Nature Watch
November 30, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
Winter has arrived, at least in the form of sustained cold temperatures. Many of us who live in the north enjoy the fact that we get four seasons, so bundle up and get outside if you can! Freezing temperatures tend to slow things down, but by no means do they bring wildlife activity to a stop. In fact, if you’ve always wanted to try birding, winter is a great time to start because there are fewer species around and they tend to congregate around feeders.
If you’re near a lake or river, you might also see birds congregating on open, ice-free water. Before everything freezes up, you might chance to see a swan, for example. Who can forget the classic tale of the ugly duckling, where a homely little chick grows into a beautiful white swan? That “duckling” in the story was no duck, of course, but was really a Mute Swan chick passing through its natural brownish phase before turning white.
The Mute Swan, commonly found along the East Coast and in parts of the Midwest, is the bird typically featured in artwork and folklore as a symbol of grace and beauty. However, many people consider this nonnative bird undesirable in North America because it harasses native waterfowl and can uproot large quantities of aquatic vegetation.
Mute Swans were brought to the United States from Eurasia in the 1800s as ornamental additions to estates, parks and zoos. Over the years, many were released or escaped captivity and, by the 1970s, a resident population had established itself in Wisconsin and has been growing ever since.
By contrast, native Trumpeter Swans were almost wiped out during the nineteenth century when they were hunted for their meat and feathers. Swan skins were sold in the fur trade to Europe where they were used to make ladies’ powder puffs and feathers were used to adorn fashionable hats. By 1900, it was widely believed that the species had become extinct. Fortunately, a small nonmigratory population of Trumpeters survived in remote mountain valleys of Montana, Idaho, and Wyoming. In the 1930s, a national effort was organized to protect these birds and in the decades since to reestablish populations in other states.
In 1989, trumpeter swans were reintroduced to Wisconsin and placed on the state endangered species list. State natural resource agencies in Minnesota and Michigan had also initiated Trumpeter Swan recovery programs in the 1980s, and together, the three states are now establishing flocks that will help create a migratory and breeding population in the Midwest.
Both the Trumpeter and Mute Swans can be found in marshes, lakes and prairies, and both are about the same size, with a wingspan of up to eight feet and weight of up to 30 pounds.
How can you tell the difference? The Mute Swan is best identified by its orange bill and prominent black fleshy knob extending from the base of the bill to the forehead. Additionally, Mute Swans typically hold their necks in an S-curve with the bill pointed downward. Contrary to its name, this bird is not silent, but utters a variety of calls, including grunts and snorts. By contrast, the Trumpeter Swan has a broad, black bill and holds its neck erect and head upright. True to its name, the Trumpeter’s voice sounds like a horn.
Start a winter birding journal this year! Chickadees, juncos, and nuthatches should easily get you started. Build your birding skills and confidence, then try to identify and get familiar with 10 bird species this winter.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
Friday, November 30, 2007
Friday, November 23, 2007
Snowshoe Hare
Nature Watch
November 23, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
I saw the first white snowshoe hare of the season today. I was walking down near the Namekagon River when I noticed this white lump just inside the tree line along the road. There were still a few brown spots on the shoulder and back, but the hare had otherwise changed over to his beautiful, pure white winter coat that will eventually help it to disappear against the winter snow.
The change in fur color from brown to white in the fall and back again in the spring is just one difference between hares and rabbits, but it is the most easily observed. Because of this unique transition, the snowshoe hare is also known as the “varying hare.” Why do hares molt this way and rabbits do not? The answer is mostly related to climate.
The geographic ranges of hares and rabbits overlap here in Wisconsin, but hares are considered a northern species, while rabbits are a southern species. Farther north, the durations of summer and winter are more equal, so hares change colors to match their environment. The same is true of other far northern species, such as the weasels (which are also found in Wisconsin), ptarmigan, arctic fox, and collared lemmings. But the change in color has to also keep the hare warm, and white is typically known as a color that reflects sunlight. The answer to this riddle is that white in the natural world is not a color but is the absence of color, or pigment. The cells in white hairs are empty of pigment and are instead filled with air, which provides thermal insulation. The change from brown to white fur begins as the amount of daylight decreases. The ears and feet are the first to change, and the whole animal is white after about 10 weeks.
Snowshoe hares (waabooz in Ojibwemowin) are found in the northern half of Wisconsin where they prefer spruce and cedar swamps and other thickly-vegetated coniferous woodlands. Up to two litters of one to seven (usually two to four) hares are born each year between May and August. Hares primarily feed on grasses, tree buds, and other plant material, but they are also known to feed on the meat of dead animals, including other hares. They do not kill other animals, but they will scavenge carcasses whose insides have been exposed. Main predators of snowshoe hares in Wisconsin include weasels, gray and red foxes, coyotes, wolves, bobcats, and mink.
A popular topic of study for biologists has been the dramatic fluctuations of snowshoe hare populations. Hares are known to go from periods of great abundance (almost overabundance) to periods when there seem to be none left anywhere. The severity of the fluctuation varies geographically, and this is thought to be caused by the relative number of other species that occupy the same level as the hare in the food chain. In other words, if there are fewer links in the food chain (such as in far northern latitudes), and the hare is a main source of food for larger predators, than over-consumption by predators can cause the hare population to decline sharply, and the predator population will soon follow. On the other hand, if there are other animals (such as cottontail rabbits here in Wisconsin) for predators to feed on, then the hare population does not change so dramatically. It is estimated that as many as 85% of hares do not live for more than one year. The common life span for those that do survive is five years.
The change of seasons is upon us. The varying hare has shown that winter is indeed on the way, even if we do not see much snow yet. But the lack of snow makes it easier for us to see this fascinating animal if we look hard enough.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
November 23, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
I saw the first white snowshoe hare of the season today. I was walking down near the Namekagon River when I noticed this white lump just inside the tree line along the road. There were still a few brown spots on the shoulder and back, but the hare had otherwise changed over to his beautiful, pure white winter coat that will eventually help it to disappear against the winter snow.
The change in fur color from brown to white in the fall and back again in the spring is just one difference between hares and rabbits, but it is the most easily observed. Because of this unique transition, the snowshoe hare is also known as the “varying hare.” Why do hares molt this way and rabbits do not? The answer is mostly related to climate.
The geographic ranges of hares and rabbits overlap here in Wisconsin, but hares are considered a northern species, while rabbits are a southern species. Farther north, the durations of summer and winter are more equal, so hares change colors to match their environment. The same is true of other far northern species, such as the weasels (which are also found in Wisconsin), ptarmigan, arctic fox, and collared lemmings. But the change in color has to also keep the hare warm, and white is typically known as a color that reflects sunlight. The answer to this riddle is that white in the natural world is not a color but is the absence of color, or pigment. The cells in white hairs are empty of pigment and are instead filled with air, which provides thermal insulation. The change from brown to white fur begins as the amount of daylight decreases. The ears and feet are the first to change, and the whole animal is white after about 10 weeks.
Snowshoe hares (waabooz in Ojibwemowin) are found in the northern half of Wisconsin where they prefer spruce and cedar swamps and other thickly-vegetated coniferous woodlands. Up to two litters of one to seven (usually two to four) hares are born each year between May and August. Hares primarily feed on grasses, tree buds, and other plant material, but they are also known to feed on the meat of dead animals, including other hares. They do not kill other animals, but they will scavenge carcasses whose insides have been exposed. Main predators of snowshoe hares in Wisconsin include weasels, gray and red foxes, coyotes, wolves, bobcats, and mink.
A popular topic of study for biologists has been the dramatic fluctuations of snowshoe hare populations. Hares are known to go from periods of great abundance (almost overabundance) to periods when there seem to be none left anywhere. The severity of the fluctuation varies geographically, and this is thought to be caused by the relative number of other species that occupy the same level as the hare in the food chain. In other words, if there are fewer links in the food chain (such as in far northern latitudes), and the hare is a main source of food for larger predators, than over-consumption by predators can cause the hare population to decline sharply, and the predator population will soon follow. On the other hand, if there are other animals (such as cottontail rabbits here in Wisconsin) for predators to feed on, then the hare population does not change so dramatically. It is estimated that as many as 85% of hares do not live for more than one year. The common life span for those that do survive is five years.
The change of seasons is upon us. The varying hare has shown that winter is indeed on the way, even if we do not see much snow yet. But the lack of snow makes it easier for us to see this fascinating animal if we look hard enough.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
Wednesday, November 14, 2007
Snow Buntings
Nature Watch
November 14, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
The snow buntings have arrived. You may have noticed the flocks of “little brown birds” along road shoulders and in open fields that are more white than brown. They are hard to see when they are on the ground, but when the flock takes flight, it looks as if a tuft of snow has blown up in the wind.
Snow buntings (Plectrophenax nivalis) are common winter visitors to the northern United States and the higher elevations of northern Colorado and Utah. They come to us from above the Arctic Circle, where they are found in Alaska, across northern Canada, Greenland, Iceland, and very northern Eurasia. They nest on tundra, rocky coastlines, and in the crevices and cracks of rocky mountain slopes. When they are here, we find them feeding on the seeds of grasses and other plants along roads, in fields, and along lakeshores, places most like the open tundra they are accustomed to further north. They are usually seen in flocks, often mixed with other winter birds such as horned larks.
Snow buntings are well-named. The species name nivalis comes from the Latin word, nivis, meaning “snow.” Adults have pure white heads, breasts, and bellies. Over the rest of the body, white is broken up by black on the back, shoulders, wing tips, and tail during the breeding season. Some of this coloration is retained over the winter, but when we see them here, the black has given way to more white on the back and tail along with some light tan on the head and back. Still, the white coloration is very striking, making this little bird difficult to confuse with any of the other winter finches.
Snow buntings are also well-named because, in our region, they portend the arrival of snow. Usually, when the snow buntings arrive, we see snow mixed with the rain or blowing in the cold north wind within a week. This year, just three days passed between my first sighting of the buntings and the first falling of snow. This is in contrast to the Koyukon people of northwestern Alaska who regard snow buntings as a sign of spring, “the certain proof that winter is vanquished at last,” according to writer and anthropologist Richard Nelson. But even in that far land, snow buntings are only visitors that “twitter over the drifts in April and vanish to places farther north a few weeks after [they] appear.”
How do such small birds survive in a bitterly cold place like the Arctic tundra? In her book, For the Birds, Laura Erickson writes that snow buntings can easily survive temperatures of 40 degrees below zero. But during periods of severe cold, the snow bunting, like the ruffed grouse, will stay warm by diving into a snow bank and burrowing in. Also, the black feathers on its back help the bunting to retain heat from the sun. Think of when you wear a black shirt on a sunny day. The black color absorbs light and heat, keeping you warmer than if you were wearing a lighter-colored shirt. Throughout the winter, buntings will use their bills to preen their feathers and they will “snow-bathe” flapping and rubbing in the snow. This actually causes the white and tan colors on their feathers to wear off, exposing the black feathers beneath. By spring, when they head north, the snow bunting is once again brilliantly black and white.
Snow buntings are just one of the winter visitors to our region. Others include the horned lark, common redpoll, pine siskin, and bohemian waxwing (the larger cousin of our resident cedar waxwing). Other, larger birds such as owls attract more attention, but the smaller snow bunting is one of the first to arrive, a special category for a beautiful little bird.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
November 14, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
The snow buntings have arrived. You may have noticed the flocks of “little brown birds” along road shoulders and in open fields that are more white than brown. They are hard to see when they are on the ground, but when the flock takes flight, it looks as if a tuft of snow has blown up in the wind.
Snow buntings (Plectrophenax nivalis) are common winter visitors to the northern United States and the higher elevations of northern Colorado and Utah. They come to us from above the Arctic Circle, where they are found in Alaska, across northern Canada, Greenland, Iceland, and very northern Eurasia. They nest on tundra, rocky coastlines, and in the crevices and cracks of rocky mountain slopes. When they are here, we find them feeding on the seeds of grasses and other plants along roads, in fields, and along lakeshores, places most like the open tundra they are accustomed to further north. They are usually seen in flocks, often mixed with other winter birds such as horned larks.
Snow buntings are well-named. The species name nivalis comes from the Latin word, nivis, meaning “snow.” Adults have pure white heads, breasts, and bellies. Over the rest of the body, white is broken up by black on the back, shoulders, wing tips, and tail during the breeding season. Some of this coloration is retained over the winter, but when we see them here, the black has given way to more white on the back and tail along with some light tan on the head and back. Still, the white coloration is very striking, making this little bird difficult to confuse with any of the other winter finches.
Snow buntings are also well-named because, in our region, they portend the arrival of snow. Usually, when the snow buntings arrive, we see snow mixed with the rain or blowing in the cold north wind within a week. This year, just three days passed between my first sighting of the buntings and the first falling of snow. This is in contrast to the Koyukon people of northwestern Alaska who regard snow buntings as a sign of spring, “the certain proof that winter is vanquished at last,” according to writer and anthropologist Richard Nelson. But even in that far land, snow buntings are only visitors that “twitter over the drifts in April and vanish to places farther north a few weeks after [they] appear.”
How do such small birds survive in a bitterly cold place like the Arctic tundra? In her book, For the Birds, Laura Erickson writes that snow buntings can easily survive temperatures of 40 degrees below zero. But during periods of severe cold, the snow bunting, like the ruffed grouse, will stay warm by diving into a snow bank and burrowing in. Also, the black feathers on its back help the bunting to retain heat from the sun. Think of when you wear a black shirt on a sunny day. The black color absorbs light and heat, keeping you warmer than if you were wearing a lighter-colored shirt. Throughout the winter, buntings will use their bills to preen their feathers and they will “snow-bathe” flapping and rubbing in the snow. This actually causes the white and tan colors on their feathers to wear off, exposing the black feathers beneath. By spring, when they head north, the snow bunting is once again brilliantly black and white.
Snow buntings are just one of the winter visitors to our region. Others include the horned lark, common redpoll, pine siskin, and bohemian waxwing (the larger cousin of our resident cedar waxwing). Other, larger birds such as owls attract more attention, but the smaller snow bunting is one of the first to arrive, a special category for a beautiful little bird.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
Friday, November 9, 2007
Winter Questions
Nature Watch
November 9, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
Well, we’ve got our first taste of it and more is around the corner: snow. Love it or hate it, snow is part of our lives here in the north, so let’s take a moment to think about and appreciate some of its unique properties.
Why is snow white?
Most natural materials absorb some wavelengths of sunlight and reflect others, which in turn gives these materials their color. Snow reflects most of the sunlight—and visible sunlight is white. The complex structure of snow crystals results in countless tiny surfaces from which visible light is reflected.
How big can snowflakes get?
Snowflakes are aggregations of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions—usually involving near-freezing temperatures, light winds, and unstable atmospheric conditions—much larger and irregular flakes close to two inches across can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.
Is it ever too cold to snow?
It can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground—typically 15°F or warmer. Warmer air can hold more water vapor, and that vapor is what makes snow.
When is it too warm to snow?
Snow forms when the atmospheric temperature is at or below freezing and there is little moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground. However, snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake. This cooling retards melting. As a general rule, though, snow will not form if the groud temperature is 41°F or warmer.
Is it true that there is one inch of water in every ten inches of snow that falls?
The water content of snow is quite variable. While many snows that fall at temperatures close to 32°F and snows accompanied by strong winds do contain approximately one inch of water per 10 inches of snowfall, that ratio is not always accurate. Ten inches of fresh snow can contain as little as one-tenth of an inch of water or as much as four inches of water, depending on the snow’s crystal structure, the wind speed, temperature, and other factors.
Why is snow a good insulator?
Fresh, undisturbed snow is composed of a high percentage of air trapped among the lattice structure of the accumulated snow crystals. Since the air can barely move, heat transfer is greatly reduced. Fresh, uncompacted snow typically is 90-95 percent trapped air.
Why do weather forecasters have so much trouble forecasting snow?
Snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make huge differences in snow forecasts.
Why does snow crunch when you step on it?
A layer of snow is composed of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer—a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures—closer to melting—this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It’s difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
November 9, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
Well, we’ve got our first taste of it and more is around the corner: snow. Love it or hate it, snow is part of our lives here in the north, so let’s take a moment to think about and appreciate some of its unique properties.
Why is snow white?
Most natural materials absorb some wavelengths of sunlight and reflect others, which in turn gives these materials their color. Snow reflects most of the sunlight—and visible sunlight is white. The complex structure of snow crystals results in countless tiny surfaces from which visible light is reflected.
How big can snowflakes get?
Snowflakes are aggregations of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions—usually involving near-freezing temperatures, light winds, and unstable atmospheric conditions—much larger and irregular flakes close to two inches across can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.
Is it ever too cold to snow?
It can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground—typically 15°F or warmer. Warmer air can hold more water vapor, and that vapor is what makes snow.
When is it too warm to snow?
Snow forms when the atmospheric temperature is at or below freezing and there is little moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground. However, snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake. This cooling retards melting. As a general rule, though, snow will not form if the groud temperature is 41°F or warmer.
Is it true that there is one inch of water in every ten inches of snow that falls?
The water content of snow is quite variable. While many snows that fall at temperatures close to 32°F and snows accompanied by strong winds do contain approximately one inch of water per 10 inches of snowfall, that ratio is not always accurate. Ten inches of fresh snow can contain as little as one-tenth of an inch of water or as much as four inches of water, depending on the snow’s crystal structure, the wind speed, temperature, and other factors.
Why is snow a good insulator?
Fresh, undisturbed snow is composed of a high percentage of air trapped among the lattice structure of the accumulated snow crystals. Since the air can barely move, heat transfer is greatly reduced. Fresh, uncompacted snow typically is 90-95 percent trapped air.
Why do weather forecasters have so much trouble forecasting snow?
Snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make huge differences in snow forecasts.
Why does snow crunch when you step on it?
A layer of snow is composed of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer—a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures—closer to melting—this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It’s difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
Friday, November 2, 2007
Winter Coming
Nature Watch
November 2, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
Chilly fall days give us a warning that winter is coming, and in response plants and animals (including we humans) adjustment accordingly.
The changing of the season also affects the behavior of bodies of water. You may have heard people referring to lakes “turning” this time of year, and wondered what that means.
It’s a twice-yearly phenomenon related to water and air temperature—here’s how it works: In late summer, lake surface waters reach their annual maximum temperatures. Deeper waters are cooler—in many lakes, there is a definite stratification or layering of water temperatures that you would feel if you were swimming on the surface and then dove down deep. The warmest, least dense waters lie on top; water temperature decreases with depth, reaching its minimum temperature at the greatest lake depths.
In the summer, a deep lake will have three layers in the water column: the upper, warmest water (the epilimnion); a thin middle layer, where temperatures rapidly decrease (the thermocline or mesolimnion); and the deepest, coldest water (hypolimnion).
In autumn, cooler air temperatures and diminished hours of sunlight result in a loss of heat from the lake’s upper water layer. As these waters cool, they become more dense and when they reach about 50 degrees F, they sink into the middle layer below, erasing the temperature stratification that had developed during summer.
Eventually, all the lake water reaches a uniform temperature, and surface winds then mix all the water. When the winds are strong and fairly constant in direction for an extended time, they establish a water circulation pattern—as surface waters are blown downwind, waters from below must rise along the upwind shore to replace those waters pushed across the surface. To complete the circuit, the downwind shore surface waters, piled up by the wind, sink to replace the rising bottom waters.
In time, the resulting circulations will completely overturn and mix all the lake’s water—hence “fall turning.” The phenomenon can at times produce a rotten-egg odor, since the deep waters, which are low in oxygen and high in sulphur, rise to the surface and release sulphurous gases into the air. The turnover also mixes atmospheric oxygen into the lake water, replenishing the oxygen in deep waters and allowing fish to return to the depths where many will spend the winter.
Of course, after the fall turning cools the lakewater and as winter approaches, surface waters approach the freezing mark. Unlike most compounds, water reaches its maximum density as a liquid just before becoming a solid. Under normal conditions, freshwater is most dense at 39 degrees F, and ice, being less dense than liquid water, floats. So as lake waters cool, they sink when they reach 39 degrees. Colder water remains above, and is eventually covered with ice.
In the spring, the cycle happens again in reverse. Ice cover melts, and cold surface waters warm until they reach the temperatures of the bottom waters. Winds blowing over the lake again set up a full circulation system; as warming continues, the three water layers again become established, and a full turn of the cycle is complete.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
November 2, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
Chilly fall days give us a warning that winter is coming, and in response plants and animals (including we humans) adjustment accordingly.
The changing of the season also affects the behavior of bodies of water. You may have heard people referring to lakes “turning” this time of year, and wondered what that means.
It’s a twice-yearly phenomenon related to water and air temperature—here’s how it works: In late summer, lake surface waters reach their annual maximum temperatures. Deeper waters are cooler—in many lakes, there is a definite stratification or layering of water temperatures that you would feel if you were swimming on the surface and then dove down deep. The warmest, least dense waters lie on top; water temperature decreases with depth, reaching its minimum temperature at the greatest lake depths.
In the summer, a deep lake will have three layers in the water column: the upper, warmest water (the epilimnion); a thin middle layer, where temperatures rapidly decrease (the thermocline or mesolimnion); and the deepest, coldest water (hypolimnion).
In autumn, cooler air temperatures and diminished hours of sunlight result in a loss of heat from the lake’s upper water layer. As these waters cool, they become more dense and when they reach about 50 degrees F, they sink into the middle layer below, erasing the temperature stratification that had developed during summer.
Eventually, all the lake water reaches a uniform temperature, and surface winds then mix all the water. When the winds are strong and fairly constant in direction for an extended time, they establish a water circulation pattern—as surface waters are blown downwind, waters from below must rise along the upwind shore to replace those waters pushed across the surface. To complete the circuit, the downwind shore surface waters, piled up by the wind, sink to replace the rising bottom waters.
In time, the resulting circulations will completely overturn and mix all the lake’s water—hence “fall turning.” The phenomenon can at times produce a rotten-egg odor, since the deep waters, which are low in oxygen and high in sulphur, rise to the surface and release sulphurous gases into the air. The turnover also mixes atmospheric oxygen into the lake water, replenishing the oxygen in deep waters and allowing fish to return to the depths where many will spend the winter.
Of course, after the fall turning cools the lakewater and as winter approaches, surface waters approach the freezing mark. Unlike most compounds, water reaches its maximum density as a liquid just before becoming a solid. Under normal conditions, freshwater is most dense at 39 degrees F, and ice, being less dense than liquid water, floats. So as lake waters cool, they sink when they reach 39 degrees. Colder water remains above, and is eventually covered with ice.
In the spring, the cycle happens again in reverse. Ice cover melts, and cold surface waters warm until they reach the temperatures of the bottom waters. Winds blowing over the lake again set up a full circulation system; as warming continues, the three water layers again become established, and a full turn of the cycle is complete.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.
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