Why is the sky blue?

When a child colors a picture of the sky, the sky is blue and the sun is yellow. Why isn’t the sky green and the sun purple? 

The sky appears more blue in the mountains because there are fewer aerosols to scatter white light

The color of the sky is determined by what is in the atmosphere. Without an atmosphere the sky would be black, like what astronauts see from the moon.

Sunlight consists of a broad spectrum of electromagnetic radiation traveling at different wavelengths. We see the visible light portion of the spectrum (violet, indigo, blue, green, yellow, orange and red). The sunlight travels in a straight line until something reflects it (mirror), bends it (prism) or scatters it (gas molecule). 

The atmosphere is filled with different components, such as gas molecules, water droplets and dust, and each component scatters different wavelengths of light depending on the component’s size. 

Our eyes perceive clouds as white because water droplets in clouds scatter white light

Aerosols (such as soot, water droplets and dust) are larger components in the atmosphere and scatter all wavelengths of light. Gas molecules are smaller and scatter shorter wavelengths and blue light has a short wavelength. When blue light is scattered it goes in all directions and that is the light that strikes our eyes. 

Other colors of light pass through the atmosphere without scattering and aren’t visible until they reflect off an object. Then we see the yellow daffodils, green grass and purple plums. 

If blue light scatters in all directions, why isn’t the sky uniformly blue on a clear day? You may have noticed that closer to the horizon the sky is a lighter blue or even whitish. 

The sky near the horizon often appears a lighter blue 

When the sun is directly overhead, sunlight passes through the least amount of atmosphere. The more atmosphere sunlight passes through, the more chances there are for the light to be scattered and rescattered in all directions. This scattering mixes the scattered colors together again so we see more white and less blue. 

The same reason applies to why clouds are white and hazy days are whitish. On cloudy or hazy days, there are more aerosols in the atmosphere (such as water droplets) and these aerosols scatter all wavelengths of light, causing them scatter, rescatter and mix back together. 

Scattering also influences the color of sunsets. Since the sun is low on the horizon, sunlight has to travel a greater distance to reach our eyes. This extra distance provides more opportunities for blue light to scatter in every direction. So much scattering happens that red and yellow light is all that is left to reach our eyes. 

Sometimes the whole western sky appears red, especially if it is smoky from a forest fire, because large particles (like soot) reflect and scatter more of the red and yellow light. If there are fewer particles in the atmosphere, the sunset will be more yellow or orange. 

Light traveling a longer distance causes sunsets to be red, orange and yellow  

After the sun sets, the sky sometimes turns a purple hue. The purple hue is a result of red light reflecting off aerosols higher in the atmosphere and mixing with scattered blue light to reach our eyes as purple.

However, during and right after sunset, the entire sky doesn’t turn red, orange or yellow--the sky overhead is still blue. The sky is still blue because of the ozone. Ozone absorbs the longer wavelengths of light (red, yellow and orange) and leaves the shorter wavelengths (blue). At sunset the light travels its longest distance in the atmosphere and the ozone filters out most of the wavelengths, leaving the sky blue overhead. 

A mixture of red light and blue light turns the sky purple after sunsets

The color of the sky can change color for a variety of reasons. But we can be certain the sky will be blue on a clear day and the sun yellow, just as kids depict it. 

Note: Published in the Bonners Ferry Herald on Feb. 27, 2014.

First Robin

Despite cool morning temperatures in the teens, the robins are back. This morning I saw a flock of eight robins in the tree tops. They weren't singing their cheery song yet.

Looking back at my calendar, this year's robins are five days earlier than last year when I saw the first one on Feb. 27. In 2011, I saw the first robin on March 9.

Stromatolites a window into Earth’s history

Fossils offer a glimpse of what organisms have lived on Earth, such as woolly mammoths and Tyrannosaurus rex, and most don’t exist today. Some fossils resemble modern-day counterparts, such as ferns and petrified wood, and others have living examples, such as stromatolites. 

The irregular lumps of rock at the base of the Kootenai River swinging bridge upstream of Troy, Montana are fossilized stromatolites.

Stromatolites are structures created by cyanobacteria (also known as blue-green algae). The internal structure resembles a cabbage while the outside can look mushroom-shaped, loaf-shaped or cauliflower-shaped. 

Cyanobacteria create stromatolites by growing in layers in shallow marine water. Cyanobacteria grow in mats with nearly three billion cyanobacteria covering one square meter. As sediment is deposited over the cyanobacteria from tides and wave action, the cyanobacteria grow up through the sediment. Layers of sediment then alternate with layers of cyanobacteria.

If you’ve noticed the rocks around the base of the Kootenai River swinging bridge, they look like cream and black cabbages sliced open--these are fossilized stromatolites. The black layers are carbon-rich layers from when there was little deposition of sediment and the creamy layers are from periods of higher deposition. 

Alternating organic-rich (dark) and sediment-rich (light) layers can be seen in this stromatolite

Stromatolites grow slowly, so slowly that it can take 100 years for five centimeters of growth or 2,000 years for a stromatolite to reach one-meter high.

When living stromatolites were discovered in 1956 by scientists in Shark Bay, Australia, they were the first ever recorded examples of a structure previously only found as a fossil in ancient rock. 

Stromatolites are one of the oldest fossils on Earth. Worldwide, the oldest fossilized stromatolites are found in South Africa and date back 3.2 billion years. The stromatolites around the Kootenai bridge are part of the Belt formation, a Precambrian sedimentary formation dated between 600 million and 800 million years old.  

The roughly concentric circles are fossilized stromatolites

Stromatolites and the cyanobacteria that created them played a crucial role in shaping the atmosphere of Earth. Like all green plants, cyanobacteria absorb carbon dioxide from the atmosphere, use the carbon to build tissue and then release the oxygen. 

During the Precambrian, the atmosphere contained very little oxygen. With the growth of stromatolites and the spread of cyanobacteria around the Earth, the atmosphere became more oxygen-rich and less carbon-rich. 

The harsh conditions of the Precambrian, with its carbon-rich atmosphere, hot temperatures and intense ultraviolet radiation set the stage for cyanobacteria to thrive at that time since little else could.

Living stromatolites are found in three places on Earth today: Shark Bay and two places in the Bahamas. Cyanobacteria thrive in Shark Bay because the water is twice as salty as normal seawater due to the restricted flow of the bay. In the Bahamas, stromatolites are found in sub-tidal channels where the currents are very strong and few animals can survive. 

The internal layers of stromatolites can be a variety of shapes

Burrowing and grazing marine animals are the demise of stromatolites because they destroy the layers. Therefore, as marine animals populated the oceans, the range of stromatolites decreased to places that were too hostile for animals to survive. 

While there may only be a few places on Earth to view living stromatolites, those places offer an opportunity to study a living example of a fossil and determine what affects growth. Scientists have determined tides, temperature and sunlight control the growth of cyanobacteria. So not only do scientists have insight into the conditions on Earth three billion years ago but living stromatolites are keeping a diary of the current conditions on Earth. 

Note: Published in the Bonners Ferry Herald on Feb. 20, 2014.

Caching allows early nesting for Clark’s nutcracker

Laying eggs in March would be a risky venture for most birds because of the lack of food available to feed nestlings. But an alpine resident--the Clark’s nutcracker-- has a system figured out to circumvent that issue.

The Clark's nutcracker is a member of the corvid family which includes gray jays, crows and ravens

Like the gray jay, the Clark’s nutcracker caches food when food is available. With a diet consisting mostly of conifer seeds, a Clark’s nutcracker caches between 35,000 to 98,000 seeds between late summer and fall. These cached seeds are the nutcracker’s main food source until the cones are ripe again the next year. Nutcrackers also opportunistically forage for insects, spiders, small birds, small mammals and carrion.

In the northern Rockies, Clark’s nutcrackers mainly forage for the high-protein seeds of whitebark pine, ponderosa pine and Douglas fir. A diet of different seeds is a survival strategy because both ponderosa pine and whitebark pine are masting species (produce a large cone crop every few years). 

Since nutcrackers cache seeds, they keep a home range. The home range isn’t usually productive enough to harvest 98,000 seeds, so the nutcracker forages up to 20 miles away for seeds. 

Obtaining the seeds isn’t easy, which is why the Clark’s nutcracker has a sharp, sturdy bill. The nutcracker hammers into the ripe cone, which on whitebark pine is closed when mature, to pluck out the seeds. 

The Clark's nutcracker uses it long, sharp bill to hammer open ripe cones to obtain seeds 

The Clark’s nutcracker doesn’t carry one seed or even two seeds at a time back to its home range when foraging. Instead, the nutcracker has a pouch beneath its tongue that can hold between 30 to 150 seeds. Once full, the nutcracker flies back to its home range to cache the seeds.

Caches can be in a trench in the ground, in crevices of bark, among tree roots, near fallen logs or branches, or next to rocks. To minimize spoilage and raiding of a large cache by competitors (such as squirrels), the nutcracker places one to 15 seeds in each cache. 

Snow covers the caches on or beneath the soil, so nutcrackers cache more seeds in above ground locations for access during the winter. For the caches beneath the soil, the nutcracker uses it bill to swipe away soil to create a trench. Then it places a few seeds in the trench and pushes soil back over the seeds with its bill. 

With an amazing memory, Clark’s nutcrackers can remember the location of the caches for nearly nine months--long enough for the first cones to ripen the next summer. If cached in a suitable location, the forgotten seeds will germinate and grow. 

Clark's nutcrackers live in alpine and subalpine habitat

The caches of seeds enable the Clark’s nutcracker to court in January or February when most birds are just trying to survive winter. They lay eggs in March (earlier than any other songbird species) with both the male and female incubating the eggs. The male even develops a brood patch on its chest like the female to keep the eggs warm. When a parent isn’t incubating, it goes off to retrieve seeds from one of its caches.

Once the eggs have hatched, the parents utilize the seeds from their caches to feed the young. Researchers speculate that the early nesting allows the young enough time to mature so they can be ready to cache their own seeds once the cones are ripe in late summer. 

Note: Published in the Bonners Ferry Herald on Feb. 13, 2014.

Sweet cottonwood fragrance will announce spring

One day in early spring, the sweet fragrance of cottonwood buds will fill the air. The large pointed buds of black cottonwoods are filled with a sticky, reddish substance that emits a sweet resinous fragrance.

Cottonwoods grow on some of the islands in the Kootenai River and are identified by their thick, deeply-furrowed gray bark

The fragrance is only the beginning of noticeable features for black cottonwoods. The fragrance precedes the large, broad, almost triangular leaves that are deep green above and silvery white on the underside. 

Most noticeable is in May or June when cottonwoods release plumes of white, cottony seeds--hence how cottonwoods received their name. The millions of seeds released will perpetuate the species if they land in a moist location and germinate.

The bright green leaves begin to emerge after the catkins (flowers) have emerged

Black cottonwood use and store a large quantity of water and, therefore, generally grow in moist areas such as riverbanks, gravel bars, lake shores, swamps and springs. Cottonwoods take in large enough quantities of water to “bleed” water when they are cut and barely float when they topple into water because they are so saturated. 

Being in moist areas allows for cottonwood to grow rapidly--so rapidly that they are the fastest growing tree and largest broadleaf tree in the Inland Northwest. The rapid growth doesn’t produce durable wood, so the wood is weak. Large upper limbs are extremely susceptible to breaking off during windstorms or under a considerable amount of snow or ice. 

However, broken branches and remaining stumps help perpetuate the species. New trees can sprout from cut stumps, limbs partially buried in soil or from surface roots. As a shade-intolerant pioneer species, cottonwood rely on their rapid growth to maintain the dominant position on barren soil, such as gravel bars disturbed by flood waters. 

A bald eagle perches in a cottonwood snag

Cottonwoods are an important component of river ecosystems. A cottonwood’s root system helps stabilize river banks and islands. Their heavy crown shades the water and keeps it cool for fish. 

Even trees toppled by erosion or old age are important as habitat. When cottonwoods topple into a body of water, they provide important aquatic habitat for fish. The decaying leaves also provide a source of nutrients for caddisflies, mayflies and other insects.

In addition to fish, other wildlife utilize cottonwoods in many ways. The large upper limbs of cottonwood provide sites for the huge platform stick nests of bald eagles and osprey. Colonies of blue herons also build large stick nests in the crowns of cottonwoods. 

Eagles commonly build their large nests in the upper limbs of cottonwood trees

Rotten trunks or places where limbs have broken off offer cavities for woodpeckers, great horned owls, wood ducks, flying squirrels, raccoons and numerous songbirds. 

Beavers easily gnaw through the weak wood to topple trees for food and for building lodges and dams.

Deer, moose and elk eat the twigs and buds and they can hinder the growth of cottonwoods even more by rubbing their antlers on young saplings.

If a cottonwood can survive the elk, deer, voles, mice and beavers for 10 years, then the cottonwood will be old enough to begin to flower.

Most cottonwoods don’t live past 200 years old but in those 190 years of flowering, they produce billions of cottony seeds that drift through the air and pile up like snow drifts in the summer. Maybe only a few hundred of those seeds will germinate and survive long enough to emit the sweet fragrance that announces spring. 

Male and female flowers are produced on separate trees and the female catkins (flowers) give rise to the plumes of cottony seeds

Note: Published in the Bonners Ferry Herald on Feb. 6, 2014.