Saturday, December 28, 2013

Black and White

Frosty air stung my cheeks and my breath froze to my eyelashes as I zoomed down the trail. Fading sunlight and frigid temperatures haven’t kept me from skiing! An intricate pattern of blacks and whites decorated the thick winter woods. Each twig carried a shadow of white snow. Wind-driven snow accentuated the furrowed bark of trees.  Subtle shadows graced the drifted terrain. I was skiing through an Ansel Adams photograph.

The pale sunlight gradually faded behind the hills, and a gibbous moon rose to take its place. I barely noticed the change at first, since the moon (reflecting light from the sun back toward us) was so bright. During the long days of summer—when bedtime comes before twilight, and mosquitoes flock at dusk—I seldom get outside after dark. Skiing under the stars, with trees casting moon shadows across my path, was a lovely treat. The contrast of trees and snow made it easy to navigate. I left my headlamp in my pocket.

As my purple-mitten clad hands swung in an easy rhythm, I noticed their color fading toward charcoal gray, finally matching the color scheme of the forest. Despite being mostly limited to seeing in grayscale now, it was still impressive that I could see at all. According to the American Optometric Association (AOA), the sensing capabilities of the human eye reach across nine orders of magnitude. This means that the darkest light we can see is one-billionth as bright as the brightest light we can see.

Two different types of light receptors in our eyes make this possible. Our cone cells function in daylight, and give us color vision with a high resolution of detail (20/20 vision if we’re lucky). Rod cells work even in very dim light, but provide limited resolution (20/200 vision), and a black and white palate. Cones are clustered in the center of our retinas, while rods form a donut around the periphery of the retina.

(A freaky side effect of our rod and cone arrangement is that if you stare directly at a small object in the dark, it will disappear! A tree trunk might be severed with its top half floating, your friend’s face might go dark, or a star you just saw might blink out. These optical illusions are caused by a small blind spot in the middle of our night vision, where the cones would normally focus if they had sufficient light. If you just look slightly to the side, the objects will reappear.)

Humans have several additional adaptions that help us to see across such a wide spectrum of light levels. Our pupils can dilate to let in more light at night, or contract to physically protect our eyes from damaging amounts of light during the day. The diameter of the pupil can contract to 1.5mm and expand to 8mm, which equates to a 30-fold range in the quantity of light entering the eye. After skiing for more than half an hour in low light conditions, my pupils were probably widely dilated.

A chemical called rhodopsin is another key to our night vision. Rods use rhodopsin to absorb photons and perceive light by converting it to electrical activity, which initiates visual impulses in the brain. (Cones use their own specific photopigment in the same way.)
When you expose your eyes to bright light, photons actually split the rhodopsin into two other chemicals. This reduces the sensitivity of our retina and protects our eyes from damage to intense light. It also diminishes our ability to see in low light. (Have you ever noticed how turning on a flashlight temporarily ruins your night vision?) Although it takes several minutes, the chemicals eventually recombine into rhodopsin and our night vision returns.

The moonlight provided plenty of light for skiing. Rods can function even on an overcast night with no moon, and the high contrast between snow and trees gave me confidence in avoiding collisions. As moon shadows from the trees slipped by under my skis, I looked down to admire the patterns. I was surprised to find that my skis still looked red!

Back inside, I did a little research and discovered that with any color except red, as you turn down the lights, you can watch as first the color turns gray (cones stop working), and then you lose the sensation of light (rods stop working). This is called the photochromatic interval. With red, however, the color and sensation of light disappear at the same time. And, as it turns out, cones can function a little even with only 50% moonlight!

As wonderful as human vision is to me, I know that many animals see in ways that we can only imagine. Many critters have better night vision, can see more colors than we do, and can even see faster than humans. I will explore those super senses in a future article.

With Winter Solstice and the shortest days of the year behind us, we’ll soon have increasing amounts of light to see by. For now, our grayscale night vision melds perfectly with the black and white winter world. By summer, when our cones have 15 hours and 43 minutes of daylight to see by, the Northwoods will once again be full of color.

Friday, December 27, 2013

A Visit to the Hawkeye State

My journey home for the holidays used to mean a cross-country flight. Since I moved back to the Midwest, it means a long drive over some of the most beautiful rural highways in Wisconsin, Minnesota, and Iowa. Long ago, my dad entertained us on similar car trips by watching for raptors all across the Hawkeye State.

Red-tails on power lines, kestrels on road signs, harriers soaring over fields, eagles near the river, and owls on fence posts captured our attention. If the traffic was light and a field driveway was near, Dad would “flip a Louie,” get out his camera with the long lens (nicknamed “Big Bertha,”) and go back to see if he could get a good shot. We’d hold our breath and try not to wiggle the car. Sometimes the raptor was cooperative, and posed for a while, blinking in the sun. Sometimes they took off just as Dad was about to push the trigger on the motor drive.

My trip home for the holidays this year included many raptor sightings, and while I didn’t do any U-turns to photograph them, I admired their calm grace and their high hunting posts in roadside trees. It is hard to believe that birds of prey could be looking for mice from way up there, but I know they can see between four and eight times as well as us humans. According to one source, “If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building.”

How do they do that?

Accommodation is one trick birds use to focus on objects at a variety of distances. This simply means that tiny muscles around the eye alter the curve of the lens so that it can focus on objects that are far or near. It’s like using the focus wheel on binoculars. Humans share this adaptation for focusing. You may have noticed the time-delayed focus as you stare at an object to see it better and your muscles automatically curve the lens after a second. The amazing thing about raptors is that they can change the shaped of their cornea as well as their lens. This gives them an even more precise focus on the world.

I experienced the lack of accommodation when my holiday journey detoured to the eye doctor’s for my annual checkup. As I sat in the waiting room waiting for the drops to finish dilating my eyes, I noticed that I was having increasing difficulty reading the magazine. I asked, and Dr. Landis explained, that the numbing drops don’t just stop the muscles around my pupils from working, they also stop my accommodation muscles from working.

While squinting in the bright snow and frustrated at blurred vision on the way home from town, I was still able to spot a rough-legged hawk soaring over the corn stubble. Their feathered or “rough” legs are well suited for both summers in the Arctic, and the relatively mild winters of Iowa. Hunting on the wing presents some challenges, though, and I can’t imagine being able to see details on the ground from such a height. Raptors and other birds not only use muscles to focus their eyes, they also have more light-receptor cells to focus the images on.

Humans have a fovea, or focal point, at the center of our retina with 200,000 light-receiving cone cells per millimeter. This provides for our good color vision. Eagles (and other raptors), on the other hand, have about a million cones per millimeter in their central fovea. That gives them much higher resolution vision. Not only is their fovea packed more densely with cones, it is also deeper than ours, so it may act like a telephoto lens and give them extra magnification in the center of their field of view.

Moreover, raptors actually have TWO fovea. In addition to their central concentration of cones, they have a lateral fovea that allows them to keep the horizon and the ground in focus simultaneously.

All this extra visual resolution gives hawks and eagles somewhere between 20/5 and 20/2 vision. At best, the physical properties of human eyeballs limit us to 20/10 or 20/8 vision. In other words, what a normal person could see at eight feet, an exceptional person could see at twenty feet. Or, what a normal person could see at TWO feet, a hawk could see at TWENTY feet.

Scientists consider bird vision to be the finest in the animal kingdom, and raptors are at the top of their class. But raptors don’t lead the class in every respect. Smaller birds may see faster than raptors, and receive more colors. Migratory birds may be able to see polarized light and the Earth’s magnetic field. While owls are raptors, and do have excellent eyesight, their eyes are tuned for night vision.

Over the holiday, eagles and hawks soared past our windows on a regular basis. Each time, several sets of human eyes focused eagerly on the majestic visitor, still registering barely more than a blur of flight. It is fun to imagine just how differently various creatures can see the world. Wouldn’t it be fun if we could truly be in the hawk eye state?

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot,

Friday, December 13, 2013

There is No Place Like Home

Uncountable stars twinkled brightly as I stepped into the night. Head tilted back, I reveled in a moment of wonder. The fingernail Moon hung among the stars—a tiny sliver of light in the Universe. Venus, named after the Roman goddess of love and beauty, shone brightly just below the moon. As I inhaled deeply, my nostrils froze together, and the cold, dry air in my throat triggered a coughing fit. There went my moment of awe!

Winter stargazing means clear skies and frigid temperatures. The clearest nights are also the coldest. Why? All day, the Earth absorbs energy from the Sun through visible and infrared light. Blacktop roads soak up rays, dark trees warm up, even pale snow captures a little bit of the Sun’s energy. Then, all night long, the Earth radiates heat back up toward the sky.

Clouds are very efficient blankets for our planet. They can trap heat and radiate it back down toward the Earth. If there are no clouds, however, the heat escapes. So, clear nights get colder. In addition, cold air can’t hold as much moisture as warm air, so the number of water molecules standing between our eyes and the stars is reduced even further.

All of this makes winter a great time for contemplating Earth’s place in the Universe. Why are we standing here instead of on Venus or Mars? What makes the Earth special?

First, consider the Moon. While we might think of our Moon as just a lowly satellite revolving around us, it has been integral to the history of our planet. The Moon stabilizes the Earth’s rotation, which keeps our seasons (caused by a slight tilt of the Earth), from going to the extremes. Even its formation was important. Long ago (scientists think), collisions with asteroids flung lighter material away from the Earth, where they coalesced into the Moon. This means that our core is much denser than that of Venus. Today, Venus’s lower density means that its interior is entirely liquid, and oddly calm.

In contrast, the denser Earth has a swirling core that is part liquid and part solid. The movement of our core generates the Earth’s magnetic field. Without our magnetic field, we would be bombarded by harmful radiation from the Sun. Without a magnetic field, solar radiation drove away the water on Venus.

So, instead of fluffy white clouds of water vapor, Venus has opaque clouds of sulfuric acid, and a runaway greenhouse effect. (You could never stargaze on Venus!) The average temperature on Venus is now 864 degrees Fahrenheit. That almost sounds inviting when our mercury dips below zero!

Mars, on the other side, has a different problem. Mars's atmosphere is about 100 times thinner than Earth's, so it lacks a thermal blanket. The stars might always be bright on Mars, but the nights are always cold. During the day, the temperatures on Mars can reach a balmy 70 degrees Fahrenheit. However, its average temperature is -80 degrees, and the night-time poles can dip to -225 degrees.

What factors led to our “just right” temperatures and atmosphere? Our mass (not too big and hot like Jupiter, not too small and cold like Mars) is one parameter. The gravity on Earth doesn’t just press us back into our beds in the morning; it also tugs at the atmospheric blanket and prevents it from slipping away. Mars has a smaller mass than the Earth, and therefore lacks sufficient gravity to hold onto its atmosphere.

Our distance from the Sun is also ideal, since it partly controls how much radiation we get from our closest star. Not too far, not too close, we got it just right.
“…dear star, that just happens to be where you are in the universe to keep us from ever-darkness, to ease us with warm touching, to hold us in the great hands of light…” –Mary Oliver, from “Why I Wake Early.”
In the end, almost all the factors that make the Earth special relate to the presence of liquid water, and our ability to retain it. Liquid water, and the seasonal presence of solid-state water (hooray for fishable ice and skiable snow!), are certainly important in my life!

On that starry night, the scarcity of liquid water above me was both a blessing and a curse. The chance to gaze out at the sparkling Universe emphasized that there is no place like home…even if home is where your nostrils freeze together.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot,

Night Magic

Headlamp stretched over wool hat, I strapped on snowshoes (fresh from basement storage) and ventured into the snowy dark.

A few flakes glittered through the air, but mostly the snow clung to trees and heaped on the ground. My light reflected brightly off white in all directions as I walked along in its bubble. The only sounds came from my snowshoes--creaking, squeaking, and shuffling.

Trees leaned out over the driveway, reaching their ice-encased, snow-frosted twigs toward my path. The lower edges of the iced twigs were scalloped with frozen droplets. Falling temperatures had slowed the drips to stillness. Unable to resist, I licked some fluffy snow-frosting off a birch twig.

Amazed at the beauty caught in every movement of my headlamp, I swept the light around in a wider arc, taking in the intricate patterns of twigs, needles, and snow. A ways off in an open area, the light caught something brighter. Two green eyes shone back at me.

Eyeshine is caused by a layer of tissue called the tapetum lucidum (which means “bright tapestry” in Latin). This layer sits behind the retina, and increases the light available to the animal’s photoreceptors by reflecting visible light back through the retina. Deep sea creatures and nocturnal animals use the tapetum lucidum to increase their night vision.

I was hoping that the two green orbs belonged to the neighborhood bobcat, but as the eyes moved, I could just barely make out the profile of a deer against the snow. Even so, I couldn’t stop my brain from imagining a monster or goblin behind those glowing spheres.

Undaunted by my overactive imagination, I followed a wandering herd of snowed-in deer tracks out the driveway and onto the trail. A snow-laden balsam arched across the trail at waist height, its tip buried under the crust. I gently swung it forward like a gate, and entered a tunnel fit for dwarves

This section of trail sneaks through a thicket of balsam on an old road grade. It is always dark and narrow. Tonight, snowy balsam branches hung especially low and close as I bent down to shuffle through. The passageway heightened my sense of expectation and suspense, as if I really might pop out into the Narnian Empire at any time.

Instead, the trail entered a spacious hemlock grove. As the trees opened up, the dark closed in. Through the open understory, I caught the shining green eyes of four more deer. As they bounded away, a soft whisper of wind tinkled through the treetops. Snowflakes drifted down. The whisper crescendoed to a rush of air, and bigger clumps of snow fell, plopping all around me. As I put up my hood and leaned toward the trunk of a large hemlock, I imagined Ents in a snowball fight. When the dull thumps of falling snow had subsided, I continued on through the aftermath of drifting clouds of crystals.

A cute string of mouse tracks made me grateful for another sign of life. Most small mammals are hiding out under the thick, fresh snow, where a new world has just developed beneath our feet. This ephemeral habitat is called the subnivean layer.

The subnivean layer, like so much of life on Earth, owes its existence to the unique chemistry of water. When frozen, water becomes light and airy, a wonderful insulator. Just as down feathers in your jacket trap a layer of air next to your body, retaining the heat you radiate, a six-inch layer of snow traps air that retains heat from the Earth.

Because of this insulation and radiating heat, a thin zone opens up under the snow, right at the surface of the ground, which stays at a pretty stable 32 degrees Fahrenheit. This becomes even more important as the temperature plunges into the single digits, and then below zero. Without snow to insulate the ground, frost burrows more deeply.

Tree roots, invertebrates, and the myriad little critters in the upper reaches of the soil suffer in cold, dry winters. Snow provides not only provides warmth, it also facilitates easy access to food, and gives cover from predators. “To the mouse, snow means freedom from want and fear,” wrote Aldo Leopold in A Sand County Almanac.

Red squirrels also use the subnivean layer to escape want and fear. In the fall, they cache seeds near the ground, and then use their great sense of smell to find them again, even below four meters of snow. During cold spells, squirrels will dig themselves a little snow den and hang out near their pantry, safe from the searching eyes of predators. I stepped over a small hole excavated by a hungry squirrel, and peeked down into the dry, leafy carpet of the subnivean zone. His tracks still ran across the top of the snow, but with temperature forecasted to plunge, my guess is that he will take a dive, too.

I emerged from the woods onto the gravel road and turned off my light. The world went gray. A grove of balsams—their drooping branches and conical shape perfectly adapted to the heavy snow—stood like statues in the White Witch’s courtyard. A rosy-pink glow in the northern sky gave an otherworldly aura to the night.

Beams of warm yellow light beckoned me back inside, but I hesitated, reluctant to leave this magical world (my snowy backyard) behind.

“And if you have not been enchanted by this adventure-Your life-What would do for you?” –Mary Oliver

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot,

Minerals all around us

Is it frozen yet? Every morning for the past few weeks, I’ve peered into the gray dawn, looking for the absence of waves on the lake. If there wasn’t a choppy surface, then I stared harder still, trying to determine if the reflection was due to calm water, or a skim of ice. One morning the pink clouds reflected on both ice and liquid, as a thin sheet filled my bay. The next morning, I couldn’t see any open water.

I gave the ice another frigid day and night before venturing down to its edge. Wind had piled that first layer of ice up against the shore, and now the round pancakes of ice chunks were frozen together in a jumbled mess. Cautiously, I stepped out away from the shore, and stopped a minute to listen for cracking, and feel for shifting. Solid. I shuffled out toward the smooth ice.

Two inches of clear ice covered the deeper water. Silvery bubbles and crystalline patterns decorated the black surface. Just under the ice, little blobs of mint-green algae bobbed gently, barely moving in a gentle current. Even as the ice was freezing, these little producers had been carrying out photosynthesis and releasing oxygen bubbles, which left shimmering vertical paths in the ice as they rose toward (but never reaching) the surface.

Strange as it may seem, the surface of my lake has just become a mineral. Mineral is a geologic term, describing solid substances with certain characteristics. Minerals are all around us, but mostly overlooked. Let’s take a closer look at ice to see how it fits.

The first criterion in the definition of a mineral is that it must be a natural occurring, inorganic substance. The ice on my lake is natural occurring, of course, but the ice cubes in my freezer are not. Diamonds made in laboratories are not technically minerals, either. Ice doesn’t contain carbon and is not derived from living things, which means it is also inorganic.

Minerals must be crystalline solids, meaning that they have to have an orderly internal arrangement of atoms. This means that water is not a mineral, the same way that volcanic lava is not a mineral, since they are both liquids. As they cool, though, their molecules arrange themselves into distinct geometric patterns. You can see this in the hexagonal symmetry of snowflakes.

Each mineral is made of a particular mix of chemical elements, and has a definite chemical composition. The chemical composition of ice is, of course, two hydrogen atoms and one oxygen atom per molecule, written as H2O. Apatite, a mineral in your tooth enamel, is more complicated, and contains set amounts of three elements, plus variable amounts of four more.

Speaking of appetite, we are surrounded by minerals not only on a frozen lake, but also at the dinner table (especially if we relax the definition of mineral to include human-altered forms). The salt in your shaker is tiny crystals of the mineral halite (also called sodium chloride, NaCl,). The leavening in your pumpkin bread is sodium bicarbonate (NaHCO3). Both the pumpkin and the turkey contain small amounts of calcium and iron.

As you dig into the food on your plate (ceramic plate containing feldspar and silica), with your grandmother’s set of heirloom flatware (silver) or your regular silverware (made of iron, nickel, molybdenum, chrome, strontium, and neodymium), be careful not to chip a tooth filling (gold)!

Give thanks for the screws (iron and zinc) that hold the feasting table together, the drywall (gypsum) that forms your cozy room, and the insulation (vermiculite) that keeps you warm. Don’t forget the electric wires (copper) that power your light bulbs (tungsten, silica), and the plumbing (copper) that carries the remains of the feast away.

As you settle your stomach onto the couch with a swig of pepto (bismuth), and maybe crack a beer can (aluminum), I encourage you to give minerals a second thought. They are all around us, bringing the beauty of snowflakes, the fun of ice fishing, and the grace of good health.

“When we cut the ripe [pumpkin], should we not give it thanks? And should we not thank the knife also? We do not live in a simple world.”

From “At the River Clarion,” by Mary Oliver.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot,