Ice and Snow Study Guide (page 2)
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.
Ice Sheets on the Continents
One very important type of ice remains: continental ice sheets. Only two places on Earth have very large continental ice sheets, but in the past, during ice ages, the ice sheets were much more extensive.
Ice sheets are to glaciers what the urban sprawl of Los Angeles is to a single city street. Today, ice sheets cover almost all of Greenland in the Northern Hemi sphere and Antarctica in the southern. They are massive almost beyond belief, with ice miles in depth (or height, depending on where you start to measure).
Recall that the continental ice sheets of Greenland and Antarctica amount to 2.2% of the world's water (Ice caps are another term for ice sheets because they cap the rock they are on.) . Also recall that the ocean has 97.2% of the world's water.
Let's calculate how much sea level would rise if Greenland and Antarctica melted. For simplicity, assume that all the 2.2% of glaciers and ice caps is in Greenland and Antarctica. That's a good assumption. You'll be walked through the calculation here, just up to the answer, and then be asked to compute the answer for one of the practice questions. If Greenland and Antarctica melted, using the assumption just stated, the ocean would rise in sea level by a fraction of of its current volume. Disregarding the issue of continental shelves, we will assume that the ocean has a constant depth of 4,000 meters (again, not a bad assumption). So, how much would sea level rise if Greenland and Antarctica melted?
One of the great discoveries of the earth sciences over the last hundred years or so is the recognition that long ago, ice sheets covered massive parts of northern North America, Europe, and Russia. This understanding is still being refined today by hundreds or more specialists around the world who study Earth's past ice ages. Such intensive study is required not only because earth's history is fascinating, but because Earth's past might give us clues to Earth's future.
The earliest evidence for past ice ages is in the form of geological formations left by the great ice sheets (mountain glaciers leave some of these, too, but the results are smaller). Examples of this geological evidence include moraines, which are piles of rubble left by the ice sheet (or glacier) as the ice pushes for ward across the land. The final, farthest rubble pile left by the advance of an ice sheet (or a glacier) is called the terminal moraine. Much of Long Island, New York State, is a giant terminal moraine from a past ice age.
Streams of meltwater that flow from a glacier or in a tunnel under an ice sheet can create a ridge of gravel called an esker. When some of the finely ground glacial material is blown by the prevailing winds into an elongated hill (or series of hills), we have what is called a drumlin.
Geological features like these, found in the United States, presented the early modern geologists with evidence that giant ice sheets had once covered most of Canada and a good deal of the northern United States. We now know that the ice sheets during the last ice age were very thick. New York City, for example (or Manhattan Island, because New York City did not exist), as recently as 20,000 year ago was covered with an ice sheet a mile thick. How do we know the thickness? The geological evidence can provide some sense of the lateral extent of the ice, but what evidence do we have for its thickness?
Scientists can get numbers for the ice's volume on the continents during the last ice age by using oxygen isotopes. We have already discussed the science behind the radioactive isotope of carbon (C-14) and other radioisotopes (such as potassium-40) that are used for dating the buildings of ancient cliff dwellings and the birth of igneous rocks. In Lesson 8, the stable isotope of carbon (C-13) was briefly noted. Many elements have stable isotopes as well as radioactive ones. Ordinary oxygen is O-16, with eight protons and eight neutrons in its nucleus. Oxygen also has a stable isotope, O-18, with eight protons and ten neutrons in the nucleus.
Water (H2O) contains both O-16 and O-18. The oxygen in water is mostly O-16, but small fractions of the water molecules contain O-18. This "heavy" water is everywhere, in the ocean, in the rain, in your body. When water evaporates from the ocean, the heavy water stays behind with just a little more statistical propensity than the water that contains O-16. That means that water vapor in the sky contains a higher percentage of O-16 water than water in the ocean. That means, in turn, that rain or snow contains a higher percentage of O-16 water than water in the ocean.
When reservoirs of water are all in balance, as they are most of the time, the lighter water in the rain or snow returns to the ocean, so the ocean is not permanently altered. But during the development of an ice age, in which ice sheets on the continents are growing, the water that is locked up as ice, which fell as snow from the sky, is lighter (it has more O-16) than the water in the ocean. During the ice age, so much water was locked up in the ice sheets that the ocean's ratio of O-18 to O-16 was changed. If there was more O-16 locked up in the ice sheets, can you tell whether the ratio of O-18 to O-16 in the ocean increased or decreased?
The ratio of O-18 to O-16 in the ocean increased. The ocean's water became slightly "heavier." That ratio can be measured today, even though the ancient ocean is history. Organisms that make calcium carbonate shells in the ocean use carbonate from the ocean water. The carbonate (CO32–) is formed by chemical reactions between carbon dioxide and water molecules in the ocean, and therefore, the oxygen isotopes in the carbonate molecules have the same ratio of O-18 to O-16 as does the ocean. The carbonate shells fall to the bottom when the creatures that make them die. For the most part, the shells are preserved in the ocean bottom sediments. Scientists drill into and retrieve these sediments, take them back to their laboratories, and measure the isotope contents of the shells to determine the isotopes of the ancient ocean at various times.
By measuring the ratio of O-18 and O-16 in the shells, which equals that of the ancient ocean in which the shells grew, it is possible to calculate how much ice (with a reduced ratio of O-18 to O-16) would have been on the continents to increase the ocean water's ratio to the value that scientists measure. This technique has shown that so much ice was on land during the middle of the last ice age that sea level was down by about 150 meters! This means that the continents went further out than they are now, out to the edges of the continental shelves in many places.
When did the ice age occur? From studies of geological "calling cards" of the ice sheets, as noted previously, as well as the all-important modern methods of oxygen isotopes (and other techniques), it is known that the ice sheets waxed and waned in a large cycle that lasted about 100,000 years in length. In between the ice ages were warmer times, like the one we live in now. The most recent ice age ended about 10,000 to 12,000 years ago.
Many scientists of climate (climatologists) say we are just about at the start of another ice age and claim that our in-between warm period is coming to an end. Other climatologists say that because we are now increasing the greenhouse gas carbon dioxide, the earth will not be following the same rules that determined its climate in the past. Still other climatologists point to the fact that although most of the warm intervals between ice ages last 10,000 to 12,000 years, some times those warm intervals are longer, up to 20,000 years, and that might be the natural case for us now. Only time will tell.
Practice problems of this concept can be found at: Ice and Snow Practice Questions
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