Page 3 of 10
Although water in an ice-cube tray will turn solid around 32ºF, pure liquid water in clean air never freezes at that temperature. The formation of ice in the air depends on water molecules latching together in a very specific hexagonal pattern that resembles three-dimensional chicken wire. Once enough water molecules organize into that pattern, an ice crystal grows rapidly. The droplet freezes in an instant. But getting that hexagonal-patterned ice embryo to form in the first place is difficult. At just below 32 degrees, more than 100,000 water molecules need to latch together before the crystal becomes stable enough to grow on its own. Since water molecules are constantly dancing around with thermal energy, the likelihood of enough water molecules happening by chance to strike this collective pose is exceedingly low, even at much colder temperatures.
Some particles facilitate the process, however, through a process called nucleation. A little mineral crystal can act as a template, coaxing water molecules on its surface to organize into the hexagonal lattice of an ice crystal. There is plenty of microscopic junk that allows the water in a puddle in your backyard (or even in your ice-cube tray) to freeze just below 32 degrees. But a cloud droplet that is just slightly wider than a red blood cell may contain only one such particle. In order for this particle to nucleate ice crystals, it needs to have just the right shape and give off minute attractive and repulsive atomic forces in just the right places so that the H's and O's in those H2O molecules stick to the particle in the right hexagonal pattern.
Clouds at –30 or –35ºF are often entirely liquid because they do not contain any efficient ice-nucleating particles. And yet scientists also see clouds that are much warmer, even 10ºF, that are full of ice and gushing out rain or snow. These clouds obviously contain something that efficiently nucleates ice at those much warmer temperatures.
For decades scientists could not put their finger on what that mystery particle was. In the mid-1960s Gabor Vali, a Hungarian-born Ph.D. student in physics at McGill University in Montreal, devoted most of his waking hours to looking for it. Vali spent months collecting snow and rainwater by the gallon. He brought it to the lab and squeezed it by syringe, drop by drop, onto sheets of aluminum foil, 100 drops per sheet. He cooled the sheets by 2ºF per minute, taking a photo every 30 seconds. Later he projected the photos on a wall and looked to see the temperature at which each drop had frozen.
Vali did 10 experiments a day, five or six days a week, for several years—"many hundreds of thousands of drops," he says. Each drop froze at a different temperature depending on what specks were floating inside it. A few drops froze at 23ºF, but not frequently enough to explain how efficiently ice crystals form in warm clouds. Vali was missing something.