Montanans and people everywhere have a love-hate relationship with water's solid form. Some wait impatiently for it to appear. Others take their last tragic breath because they misread it. This inorganic mineral falls from the sky, forms on our roads, in our mountains and on our waterways. Even snow lovers are technically ice lovers, since their powder playground is really covered with a form of ice.
All types of frozen H2O are ice of one kind or another. Scientists define 15 different types of it, known as crystalline phases of water. The most common, "Ih," a hexagonal molecular structure, forms when water cools below 32 degrees Fahrenheit at standard atmospheric pressure—which is most of the frozen water on the surface of Earth. The 14 other varieties are in outer space, high in the atmosphere, buried in polar ice caps or deep underground.
The bonds between water molecules create the structure and strength of a particular patch of ice. Its thickness—and the amount of air and other substances suspended in it—primarily determines its appearance. Pure ice looks blue because, like water, it absorbs the red end of the light spectrum and reflects the blue. The thicker the ice, the bluer it appears; the more air in it, the whiter. Sediments and algae can turn ice other hues, such as brown, gray or green.
Black ice is another story. This invisible and potentially lethal roadway coating is not black at all, but instead is so thin it's transparent, exposing the dark pavement.
Black ice also forms in the mountains: It's notoriously hard, dense and difficult to climb. Verglas, also very dangerous for climbers, is a clear frozen glaze that makes rocks extremely slick.
What makes ice so slippery? The common notion is that pressure or friction creates a film of water that makes it seem slick. In this view, friction from a gliding skate slightly melts the ice next to the blade, allowing you to slide across a rink.
The more accepted scientific theory holds that water molecules at the interface between the ice and the air cannot strongly bond with either, so they remain in a perpetually liquid and lubricating state. In other words, ice is slippery whether you touch it or not.
Because it's nine percent less dense than water, ice also floats—which means it covers the surface first and grows downward. (Some people think rivers and lakes freeze from bottom to top, but they're all wet.) Frazil ice, or small ice shards, can accumulate on or near a riverbed; chunks of ice can pile up underwater against submerged rocks or logs. But the ice wasn't born on the bottom.
That's fortunate for Earth as we know it. The frozen layer on top of lakes, streams and oceans allows light to pass through and shelters fish, invertebrates, algae and other life forms, protecting them from subfreezing temperatures and wind chill.
Ice carves our mountain landscapes, supplies us with water and provides endless hours of outdoor entertainment. We curse it when our skis skitter sideways and kiss it when we climb a pillar. It's weak enough to dissolve in a glass of whiskey and strong enough to sit on while we fish, drink in hand. You can't get much cooler than that.