Ice and Snow Study Guide
We drink liquid water, and when the day is sweltering, we rely on ice cubes to cool off our drinks. In winters in the midlatitudes, precipitation falls as snowflakes. Some of us enjoy skiing and snowboarding on the white, solid form of water. Over hundreds of thousands of years, great continental ice sheets have waxed and waned. In this lesson, we look at the science behind ice, from snowflakes to ice sheets.
The Magic of Ice
How water turns solid is an interesting story that takes us, once again, into the molecular structure of the water molecule. Recall that the water molecule (H2O) is slightly bent. The two hydrogen atoms that are individually bonded by a bridge of shared electrons with the central, larger oxygen atom do not stick out at 180° from each other in a straight line. Instead, they are slightly bent in toward each other, making a angle of about 109° between them, with the vertex of this angle at the oxygen atom.
Now, the hydrogen atoms in a single water molecule cannot bond with each other, but those in two different water molecules can. The bond between hydrogen atoms goes by the special name of the hydrogen bond, because it is unique to hydrogen atoms. In a hydrogen bond, each hydrogen atom, with only one electron, would naturally share its electron with a neighboring hydrogen atom. But in the case we are considering of water, the hydrogen atoms are already committed to sharing their electrons with the oxygen atom. Yet the energetics to share with another hydrogen are still in place. The result is a slight attraction of their hydrogen atoms across neighboring water molecules toward each other.
The hydrogen bond is weak, much weaker than the actual shared electronic bond between the hydrogen and oxygen atoms within the water molecule itself. The weak hydrogen bond makes water, well, watery. Imagine little molecules slightly sticky toward each other, just enough to make water coherent as a liquid. But now imagine that the temperature cools, and the energy (the velocity and vibration) of each molecule gets less and less. The energy gets so weakened with further cooling that the hydrogen bonds start to have more and more influence. At what we call the freezing point of water (0° C, 32° F), the hydrogen bonds start to link the water molecules in a latticework with each other. Ice is born.
The hydrogen bonds are so weak that they can be broken by temperature in the range we live in. The breaking of the hydrogen bonds takes place when ice melts. The formation of the hydrogen bonds occurs when ice forms. The melting and freezing of water involves a transfer of energy, even though the temperature remains the same. For review, this latent heat of freezing or melting (called the latent heat of fusion) is 79 calories per gram of water.
At certain places on Earth, it is cold enough to have permanent ice all year round (more on that later), but ice can form at any latitude and longitude—if you go high enough in the atmosphere. Recall that temperatures cool as you go up in the troposphere. They cool so much that there can be snow on mountains in regions that have nice hot summers at sea level. Temperatures in the troposphere can drop so much with altitude that permanent snow and ice are found on top of Mount Kilimanjaro, which is almost right on the equator in Kenya, Africa.
The cooling of temperatures with altitude is governed by the adiabatic lapse rate. Recall that two types of lapse rates exist: dry and moist. The dry lapse rate is about 9° C per kilometer, which is applicable in places where water vapor is very low. The moist lapse rate averages about 6° C per kilometer in altitude. If the moist adiabatic lapse rate applies to a given area, would you have to go higher or not as high in the atmosphere to reach the freezing point of water, compared to another area where the dry adiabatic lapse rate holds (assume the same starting temperatures at the ground at sea level)? The answer is higher. Were you correct? Because the temperature decreases at only 6° C per kilometer in the moist adiabatic condition, you would have to go higher up to reach the same cold temperature, compared to a region where the temperature decreases at 9° C per kilometer. Does the lapse rate decrease enough to give us snow on Kilimanjaro? We'll find out in one of the practice questions.
The top of the troposphere is the tropopause, which averages about 10 km high. Using the moist adiabatic lapse rate from sea level, we can see that the tropopause is about 60° C (10 km × 6° C/km = 60° C) cooler than the ground at sea level. Because 60° C is 140° F, any location on Earth will have freezing temperatures at the tropopause, indeed, well before that height is reached.
The troposphere is the zone of the atmosphere where weather takes place. Clouds that go up that high, say, nearly to the tropopause, will be frozen and are called cirrus, the wispy very high clouds that look like faint white brushstrokes by the painter called nature. Cirrus clouds consist of tiny ice crystals, so small they stay aloft as clouds.
The fact that the atmosphere gets cold up high is the reason that hail can fall in the hot desert in summer. Air currents go up and down. In fact, the intense heating of the ground during summer can drive air currents upwards, sending water-laden air high enough for the water vapor in the air to freeze. If the currents descend, some of the frozen droplets melt a little. Then if the air rises to the atmosphere's freezing level again, more ice grows on the droplet and it enlarges. Up and down the droplets go, melting a little, freezing some more. Eventually, the droplets, round balls by this time, get too heavy for the winds to keep them aloft and they fall as hailstones. When hailstones are cut open, the layers of freezing events are visible.
Many different kinds of frozen precipitation exist. Sleet, for example, is basically frozen rain drops— messy and nasty. And then, of course, there's snow. The uniqueness of snowflakes provides us with the best metaphor for how every shape in the universe is individual, including each of us as persons. Snowflakes are, of course, hexagonal (see Figure 16.1). As unique as each might be, each has six radiating arms. All crystals made from specific substances have inherent geometries.
Salt (sodium chloride), for example, is cubic. It so happens that the geometry of the hydrogen bonds that lock together when water becomes ice creates a hexagonal pattern. A snowflake forms when water vapor freezes onto a crystal that is already growing (which started around a condensation nucleus). Different temperatures and densities of the water vapor in the atmosphere cause different nuances of crystallization in the growing snowflake. When the flakes become heavy enough, they can no longer be kept aloft and they fall to the ground. See if you can catch one on your tongue.
Snow often accumulates during the winter, sometimes for weeks, sometimes for months. The greatest accumulations are usually in the mountains. Often, it is so cold that the snow doesn't melt until the spring, in what is known as the spring runoff. The spring runoff is important for the growth of many plants during the summer, and often, the runoff hap pens slowly, little by little supplying water over a month or more.
What happens when the ice from winter does not melt entirely during the summer? Then a glacier can form in the mountains. We can see glaciers today in the United States in Glacier National Park in Montana and in Alaska. Kilimanjaro has its glaciers and the Canadian Rockies have many glaciers. Glaciers in the European Alps have names that go back centuries, admired by the tourists and local people.
Glaciers tend to grow near their higher-altitude tops because that's where they are the coldest and where more precipitation occurs. But then the weight of the glacier pushes down on the glacier at lower altitudes and on the glacier's ice that is pressed against the underlying rock. This ice can melt and part of the glacier starts to move. Ice is lost at the lower altitude and under the glacier, but new ice forms farther up the mountain at the higher elevation. Thus, mountain glaciers are dynamic systems; they can grow and shrink.
Glaciers also cause erosion. You can tell when a mountain valley once had a glacier in it because the valley is U-shaped. That's in contrast to a valley that was eroded by water alone. Then the valley is V-shaped.
Another form of ice is sea ice. Ice floats because its density is less than liquid water. This is a highly unusual behavior for a molecule or substance. For most substances, their solid forms are denser than their liquid forms. The peculiar behavior of water and ice, again, has to with the hydrogen bond, which causes the water molecules to spread apart when they lock together into the network of ice.
It's a good thing for us that water expands when it freezes, which makes ice float. If ice were denser than liquid water, then the ice in winter during the freezing of a lake would sink. More ice would then form at the surface and that in turn would also sink. Lakes could, in this different world, completely fill up with ice during a winter and the fish would die. Instead, in our world, because ice floats, the lake water is insulated. Yes, the ice grows in thickness but very slowly, because as it gets thicker, it becomes more difficult to cool the water below the ice to the freezing point. Ice on a lake insulates the water.
During winter, tremendous amounts of sea ice on the ocean grow around the continent of Antarctica and upon the Arctic Ocean. These floating ice areas come and go with the seasons, waxing and waning in areas much larger than the United States. The freezing of sea ice makes the water left behind saltier and thus more dense. This process thereby helps drive the thermohaline circulation.
Sea ice also affects climate. The white ice is an excellent reflector of sunlight. In the future, if global warming causes the extent of sea ice to decrease, more of the sun's energy will be absorbed by the darker water (compared to the white ice). This change could enhance the warming from the green house gas carbon dioxide.
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