Teacher Background
Heavy snowfall is usually associated with severe winter storms. These are large extratropical cyclones (low pressure regions found poleward of the tropics) with large pressure gradients (change of pressure with horizontal distance). Heavy amounts of snowfall can also occur in lake effect snow squalls (snow formed by cold air passing over large ice-free lakes). Lighter snow amounts can occur with smaller extratropical cyclones, beneath upper level troughs (low pressure areas generally indicated by a southward dip of the jet stream), or lake effect snow areas outside squalls. This background will first describe winter storms and lake effect snow. It will then go into snowflake formation. Metamorphism (crystalline structure change) of the snowpack will be described next. Finally pollution in the snow will be discussed.
WINTER STORMS
In winter, there is a large temperature contrast north to south across the United States. The polar front jet stream (high speed wind near the tropopause) forms above where the temperature contrast is greatest. In winter, this can be as far north as Southern Canada or as far south as Texas, depending on whether the continental polar air mass from Canada or the maritime tropical air mass from the Gulf of Mexico is dominant. Storms often form along the jet stream and move eastward. Preferred locations of formation include the lee (downwind) side of the mountains, especially in Alberta or Colorado. Other formation locations are over the water of the Pacific, Gulf of Mexico, or Atlantic. Typical paths of storms are shown in Figure 1.
Figure 1. Main tracks of cyclonic (low pressure) systems across the United States (National Research Council 1983 as shown in Moran & Morgan).
The center of each storm is the region of lowest surface pressure. This low is often associated with a cold front pushed by continental polar air and a warm front being overridden by advancing maritime tropical air (see Figure 2).
Figure 2. Shaded area represents precipitation. The region marked snow is the coldest precipitation area and is therefore most likely to have snow when there is a mixture of rain and snow.
If the low is not moving or is moving slowly, these fronts can begin to spiral counterclockwise around the low while the low is deepening (developing a lower pressure). An occlusion (the final stage in the life of a mid-latitude cyclone) will often then form when the cold front catches up to the warm front. If the low is moving fast, it has less chance to deepen. Thus the slow moving storms tend to give more snow because they have more chance to strengthen and are in the vicinity longer. The typical region of precipitation is an area around the low, a wide band ahead of the warm front, and a narrow band over the cold front (see Figure 2). When there is both rain and snow around a low, the coldest region of precipitation and therefore the region most likely to have snow is often north or northwest of the low.
Wind spirals counterclockwise and in around a low in the northern hemisphere. The greater the pressure gradient, the stronger the wind. A blizzard is defined as wind at least 52 km/hr (32 mi/hr), low temperature, and enough snow in the air to reduce visibility to under 150 m. A severe blizzard is defined as winds over 72 km/hr (45 mi/hr), temperatures below -12oC, and visibility near zero because of snow. The change of direction of the wind with time can often foretell future weather. Wind from the east often means a low is approaching. If the wind backs (changes in a counterclockwise direction i.e. east to north) in time, the low will likely pass to the south and neither front will pass overhead. Steady precipitation is likely with rain possibly changing to snow. If the wind veers (changes in a clockwise direction i.e. east to south) in time, the low will likely pass to the north. First the warm front will pass over and then the cold front. Steady precipitation will cease, followed by warming. Then brief showers will be followed by cooling.
LAKE EFFECT SNOW
Lake effect snow generally occurs only around the Great Lakes of United States and Canada, and in Northern Japan. The necessary ingredient is to have very cold air pass over a large, unfrozen body of water. Other areas either have lakes which freeze over or are not near a source region of very cold air. When this cold air moves over open water, it takes on water vapor and heats up. This causes the air to become unstable and rise. As this newly moisture-enhanced air rises, it cools enough to form clouds and snow (see section on snow crystal formation). Inland hills, such as the Tug Hill region east of Lake Ontario, cause even more rising air and thus more snow. The cold air must pass over about 80 km of unfrozen lake to yield much snow. The greater the temperature difference between the air (measured at the 850 mb level) and the lake, the greater the chance of snow. A minimum of 15oC is generally needed to have significant lake effect snow.
Snow squalls often form when the wind is parallel to the axis of a lake. Greater friction over land causes a greater turning of the wind than the lesser friction over a lake. That is, the wind crosses the isobars (line of constant pressure) from high to low pressure at roughly a 30o angle over land and 15o angle over the lake. This means that wind from over land converges with the wind from over the lake. This converging air has nowhere to go but up, so a line of rising air forms near the shoreline (see Figure 3).
Figure 3. Wind arrows. Air from over the land converges with air from over the lake (see text).
This rising air condenses water vapor giving off heat. The heat creates a band of low pressure underneath that causes the wind to converge even more. Where this narrow band crosses the shoreline, a snow squall is set up. Once the band has started, since thermal convergence takes over from frictional convergence, when the isobars change direction, the band will also change direction, giving snow to another region. Wind speed also plays a role in turning the wind and the formation of clouds. Predicting the location and movement of these bands is difficult, but new mesoscale numerical models make the job easier.
Areas to either side of the squall bands may also have significant snow, hence the typical forecast: "areas to the lee of the lake will receive six to eight inches of snow while squall areas will receive over twelve inches." The wind direction that causes lake effect snow for a given region depends on the location of the lake relative to that region. The nearest weather bureau office should be able to help you determine this wind direction for your location.
SNOW CRYSTAL FORMATION
The amount of water vapor that air can hold is a function of temperature; warm air can hold more vapor than cool air. Actual vapor pressure is that portion of the total atmospheric pressure that is due to water vapor. The amount of water vapor that air can hold at a given temperature is called the saturated vapor pressure. The relative humidity (RH) is equal to the actual vapor pressure divided by the saturated vapor pressure. When air rises, it expands and cools. As air cools the RH rises, and is equal to 100% when the air is saturated. As the air cools further, the water will condense if there is something on which to condense. An example of this is the formation of dew. If there is nothing on which to condense, the RH will rise above 100% and the air will become supersaturated. This is because water vapor is more likely to leave a highly convex surface than a flat surface (surface molecules on a convex surface are attached to fewer other molecules than surface molecules on a flat surface). Therefore, very small water drops require a higher RH to exist.
Most clouds have a RH of about 101%, which means that water drops need to be at least 0.l μm in diameter to survive. Thus drops will form only around condensation nuclei (particles on which a cloud drop can form) of that size or greater, unless the condensation nuclei are hygroscopic (attract water) like a salt crystal. Once the drops form, they can grow by condensation, but the larger they become, the slower they grow. Thus most clouds consist of drops ranging from about 10 μm to 30 μm. These drops have a large surface area for their volume so that the drag (friction against air molecules) is large compared to the gravitational attraction to the earth. Thus they fall slowly relative to the air, and since the air is likely to be rising in the cloud, the drops remain suspended in the air.
If the drops continue to rise and cool, their temperature may fall below the freezing point. To freeze, many molecules must by random motion align themselves in the hexagonal pattern of ice crystals. If the drops are very small, this is unlikely, so the water remains as supercooled droplets. As the drops get larger (say 1 mm in diameter) there are more molecules, and the likelihood of some of them aligning correctly becomes greater. Once a few have aligned themselves, others can easily join them so that the entire drop will freeze.
Water molecules may attach to crystals other than ice, but similar in shape (hexagonal) to ice, thus forming ice. These crystals are called freezing nuclei. There are natural freezing nuclei such as kaolin (a clay), and artificial freezing nuclei such as silver iodide. Ice falling from a higher cloud overhead can also form the nucleus of a larger ice crystal. If an ice crystal becomes large enough, it can break up into several new freezing nuclei.
Snow is usually formed in clouds that contain both ice crystals and supercooled water droplets. The saturated vapor pressure over water is greater than that over ice. In other words, it takes less energy for supercooled water to evaporate than for ice to sublimate (go directly from solid to vapor or vapor to solid), so that there will be more vapor given off from water than from ice. The actual vapor pressure is likely to be between the saturated vapor pressures of ice and water. Therefore the RH with respect to the water will be less than 100% while the RH with respect to the ice will be greater than 100%. This means that the ice crystals will grow by sublimation from water vapor that comes from the vaporization of water droplets.
Some ice crystals grow in the plane of the hexagon forming plates or stellar patterns, and others grow perpendicular to the hexagon forming needles or columns. Which way crystals grow depends on the temperature and humidity. Figure 4 shows the types of crystals formed in the different temperature - humidity regimes. Note that both the saturation points with respect to ice and water are shown. The more complex crystals form at higher humidity. The crystal may fall from one regime to another forming combinations of crystals like capped columns.
Figure 4. Temperature and humidity conditions for the growth of natural snow crystals of various types (Magono & Lee 1966).
A summary of the various crystal types is given in Table 1.
Table 1. Classes of Solid Precipitation (Magono & Lee 1966)
As an ice crystal starts to fall, it may collide with small supercooled water droplets that will freeze on contact. These frozen droplets are called rime. As snow crystals become more and more rimed, they will lose their former shape and eventually form large globules of rime called graupel (see Figure 4).
The types of crystals you observe will tell you about the temperature and humidity of the atmosphere overhead where the crystals were formed. The crystal types also will determine the amount of air space in the snow pack and thus the insulative properties of the snow that are important to plants and animals. You can examine these crystals outside in the cold with a magnifying glass or low powered microscope or you can create replicas of the crystals with formvar solution.
SNOWPACK STRUCTURE
Examining the snowpack shows it is not one uniform mass but is made of numerous layers. These layers are caused by individual snow events modified by environmental influences such as 1) wind and air temperature changes above the snowpack and 2) temperature gradients and the presence and movement of water as a liquid or vapor within the snowpack. Much of the history of a snowpack can be discerned by studying the layers present.
Regardless of their original shape, almost all crystals change with time after they land on the ground. The processes responsible for this change include various types of metamorphism, or changes in crystalline structure owing to environmental factors. Several types of metamorphism involve sublimation, in which the parts of the crystal change to a vapor and then change back again to a solid at another part of the original crystal or snowpack.
Metamorphism is dependent on such factors as wind, temperature, solar radiation, and humidity. The extent to which each of these affects the snowpack determines the final characteristics of the layer. Therefore, different environmental conditions result in different types of layers. Ten snow types often found are described in Table 2.
Table 2. Common Snow Types.
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Powder Snow Cloud formed crystals or parts of broken crystals
Surface Hoar Thin layer of frost formed on snow surface
Ice Pellets Sleet, hail or graupel
Wind-Beaten Snow Small snow grains rounded by tumbling in the wind and packed together (upsik)
Granular Snow Old snow in which original crystal structure is broken down into rounded grains (corn snow)
Wet snow Anything from fresh snow with a high liquid water content (i.e. packs easily) to slush
Solid Type Depth Hoar Small plates or columns with sharp edges and flat surfaces found at the bottom of the snowpack
Skeleton Type Depth Hoar Stepped surfaces, often cup shaped, found at the bottom of the snowpack (pukak or sugar snow)
Crust Snow grains strongly sintered (fused) together
Ice No individual snow grains and few air bubbles
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In general, the layers within the snowpack represent different precipitation events. The thickness of each layer is, of course, a function of the initial amount of falling snow. Drifting may cause individual layers to have varying thicknesses at different locations. If the snowpack is several weeks old, you may find a particular layer more than once in the pack as snow layers from separate snowstorms progressively build. Some will not be found at all in a particular sample.
In addition to snow falling from the sky, the snowpack can form from surface hoar that is simply frost formed on the snowpack surface. In this process, water vapor sublimates directly on the surface forming ice crystals usually as flat plates. This process occurs whenever the surface temperature of the snow is lower than the dew point temperature of the air.
Once snow crystals are on the ground, they are likely to be further changed. If the snow is windblown, delicate appendages of crystals will be broken apart, forming smaller rounded grains. Wind will pack these grains closer together forming hard upsik, or wind-beaten snow. This is the snow that the Inuit people (Eskimos) use to build igloos. Snow will change form even if it is not windblown. This change, or metamorphism, can occur in various ways: destructive or constructive, equal temperature or temperature gradient, and melt-freeze.
Destructive metamorphism generally occurs first. Dendritic and needle-like snow crystals break up into smaller grains in a fashion similar to the formation of wind-beaten snow, but less violent. Compaction from the weight of snow above and general settling are the primary causes of this destructive metamorphism.
Equal temperature metamorphism is also a form of destructive metamorphism since the original crystals are being broken down. The general cause of this crystal change is a move by the snow crystals toward a thermodynamic equilibrium state in which Gibbs Free Energy (a mathematically defined thermodynamic function of state) is a minimum. For snow crystals, this implies that the ratio of surface area to volume is a minimum. Note that a certain volume split between two equal spheres will have about 1.6 times as much surface area as the same volume in a single sphere. Also two spheres connected with a neck will have a smaller surface area than the same spheres unconnected. The specific mechanisms involve the saturated vapor pressure over flat versus convex or concave ice surfaces. Molecules on a convex surface are held less tightly to the crystal than they are on a flat surface since they are linked to fewer other molecules. Molecules on a concave surface are linked to more other molecules and are thus held more tightly. Therefore, a convex surface has a higher saturated vapor pressure than a flat surface which has a higher saturated vapor pressure than a concave surface. Since the water vapor in the air within the snowpack will be near saturation, the relative humidity with respect to the convex surface will be under 100% and the relative humidity with respect to the flat surface will be over 100%. The relative humidity with respect to the concave surface will be even greater. Thus water vapor will migrate from the more convex surface to the flat and concave surfaces. Flat surfaces will grow at the expense of the convex surfaces and concave surfaces will grow at the expense of both the convex and flat surfaces. The net result is that small grains will disappear as the large grains grow, and everywhere that two grains touch (concave surfaces), they will fuse together. The fusion of small grains at contact points is called sintering (see Figure 5).
Figure 5. Formation of sintered snow.
This leads to increased strength and hardness of the snowpack. The rate at which the small grains disappear depends on the grain size. A 100 μm radius grain can be considered flat and will last over a year if temperature permits. A 10 μm radius grain will disappear in about 4 days, while a 1 μm radius grain will take about 1 hr (see Figure 6).
Figure 6. Equal temperature metamorphism of a stellar snow crystal. The numbers give the age of the crystal in days. (LaChapelle 1969)
Temperature gradient metamorphism occurs whenever different parts of the snowpack are at different temperatures. Since new crystals are being formed, this is also called constructive metamorphism. The top of the snowpack will be near the air temperature while the bottom of the snowpack will be near the ground temperature. The latter is likely to be considerably warmer owing to the heat stored in the ground the previous summer. Warm snow has a higher saturated vapor pressure than does cold snow since in a warm solid the molecules are moving faster and are more likely to leave the solid as sublimation. Thus in a fashion similar to that for curved surfaces, water vapor will migrate from warm to cold surfaces and cold surfaces will grow at the expense of warm ones. In general, water vapor will migrate up through the snowpack since the temperature usually decreases upward. The type of crystal remaining is called depth hoar (pukak) and is generally found in the snowpack near the ground.
When the temperature gradient is small (less than 0.25EC/cm) solid type depth hoar is formed. This is characterized by small plates or columns with sharp edges, corners and flat surfaces. The result is a relatively hard layer of fine grained compact snow. When the temperature gradient is larger, skeleton type depth hoar is formed. These are larger grains of stepped or ribbed surfaces forming cups, needles, scrolls and plates. Molecules add on to the edge of steps making the steps broader, thus growing flat surfaces parallel to the internal ice planes and forming stepped pyramids (see Figure 7).
Figure 7. Depth Hoar crystals (Colbeck 1980)
These steps may range from 1 to 100 molecules in thickness depending on how rapidly molecules are added. The resulting crystals have low shear strength. Since they generally form near the ground surface where the temperature gradient is greatest (i.e. in the pukak layer), they are the cause of avalanches where the weight of snow causes whole layers of snow slip off mountain peaks. When the temperature gradient is very large (over about 1oC/cm), the resulting crystals form a hard depth hoar. A quinzhee (snow shelter made from powder snow) is an example of this metamorphism. The snow hardness has been observed to increase from 8 - 80 g/cm2 to 200 - 850 g/cm2 when the warm snow near the ground is mixed with cold snow near the air surface. Temperature gradient metamorphism most often occurs in cold climates with minimal snow cover since this causes the largest temperature gradients in the snowpack.
Melt-freeze metamorphism is caused by water, melted at the surface by warm air and solar radiation, trickling down between the ice crystals below. Water within the snowpack collects at the points of contact between the ice crystals. When this water freezes, it adds great strength to the snowpack as a snow crust. Repeated cycles of melt and freeze will greatly increase mechanical strength of the snowpack and also increase its density. Surface ice layers are caused either by melting and refreezing the surface snow or by freezing rain. In the latter case, supercooled water drops fall from the sky and freeze on impact with the cold snow surface. If the ice sheet becomes impermeable, rainfall can pool on the surface and add to the ice when it later freezes. Since snow can later fall on these ice surfaces, there may be several ice layers within the snowpack. Thus many different processes contribute to the snowpack composition and the properties of the various layers of the snowpack will be different.
Hardness of the snow, which depends on density and crystalline structure, is important to animals moving over or through the snow. Hardness can be measured in both the vertical and horizontal directions. In measuring vertical hardness, the force needed to break through the various layers of the snow is determined. This can in turn be related to the weight and foot area of animals moving over the snow (see unit on Snow Coping). The hardness of the snowpack, along with its depth and density, is important for animals living on top of the snow. Caribou avoid deep or crusty snow, seeking out soft, fluffy snow to survive the winter; moose and white tailed deer choose winter habitats based on snowpack depth within their range; and elk are limited in their winter distribution by snow depth. Vertical hardness is also important to animals that burrow into the snow. For example, various finches and grouse burrow nightly into the snow which they use as a sort of quinzhee in which to spend the night.
Horizontal hardness is more important to small animals who burrow through the snow. The depth hoar layer, where they are most likely to be, is often less hard than upper layers. In years when snowpack is scarce, or snowpack with insufficient insulative properties are present, many subnivean animals die. (See Heat Transfer Teacher Background.)
Snow density times the depth determines the amount of water equivalent (the amount of water obtained when snow is melted) on a square centimeter of surface. This is used in calculating the flood or water power potential of a given watershed, and is thus of ecological and economic importance.
ACID SNOW
A major environmental concern of our time is acid deposition. Certain chemicals that are emitted into the atmosphere as air pollutants, return to the surface as acids that can damage the environment.
An acid is a chemical that in water provides an excess of hydrogen (or hydronium) ions. Acidity is measured on a logarithmic scale of pH units (negative log to the base 10 of the hydrogen ion concentration). The scale ranges from 0 to 14 with pH of 7 being neutral. Note that a pH of 3 is 10 times as acid as a pH of 4 and 100 times as acid as a pH of 5 etc.
Acid can be deposited from the atmosphere in two ways -- as wet or as dry deposition. In the latter case, particles or gases in the atmosphere strike and stick on something on the surface such as a tree. They can then be later washed off the tree during a rain. This is why the acidity of rain under a tree may be greater than rain in the open. In wet deposition, the particle or gas is attached to or dissolved in a water drop or ice crystal and reaches the ground as rain, snow, sleet or fog. This type of deposition is generally referred to as acid precipitation.
Water that has been in the atmosphere as a drop for some time will come to equilibrium with the carbon dioxide of the air forming weak carbonic acid (H2CO3). With normal atmospheric concentrations of carbon dioxide, this will yield a pH of 5.6. Thus rain with a pH lower than 5.6 is generally considered acid. There are other naturally occurring substances that can affect the pH of rain that vary from one locality to another. For example, near a sulfur spring, sulfur could lower the pH, ammonia could raise the pH, and certain organic compounds could go either way. Thus natural rain pH can vary between 4.9 and 6.5.
Acid rain consists primarily of sulfuric acid (H2SO4) and nitric acid (HNO3) with small amounts of hydrochloric acid (HCl). In eastern US and Canada, there is typically twice as much sulfuric acid as nitric acid, while in western US and Canada, they are more equal. Both acids can affect the acidity of lakes and can cause aluminum from the lake bottom and surrounding soils to be dissolved, affecting the gills of fish. Recent research seems to show that nitric acid affects forests more than sulfuric, but both acids can leach nutrients from the soil. The best way to determine which acid is present is to measure the sulfates (SO4=) and the nitrates (NO3-) in the rain. Figures 8 and 9 give the spatial distribution of average sulfates and nitrates respectively in precipitation in North America. Since most schools do not have the capability of measuring sulfates or nitrates, we will not distinguish between them in these exercises. The spatial distribution of pH of precipitation is given in Figure 10.
Figure 8. Sulfate Ion Concentration, 2001 (NADP 2002)
Figure 9. Nitrate Ion Concentration, 2001 (NADP 2002)
Figure 10. Hydrogen Ion Concentration as pH, 2001 (NADP 2002)
Not all areas receiving acid rain are adversely affected by it. This is because some areas have limestone bedrock (primarily calcium carbonate CaCO3) that reacts with the acids to neutralize them. Other areas may be primarily granite which does not neutralize the acids, thus the pH of a lake can drop. Since two adjacent lakes can have different bedrock, they can have different buffering capacity and thus different acidity. Figure 11 shows the areas of North America that are especially sensitive to acid precipitation.
Figure 11. Total Soil Alkalinity
In the student activities, we concentrate on investigations of snow rather than rain. Snow is often less acid than rain, but its effect on the ecosystem can be greater. Most of the acid in the snow runs into streams and lakes all at once during spring snow melt, especially if the ground is frozen. Spring snow melt often coincides with fish spawning; young fish are thus killed in lakes that are not too acid the rest of the year.
The pH of polluted snow depends on the composition of the atmosphere overhead when it snowed. In general, the layers within the snowpack represent different precipitation events. The snow in each of the events was formed under different conditions of temperature and humidity (which defined the crystalline structure) and snow crystal riming. Foreign matter in the snow, including both particles and absorbed or adsorbed gases, is a function of the content of the air in which the snow was formed and through which it fell; the air contents in turn are a function of where the air came from. For example, air transported from industrial areas might be high in sulfates. This situation is further complicated since air in different levels of the atmosphere probably came from different directions. These levels might include a level of snow seeding, a level of snow forming, a level of feeder clouds that cause riming, and any level below the clouds through which the snow fell. Thus the chemical composition of various layers of the snowpack are likely to be as different from each other as the air masses through which they fell.
Most of the chemicals ending up as precipitation acid are not emitted to the atmosphere as acid. Most sulfuric acid in rain starts out as sulfur dioxide (SO2) which comes from the oxidation of sulfur in fuel (coal or oil). Some sulfur dioxide can combine with water to produce weak sulfuris acid (H2SO3), but it must be oxidized before becoming sulfuric acid. This oxidation takes place slowly as the air drifts hundreds of miles unless the gas is in contact with water droplets in a cloud or fog. The moisture causes oxidation to be much more rapid.
Nitric acid starts out as nitric oxide (NO) or nitrogen dioxide (NO2). Together these gases are labeled NOx. They come from high temperature combustion. The heat causes some of the nitrogen in the air to combine with oxygen in the air to form NOx. Thus a clean fuel such as natural gas will produce as much NOx as a dirty fuel. This is why NOx from gas stoves in tight, well-insulated houses is a problem. The major sources of NOx in this hemisphere are cars and power plants. Industry and agricultural fertilization also are important. The complex processes by which NOx becomes nitric acid are still under study. It should be mentioned, however, that NOx can combine with sunlight to produce ozone that is harmful to humans and vegetation alike.
To determine the origin of possible pollution, the winds must be traced backwards to locate where the air resided since it was last cleaned by rain. An approximation of this is to follow the track of the low pressure system causing the rain or snow in your location. Figure 1 showed typical storm tracks in the US. If upper air maps are available, the 850 mb winds could be used. The Weather Service also makes trajectory forecasts that might be available to you. Remember that scientists with the latest equipment have difficulty determining the origin of pollution, so at best, your estimate will be only an approximation. Once the earlier locations of the air has been estimated, a knowledge of air pollution sources is needed. Figure 12 shows the sulfur dioxide emissions for the US counties and Canadian provinces while Figure 13 does the same for nitrogen oxides. Figure 14 shows the location of plants emitting sulfur dioxide. Phase 1 plants are high emitters and must clean up their emissions before phase 2 plants.
Figure 12 Sulfur Dioxide Emissions in the US in 1993 (EPA 1994)
Figure 13. Nitrogen Dioxide Emissions in the US in 1999 (EPA 2000)
Figure 14. Plants which are high emitters of Sulfur Dioxide (Phase 1) and lower emitters (Phase 2)
Other toxic impurities in the snow are in the form of particles. Individual toxic compounds can be hard to measure, but the total suspended particulate matter is easily measured and can be an indication of the likelihood of toxicity. Black specks in the snow show that impurities are present.
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