Sunday, September 16, 2012

Air II

I wrote a previous essay on air — that's the clear stuff we have here in Colorado. This note is more about the atmosphere, but then that's all hot air anyway — right? So put on your physics caps and let's talk about air.

The early Greeks considered "air" to be one of four elementary substances; along with earth, fire, and water, air was viewed as a fundamental component of the universe. By the early 1800s, however, scientists such as John Dalton recognized that the atmosphere was in fact composed of several chemically distinct gases, which he was able to separate and determine the relative amounts of within the lower atmosphere. He was easily able to discern the major components of the atmosphere: nitrogen, oxygen, and a small amount of something incombustible, later shown to be argon. The development of the spectrometer in the 1920s allowed scientists to find gases that existed in much smaller concentrations in the atmosphere, such as ozone and carbon dioxide. The concentrations of these gases, while small, varied widely from place to place. In fact, atmospheric gases are often divided up into the major, constant components and the highly variable components.

Although both nitrogen and oxygen are essential to human life on the planet, they have little effect on weather and other atmospheric processes. The variable components, which make up far less than 1 percent of the atmosphere, have a much greater influence on both short-term weather and long-term climate. For example, variations in water vapor in the atmosphere are familiar to us as relative humidity. Water vapor, CO2, CH4, N2O, and SO2 all have an important property: they absorb heat emitted by the earth and thus warm the atmosphere, creating what is called the "greenhouse effect."

Without these so-called greenhouse gases, the surface of the earth would be about 30 degrees Celsius (86 degrees Fahrenheit) cooler — too cold for life to exist as we know it. Though the greenhouse effect is sometimes portrayed as a bad thing, trace amounts of gases like CO2 warm our planet’s atmosphere enough to sustain life. Global warming, on the other hand, is a separate process that can be caused by build up over time of greenhouse gases in the atmosphere.

In addition to gases, the atmosphere also contains particulate matter such as dust, volcanic ash, rain, and snow. These are, of course, highly variable and are generally less persistent than gas concentrations, but they can sometimes remain in the atmosphere for relatively long periods of time. Volcanic ash from the 1991 eruption of Mt. Pinatubo in the Philippines, for example, darkened skies around the globe for over a year.

The Earth is surrounded by a blanket of air, which we call the atmosphere. It reaches over 600 kilometers (372 miles) from the surface of the Earth, and we are only able to directly observe what occurs fairly close to the ground. Early attempts at studying the nature of the atmosphere used clues from the weather, the beautiful colored sunsets and sunrises, and the twinkling of stars. With the use of sensitive instruments from space, we are able to get a better view of the functioning of our atmosphere. At sea level, the pressure of air is basically the effect of the weight of all the air above it. That weight is about 15 pounds per square inch. As you go higher up in the atmosphere, the pressure drops and the air becomes too thin to breathe.

Life on Earth is supported by the atmosphere, solar energy, and our planet's magnetic fields. The atmosphere absorbs the energy from the Sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects us from high-energy radiation and the frigid vacuum of space.

The envelope of gas surrounding the Earth changes from the ground up. Four distinct layers have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. NASA cites a fifth layer at the very edge of space. It is actually very hot in this layer, but it is really a physics definition of heat, since the air is so thin you would not notice the temperature, besides the fact you would die of suffocation, blow up like a balloon, and generally be too dead to care if your tropical shirt is appropriate attire.

The Troposphere

The lowest part of the atmosphere is called the troposphere and it extends from the surface up to about 10 - 15 km (6 - 9 miles). This part of the atmosphere is the most dense. As you climb higher is this layer, the temperature drops from about 17 to -52 degrees Celsius (63 to - 62 degrees Fahrenheit). Almost all weather is in this region. The tropopause separates the troposphere from the next layer. The tropopause and the troposphere are known as the lower atmosphere. The gases in this region are predominantly molecular Oxygen (O2) and molecular Nitrogen (N2). All weather is confined to this lower region and it contains 90% of the Earth's atmosphere and 99% of the water vapor. The highest mountains are still within the troposphere and all of our normal day-to-day activities occur here. The high altitude jet stream is found near the tropopause at the the upper end of this region.

The Stratosphere and Ozone Layer

The stratosphere starts just above the troposphere and extends to 50 kilometers (31 miles) high. Compared to the troposphere, this part of the atmosphere is dry and less dense. The temperature in this region increases gradually to -3 degrees Celsius (27 degrees F), due to the absorbtion of ultraviolet radiation. The ozone layer, which absorbs and scatters the solar ultraviolet radiation, is in this layer. Ninety-nine percent of "air" is located in the troposphere and stratosphere. The stratopause separates the stratosphere from the next layer.

The gas is still dense enough that hot air balloons can ascend to altitudes of 15 - 20 km (9 - 12 miles) and Helium balloons to nearly 35 km (22 miles), but the air thins rapidly and the gas composition changes slightly as the altitude increases. Within the stratosphere, incoming solar radiation at wavelengths below 240 nm. is able to break up (or dissociate) molecular Oxygen (O2) into individual Oxygen atoms, each of which, in turn, may combine with an Oxygen molecule (O2), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms (O3). This gas reaches a peak density of a few parts per million at an altitude of about 25 km (16 miles).

The gas becomes increasingly rarefied at higher altitudes. At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The existence of charged particles at this altitude and above, signals the beginning of the ionosphere a region having the properties of a gas and of a plasma.

At the outer reaches of the Earth's environment, solar radiation strikes the atmosphere with a power density of 1370 Watts per meter squared or 0.137 Watts per cm squared, a value known as the "solar constant." This intense level of radiation is spread over a broad spectrum ranging from radio frequencies through infrared (IR) radiation and visible light to X-rays. Solar radiation at ultraviolet (UV) and shorter wavelengths is considered to be "ionizing" since photons of energy at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision.

Incoming solar radiation is incident on a gas atom (or molecule). In the process, part of this radiation is absorbed by the atom and a free electron and a positively charged ion are produced. (Cosmic rays and solar wind particles also play a role in this process, but their effect is minor compared with that due to the sun's electromagnetic radiation.)

At the highest levels of the Earth's outer atmosphere, solar radiation is very strong, but there are few atoms to interact with, so ionization is small. As the altitude decreases, more gas atoms are present so the ionization process increases. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together. The point of balance between these two processes determines the degree of "ionization" present at any given time.

This layer moves up and down at sunset and sunrise causing some layers to combine and become better radio wave reflectors during the night. The changes in radio propagation at sunset is the reason that AM radio stations must reduce power and why you can get so many far away AM stations only at night. During the day the reflective layers separate under the direct effect of solar radiation, and you can usually only receive fairly close stations during the day, even though they increase their broadcast power. Of course, now that everyone has a CD player in their car, who listens to AM radio anyway?

At still lower altitudes, the number of gas atoms (and molecules) increases further and there is more opportunity for absorption of energy from a photon of UV solar radiation. However, the intensity of this radiation is smaller at these lower altitudes because some of it was absorbed at the higher levels. A point is reached, therefore, where lower radiation, greater gas density and greater recombination rates balance out and the ionization rate begins to decrease with decreasing altitude. This leads to the formation of ionization peaks or layers (also called "Heaviside" layers after the scientist who first proposed their existence).

The Mesosphere and the Ionisphere

The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. In this region, the temperatures again fall as low as -93 degrees Celsius (-135 degrees F) as you increase in altitude. The chemicals are in an excited state, as they absorb energy from the Sun. The mesopause separates the mesophere from the thermosphere.

The regions of the stratosphere and the mesosphere, along with the stratopause and mesopause, are called the middle atmosphere by scientists.

Auroras and "Northern Lights"

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 miles), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule.

Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. The very top of the atmosphere is both a higher percentage of oxygen, and so thin that such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth's magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.

The Thermosphere

The thermosphere starts just above the mesosphere and extends to 600 kilometers (372 miles) high. The temperatures go up as you increase in altitude due to the Sun's energy. Temperatures in this region can go as high as 1,727 degrees Celsius (3140 degrees F). Chemical reactions occur much faster here than on the surface of the Earth. This layer is known as the upper atmosphere.


The physical and chemical structure of the atmosphere, the way that the gases interact with solar energy, and the physical and chemical interactions between the atmosphere, land, and oceans all combine to make the atmosphere an integral part of the global biosphere.  It is literally the "air we breathe," and a whole lot more that sustains life on this rocky globe.

Water, in its different forms, cycles continuously through the lithosphere, hydrosphere, atmosphere, and biosphere. Water evaporates into the atmosphere from the land and the sea. Plants and animals use and reuse water and release water vapor into the air. Once in the air, water vapor circulates and can condense to form clouds and precipitation, which fall back to earth. At one time or another, all of the water molecules on earth have been in an ocean, a river, a plant, an animal, a cloud, a raindrop, a snowflake, or a glacier!

These processes combined with sunlight and minerals and nutrients in the soil produce food which feeds humans and animals and provide sustenance for all life. It all depends on the air around us. Air, water, food … the essentials of life.

Originally written on Feb. 8, 2011.

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