Why does frozen water appear as snow in some cases and ice in others?
Water comes in a variety of forms, or phases. Depending on temperature, the three commonly found are gas (water vapor), liquid (liquid water), and solid (ice). Snow and ice are made of the same material but snow is composed of crystals with regular shapes, while ice forms as sheets or solid chunks.
The difference between snow and ice lies in how water freezes into its solid form, and here’s how that happens.
Water vapor is invisible but can cool to form tiny liquid droplets that make a visible cloud. You can observe this if you look at the steam coming from a kettle. Close to the spout, where the temperature is high and the water is in vapor form, you will see nothing.
But a few millimeters away, a visible white steam cloud forms where the temperature has dropped enough for the vapor to become a liquid. Normal air always has water vapor in it. If there’s enough vapor and the temperature dips low enough, droplets can form and make up clouds.
If the temperature dips even more, these droplets can freeze to form the kind of tiny crystals that fall to earth as snow. The crystals form in a variety of shapes, largely determined by the temperature at which the freezing takes place. At temperatures close to the freezing point (25 to 32 degrees) they tend to be thin, hexagonal plates, while at slightly colder temperatures (21 to 25 degrees), they are more needle-shaped. As the temperature continues to drop, different shapes emerge: hollow columns (14 to 21 degrees), hexagonal plates with indentations (10 to 14 degrees), and below that, the branched shapes that children usually draw.
Snow is ice that falls in the form of these little crystals. When it lands, there are lots of spaces for air, so you get the fluffy, light material that we call snow. Just how fluffy depends on the sort of crystals.
Updrafts can push small snowflakes up into clouds, where extra layers of ice can form on them. This can happen repeatedly, building up lumps of ice that fall as hail. If you crack open a hailstone, you can often see layers that illustrate how the stone formed. Since strong air currents are needed to push large lumps of ice upward, hail tends to form only in severe thunderstorms.
When very cold vapor that has not solidified (so-called supercooled vapor) freezes onto a snowflake it can form a soft hail called graupel, which crumbles easily.
On the ground, the ice you see in winter is largely due to snow that has partially melted and then refrozen, perhaps more than once. This results in the disappearance of spaces between snow crystals as they fill up with liquid before refreezing. This leads to denser and denser collections of ice crystals with fewer and fewer air spaces, and eventually it becomes ice.
This may sound complicated, but it could be worse. At higher pressures, 15 distinct types of ice can form — all solid forms of water but different from each other in the way that diamonds and coal are different despite both being solid forms of carbon.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
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