of porcelain we used in call, all contained kaolinite and were referred to by clay wholesalers simply as “kaolin.” We used other names in class: “Grolleg” (kaolin from Cornwall, England), “New Zealand,” (kaolin from “the land down under”), and “JG” (the initials of our instructor who created this proprietary blend of Grolleg and other clays). Kaolin clay is white, fine, creamy, and smooth and is valued for their fluidity and translucence. There is no scientific evidence that the clouds formed on kaolinite manifest these same qualities but the two other main types of clay, stoneware and earthenware, are decidedly uncloud-like: coarse-grained, hard, tight, and in colors such as brown, yellow, and red.
Kaolinite is an abundant mineral found in large masses in clay beds around the world. It was named in 1637 after the place in China—Kaoling—which is considered the type locality. Kaoling means “high ridge.” Kaolinite is a member of a group of minerals in the Kaolinite-Serpentine group of rock-forming minerals. The Hudson Institute of Mineralogy’s mineral database describes kaolinite’s color as “white to cream,” its luster “waxy, pearly, dull, earthy,” its tenacity “sectile,” its cleavage “perfect.” More importantly for the clouds was kaolinites’s platy quality: “psueodohexagonal.” These perfect-enough, microscopic bits of kaolinite are found in relatively high densities in the atmosphere where the temperature is around at -5F (-15C). Here, the supercooled droplets are ready to freeze on ice nuclei and build their tiny hexagonal empires—ice crystals.