The Nature of Life

Rainbows: Creatures of Raindrop & Sunlight

Wed, 3 Sep 2008 16:41:08 -0600

Rainbows were once so common on summer afternoons that our town's stretch of U.S. Highway 50 was named Rainbow Boulevard. But in this extraordinarily dry summer, rainbows have been as rare as rain.
In early August though, a spectacular rainbow appeared like a gift in the gray curtains of a distant shower as we watched from the deck of the yurt where we were camping with my family in celebration of my dad's 80th birthday.
The sun eased toward the western horizon, slipping under the gathering clouds, and throwing a glowing arc over a nearby ridge. As the shower intensified, so did the bands of the rainbow.
It began as a short arc, but as the shower moved toward us, the arc grew with it until the rainbow stretched over the whole valley. Just as the first few drops plopped on the deck where we lounged, the sun slid over the horizon and both rain storm and rainbow dissipated.
What makes the glowing arc of vividly colored light we call a rainbow?
A rainbow is a creature of water and sunlight. Water droplets act like prisms: when sunlight enters a droplet, the water splits the light into its component rays, which we see as the visible bands of colors from red to violet.
The light actually splits twice, first as it enters the droplet, where the color spectrum is reflected off the curving wall of the back of the water droplet. The second refraction, where the light is split into individual waves again, happens as the light exits the water droplet.
What makes that prismatic effect into a rainbow isn't just one water droplet: it takes lots of them, each one refracting and reflecting the light.
Those water droplets can come from a rainstorm, a waterfall, or the spray of a hose or fountain. Once the haze of water droplets is created, the only remaining requirements for seeing a rainbow are the angle of the sun and the placement of the viewer.
To see a rainbow you must be between the sun and the water droplets. For example, if the rainstorm is in front of you, the sun must be at your back.
That explains why you won't see a rainbow if the sun is directly overhead. (At least not from earth's surface--you could see one if you were airborne.) Thus, rainbows are most common either early or late in the day.
Why are the colors always arranged from red to violet? The lowest-frequency light (the light having the widest distance between the peaks of its wavelength form) is always on the outside of a rainbow, and the highest-frequency light, on the inside.
Hence the range from red (the lowest-visible frequency) on the outside of the rainbow's arc to orange, yellow, green, blue, indigo, and violet (the highest-visible light frequency) on the inside. Except when the rainbow doubles.
Then the inner or primary rainbow (which is the brightest because it results from the first reflection of the light rays) is in the usual order, but for the second rainbow, literally a mirror image of the first, the color spectrum reverses.
The larger the water droplets, the brighter and more well-defined the bands in the rainbow.
But as droplets grow larger, their spherical shape is more likely to be flattened by collisions with other droplets. Flattening distorts the reflection which produces the rainbow, making the sides brighter than the top.
Watching that August rainbow grow from just a suggestion of color against the gray rain showers to a complete arch encompassing the whole valley, I wasn't thinking of the physics of light though: all I knew was the miracle of rain lit by the setting sun as my family gathered to celebrate my Dad's birthday.
Copyright 2008 Susan J. Tweit First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.

White Pelicans Bring Joy

Wed, 27 Aug 2008 17:11:41 -0600

A friend was biking past Sands Lake when she spotted a huge white bird riding the quiet water. The bird's size, she said, was "enormous"; it's shape, a curious combination of graceful and prehistoric, an apt description of an American white pelican.
White pelicans are spectacular, whether they are soaring overhead on nine-foot-wide wings or paddling quietly on the water's surface in search of a meal. Their pure white plumage with jet-black flight feathers and outsized orange beaks are unmistakable.
Pelicans are fish-catchers: they use the stretchy skin of the pouch under those beaks as a scoop to hold their catch. White pelicans fish while swimming, "dipping" for food as they paddle along.
They even fish cooperatively: a whole flock swims in a line or a semi-circle, beating wings and dipping bills in synchrony to herd fish into the shallows. They also fish at night, locating their meal--which includes crayfish and salamanders as well as fish--by touch with sensitive bills.
Once a white pelican has a beak-full of catch, it tips the two-gallon-capacity pouch forward to drain off the water before swallowing its food, like draining a pot of pasta. (How long, I wonder, does it take juvenile pelicans to learn how to dump out the water without losing the meal?)
Most species of pelicans are coastal birds. American white pelicans, however, summer inland in lakes and reservoirs from Colorado north to northern Manitoba, and from the Dakotas west to northeastern California.
In winter these big birds migrate south to warmer climes, including playa lakes in northern Mexico, and the coasts of Baja California and the Gulf of Mexico. Come spring, they flap back north again, arriving in Colorado in late April and early May.
In this state, their nesting colonies are on islands at Riverside Reservoir on the northeastern plains, McFarlane in North Park, and Antero Reservoir in South Park.
These huge birds court by strutting with heads erect and bills down, bowing to each other, and soaring so close together that their enormous wings nearly touch.
Nest-building is simple: a pelican sits on the ground and uses its bill to rake nearby sand and debris into a mound around its belly. The female lays one or two eggs in the depression in the center of the mound; both parents incubate the eggs by holding them under the extra-large webs of their feet.
The eggs hatch a month later and the parents trade off feeding duties. One stays with the chicks, while the other flies as far as thirty miles away to fish, returning with the day's take-out.
By the time they are ten or eleven weeks old, the gawky chicks can fly and have begun to fish for themselves.
Until they reach breeding age--about three years old--these birds wander widely, summering wherever they find good fishing. Hence the solo white pelican our friend spotted at Sands Lake.
In his book The Abstract Wild, mountain-climber Jack Turner recounts his surprise when he spied a flock of American white pelicans circling so high above the summit of Grand Teton in Wyoming that all he could see were "tiny glints, like slivers of ice . . . visible, then invisible, then visible again as the sheen of their feathers strikes just the right angle to the sun."
Why were these huge birds circling miles high in the Wyoming sky on a July afternoon? After ruling out any practical reason, Turner concludes the pelicans were riding the updrafts for much the same reason he climbs mountains: for the joy of it.
Why not? Perhaps the joy we feel when we see pelicans flying or floating with such grace simply reflects how these enormous birds inhabit this world.
Copyright 2008 Susan J. Tweit First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.

Welcoming Native Bees to the Garden

Wed, 20 Aug 2008 14:02:10 -0600

In these late-summer days, our yard is abuzz with butterflies big and small, the jewel-bright forms of hummingbirds, and several dozen kinds of bees. The latter aren't the aggressive stinging insects many fear, either bright yellow wasps and hornets with their painful sting or touchy, imported honeybees.
Or bees are natives, unassuming, easy-to-get-along-with insects vital to North America's farms and wildlands.
The 4,000 or so species of bees native to this continent pollinate wildflowers and many crop plants as well, including tomatoes, fruit and nut trees, squashes, melons, and blueberries. Without bees, these plants would produce no seeds or fruit--no food.
Unlike imported honeybees and Africanized bees, our native bee species rarely sting. They are content to go about their business collecting pollen and nectar to feed their young, and in the doing, fertilizing the flowers they visit, undisturbed by humans nearby.
Among the several dozen kinds of bees in our yard, two are particularly easy to watch, and their lives exemplify the diversity of native bees.
When I water the garden first thing in the morning, the huge yellow flowers on our summer squash vines are crammed with sleepy black bees ringed with white pin-stripes. These bees that spent the night in the closed blossoms are squash bees.
Like most native bees, squash bees are solitary-nesters. That is, they don't live in colonies like imported honeybees.
A female squash bee digs her own nest in the soil, excavating a vertical tunnel ending in several thimble-shaped chambers. She stocks each chamber with a pea-sized ball of protein- and fat-rich pollen, and then lays an egg atop this larder.
When the egg hatches, the grub-like larvae consumes its store of food, and then metamorphoses into a winged adult bee before working its way to the surface and flying off to look for a mate.
Where do squash bees find mates? Inside the flowers of squash, cucumber, or melon plants. Males wait there to pounce on incoming females; after mating, the female gathers flower pollen to provision the nests she will dig.
The males sleep off their exertion in the flower, conveniently ready to mate with visiting females the next day.
By the time the squash bees are awake, so are the fat and fuzzy bumblebees, large bees that often fly in the seemingly random fashion implied by their names. These boldly striped bees are among the few native bees that nest in colonies like imported honeybees.
Unlike honeybees, bumblebee colonies last just one season. After mating in late summer, a female bumblebee overwinters underground in a sheltered place; in spring, she looks for a suitable nest cavity.
This year, a female bumblebee decided that the buried valve box that controls the drip irrigation in the pocket park we tend was the perfect home for her colony-to-be.
She built half-dozen golden, waxy chambers about the size and shape of peas, and laid an egg inside each. She fed the grub-like hatchlings pollen and nectar until they metamorphosed into her first generation of adults--sterile female worker-bees.
Those workers built more cells and the queen laid another egg in each. By mid-summer, the colony had swelled to several dozen spherical cells attended by as many yellow and orange-striped bumblebees.
As summer winds down, the queen will lay a special generation of eggs that produce fertile adult bees of both sexes. The queen and existing workers will die in fall, leaving the new generation to mate and found next year's colony (but not, we hope, in the irrigation system valve box!).
Those bumblebees, along with the myriad other native bees in our yard, will pollinate next year's flowers.
Copyright 2008 Susan J. Tweit First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.

Watermelon snow

Wed, 13 Aug 2008 18:12:07 -0600

If you are up at high elevations this summer and see a pink snowbank, you're not hallucinating. It's watermelon snow, a tint produced by a living creature, a single-celled alga with a tongue-twisting name, Chlamydomonas nivalis.
This tiny creature, about four times the diameter of a human red blood cell spends its entire life in snowbanks (hence nivalis, Latin for "snowy").
C. nivalis thrives in the abundant light of high altitudes, producing its own food using sunlight.
But too much light--specifically ultraviolet light--can damage its cells. So the little alga shields itself with a layer of dark red pigments.
When spring sunlight melts the top layer of winter snowbanks, the algae multiplies rapidly, resulting in as many as one million individuals in a single teaspoon of meltwater. These dense crowds tint the snow reddish-pink.
A stroll across a snowbank colored by C. nivalis leaves brilliant red or pink tracks and releases a faint watermelon-like fragrance, hence the common name, watermelon snow. (In Scandinavia, it is called "blood snow.")
When the arctic explorer Sir John Ross brought back samples of meltwater from snowbanks off the coast of Greenland in 1818, the London Times reported that the liquid was "of so dark red as to resemble port wine" and concluded that the tint came from meteoric iron.
More than a century passed before scientists recognized the living organism that produced the unusual color. But not until recently has the life of this snow-loving alga come to light.
And an odd life it is: C nivalis thrives in conditions fatal to most other organisms, toxic levels of ultraviolet light, months of sub-freezing temperatures, and a summer "thaw" to water barely above freezing.
During the long dark winters, the alga survives in a thick-walled "resting" cell buried under layers of accumulating snow. As snowbanks warm in spring, meltwater and windblown nutrients stimulate the cell to split, releasing the next generation.
These green cells swim to the surface of the snow propelled by two whiplike flagella (the algal equivalent of tails). After reaching the brilliant light and abundant water of the top few inches of the snowbank, the algae lose their flagella and grow a thick cell coat and that protective layer of red pigment.
There they float, photosynthesizing and storing sugars, until the snow either vanishes or refreezes, signaling the little algae to form protective resting cells for winter.
The ability to thrive in the harsh environment of snowbanks is unusual, but not unique. C. nivalis is simply the most visible member of a snowbank-loving community including sixty other species of microscopic algae and an array of tiny herbivores: protozoans, ciliates, rotifers, nematodes, and springtails.
These minute grazers eat the algae, plus pollen and other wind-blown detritus that accumulates atop the snow. Alpine birds, including rock wrens and rosy finches, eat the grazers, plucking them from the snow with tweezer-like bills.
The microscopic algae that produces what we call watermelon snow may be too tiny to see with the naked eye, but new research shows the multitudes of C. nivalis soak up large amounts of carbon dioxide in photosynthesis.
Because the algae is so widespread, found on every continent except perhaps Africa, and because its populations explode in summer, researchers speculate the microscopic organisms could play a significant part in mitigating the affects of global climate change.
As a warming world means snow banks in arctic and alpine areas diminish, however, so too will populations of these miniature carbon-dioxide fixers, reducing their ability to remedy a serious problem.
Still, watermelon snow, produced by a brightly colored algae barely four times the size of a human blood cell and which remains dormant for much of the year, could have a huge impact on our global climate.
Copyright 2008 Susan J. Tweit First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.

Perseid Meteors: Summer's Shooting Stars

Mon, 4 Aug 2008 06:44:00 -0600

Set your alarm clock for the wee hours between midnight and dawn next week to watch the summer's best dazzle of shooting stars: the Persied meteor shower.
This show of streaking lights is named for the constellation Perseus, the area the meteors appear to radiate from, located in the Milky Way high overhead between the tight cluster of the Pleiades and the sideways W-shape of Cassiopeia.
The Perseids, one of the most reliable meteor showers of the year, occur when Earth brushes through the debris trails left behind in space as Comet Swift-Tuttle orbits the sun every century or so.
(This comet was last seen in 1992, and before that in 1862, the year it was first described, during the Civil War. Astronomers think Swift-Tuttle, named for Americans Lewis Swift and Horace Tuttle, who "discovered" the comet, has been orbiting the sun for at least 2,000 years and may have been observed by humans as early as 69 BC.)
As Comet Swift-Tuttle speeds on its elliptical orbit around the sun, it sheds bits of debris from its nucleus. Each time the ball of ice and what astronomer Chet Raymo calls "celestial construction junk" passes, it sheds more; these trails of space detritus are what create the Perseid meteor showers.
The ephemeral streaks of light that we call shooting stars occur when this comet-dust, ranging from sand-grain-size particles to bits the size of peas or marbles, slams into Earth's atmosphere.
Swift-Tuttle's detritus is particularly speedy as meteors go: it hits our outer atmosphere at around 133,000 miles per hour, or a bit under 40 miles per second. (These shooting stars don't risk speeding tickets though: they zip by far too fast for a standard radar gun to measure.)
At that speed, the meteors compress the air in front of them so quickly that it heats to temperatures as hot as 3,000 degrees F, and the particles vaporize almost instantly, producing those fiery trails across the night sky.
Most meteors never hit the Earth, instead burning out not long after they smack into our atmosphere around sixty miles overhead; those that do reach the planet's surface are called meteorites.
(It was a giant meteorite about the size of Comet Swift-Tuttle's nucleus, which scientists estimate measures six miles across, that slammed into Earth back in Paleozoic time, creating the massive dust clouds that cooled the planet almost instantly, killing the dinosaurs and many other species.)
Prime nights for viewing the Perseids this year will be August 11 and 12, next Monday and Tuesday, but they're visible this week too.
Best viewing times are in the wee hours of the morning after the sun sets.
Meteor-viewing in North America is always better after midnight, since that's when Earth rotates to place our continent on its "leading" side as it moves through space, meaning that our hemisphere is aiming directly into any space detritus.
So find yourself a dark place away from city lights, and get out your lawn chairs or lie on the ground, and look up at the sky. Locate Perseus by finding the silvery ribbon of the Milky Way, running more or less southwest to Northeast at this time of year. Follow the Milky Way toward the northeastern horizon, passing the sideways W-shape of Cassiopeia, and watch for brilliant streaks of light.
Be patient: this year's show is expected to bring 50 to 100 meteors an hour, or somewhere around one a minute. But they'll be spectacular, since Perseids are noted for their speed and thus, their tendency to vaporize in dazzling fireballs.
So set your alarm clocks for the wee hours and haul yourself out before dawn to see the Perseids, summer's best meteor show.
Copyright 2008 Susan J. Tweit First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.