The gases present today in Earth's atmosphere are much different than the gases that were abundant in Earth's earliest atmosphere. Billions of years ago, when Earth was a young, newly formed planet, our atmosphere consisted mostly of methane, a poisonous, explosive gas made from carbon and hydrogen atoms, and Ammonia.The atmosphere back then also consisted of some Carbon Dioxide. The composition of this primitive atmosphere changed due to many pressures exerted upon it. One of the major players in this change was the development of living creatures on the planet's surface. Even in relatively recent times, the atmosphere has been changed by the great variety of living things on and around the surface of this planet.

 

Our Present Atmosphere

 

Earth's current atmosphere contains all of the gases necessary for the continued survival of the living creatures of this wonderful world. Air is a mixture of several abundant gases including NITROGEN, OXYGEN AND CARBON DIOXIDE.

Nitrogen: This is the most abundant gas in Earth's atmosphere. It is necessary in order for living things to form proteins, molecules used in cell growth and repair. Nitrogen also dilutes the oxygen present so that rapid burning does not take place. Most plants and animals are unable to use the Nitrogen in its atmospheric form. Bacteria in the soil, called Nitrogen fixing Bacteria, are able to change the atmospheric Nitrogen into a more useable form, called Nitrates. These Nitrates are used by the plants to make proteins. Animals receive their nitrates/proteins from the plants that they consume. Nitrogen is returned to the soil and the atmosphere by another bacterial process, called decay. Decay is the breaking down of dead plants and animals into their component parts. This movement of Nitrogen from the air, to the soil, to living things, and back to the soil or the atmosphere, is called the NITROGEN CYCLE.

Oxygen: This gas IS useable directly in its atmospheric form by most plants and animals. Oxygen is essential for the process of respiration. Respiration is the process used by some living things to obtain energy by "burning" food in the presence of atmospheric oxygen. Another related process for which oxygen is also vital is known as combustion. Combustion is the process known as "burning", where fuel is combined with heat and oxygen to make a fire. Combustion will not take place in the absence of oxygen. As you can deduce, RESPIRATION is really just a specialized form of combustion, where fuel known as food is burned in a chemical reaction within the cells of living creatures. Both respiration and combustion combine the oxygen with a carbon molecule, producing a waste product called CARBON DIOXIDE.

Carbon Dioxide: The carbon in carbon dioxide helps to form the compounds that make up all living things. The amount of carbon dioxide in our atmosphere is small, but it is a very valuable gas. The process of PHOTOSYNTHESIS uses carbon dioxide to produce long chains of carbon atoms used as food by living creatures. Essentially, the process of photosynthesis breaks the carbon dioxide molecules apart, leaving the carbon to be used in the long chains used as food, and Oxygen, in the form of O₂, which is released as a waste product. Scientists are concerned about the levels of carbon dioxide in Earth's atmosphere. Carbon Dioxide contributes to the global warming phenomenon known as the GREENHOUSE EFFECT. The widespread burning of fossil fuels (which releases CO₂ into the atmosphere), combined with the systematic destruction of our rain forests, oceanic phytoplankton and local woodlands and grasslands, all combine to cause global carbon dioxide levels to climb steadily higher and higher. Unless something is done to reverse this trend, the Earth's surface will continue to get warmer and warmer. Melting polar ice caps could cause a drastic change in coastal geography, as ocean levels rise. Considering that something like 75% of the world's population live in coastal areas, this could be a big deal for literally millions and millions of people. Other effects of global warming might be a change in the growing seasons, destruction of large areas of farmland, and more severe and frequent storms such as tornadoes and hurricanes.

Earth's atmosphere is arranged in layers. The mixture of gases, the temperature, electrical and magnetic forces of the atmosphere changes as you move away from Earth's surface. Our atmosphere has no definite outer edge, rather, the air gets thinner and thinner as you move away from the surface of our wonderful planet. Eventually , the atmosphere simply fades into the vast distances between planets, called INTERPLANETARY SPACE.

The atmosphere that surrounds the Earth is not uniform. There are concentrations of gases found in different areas. Also, the air itself has different characteristics, due in part to the density and temperature of the air. The layers of air that surround the Earth are held close by GRAVITY. The upper layers of air push down upon the lower layers. This is called AIR PRESSURE. Air pressure near the surface of the planet will always be greater than the air pressure further away from the surface of a planet.

This layer of our atmosphere extends from Earth's surface up to an altitude of about 16 kilometers. The temperature at the bottom of the troposphere averages approximately 20 degrees Celsius. The air at the top of the troposphere averages about -55 degrees Celsius. As you gain altitude in the troposphere, the temperature decreases. For every kilometer that you travel above the Earth's surface, the temperature drops approximately 6.5 degrees Celsius.

The troposphere is the lowest layer of Earth's atmosphere and is the only region of the atmosphere that actually touches the Earth's surface. It is also the layer where most living things will be found. It is also the layer where ALL of our weather occurs. The air in the troposphere also contains large quantities of dust and water vapor. Because the air is denser here, because it's being squeezed so much by gravity, MOST of the air in our atmosphere ends up being in this layer. 50% of the air on Earth is located below an altitude of 5.5km.  This is also the only layer of the atmosphere that contains enough air for us to breathe.  Even as you approach the upper parts of the Troposphere, the air is just too thin to breathe.  If we were transported to the top of Mount Everest, the first thing you'd notice, other than the cold, would be that you just couldn't manage to take a deep breathe.  You'd breathe in and out as usual, but you wouldn't get that feeling that you could get enough air inside your lungs.  A little bit like breathing into a paper bag, after a while, though you're still breathing in and out, it would feel like you're not getting enough air.

At approximately 12 kilometers above the surface of the Earth the temperature stops dropping and begins to stabilize. This marks the beginning of an intermediate layer called the TROPOPAUSE. This tropopause separates the troposphere from the next layer of the atmosphere.

The stratosphere can be found between the altitudes of about 16 to 48 kilometers. The temperature range for the stratosphere is about -55 degrees Celsius at the bottom to about 0 degrees Celsius at the top of the stratosphere. As you can see, in the stratosphere, the temperature rises as you gain altitude. Just the opposite situation than we had observed in the troposphere.

The lower parts of the stratosphere interact with the upper parts of the troposphere. Several interesting features result from this interaction.

The JET STREAM is a current of fast-moving air traveling from west to east at a rate of about 320 km/hour. this jet stream has an impact upon air travel, but has an even greater impact upon our country's weather patterns. The jet stream can pull storms into an area, or keep bad weather out. The jet stream is one of the most predictable weather makers that we know about. The exact path of the jet stream varies, sometimes even on a daily basis, so figuring out what effect the jet stream will have upon our local weather can be a real challenge.

 

The Jet Stream as seen from Space

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The OZONE LAYER: The gas Ozone, and the often discussed Ozone Layer can be found in the stratosphere between the altitudes of about 16 and 60 kilometers. Ozone is a form of Oxygen, and is created in nature when lightning strikes. Ozone has a peculiar, pungent odor, and is easily recognizable after a strong thunder and lightning storm. Ozone in the upper parts of our atmosphere is responsible for the increasing temperatures there. More importantly, ozone can absorb and effectively neutralize some forms of radiant energy produced by our sun. One form of this invisible light energy is called ultraviolet radiation, or UV rays. The amount of ozone in our atmosphere is small, but it still plays an important role to all life here on Earth. Without this functioning ozone shield, the unprotected living creatures of the Earth would suffer greatly from the intense ultraviolet energies. A very small amount of UV radiation actually makes it through the ozone shield to the Earth's surface. It is responsible for your summer suntans and sunburns. Too much uv-energy is a bad thing, and has been proven a cancer causing agent in humans. Certain chemicals released into the atmosphere, such as refrigerants or propellants, can destroy the ozone in our atmosphere before it has had a chance to neutralize any harmful radiation. There was a "hole" in our ozone layer, located above Antarctica. Had this hole been located over a more populated region, it would be the biggest news story ever. Imagine not being able to walk in the summer sun!  We used to use a chemical called a Chloroflurocarbon (a CFC) in air conditioners and as propellents to push things like hair spray out of cans.  These CFCs were wreaking havoc with our Ozone supply.  Luckily, most countries have banned this chemical, and replaced it with an ozone-safe product.  Once this happened, Earth began to heal itself, and the ozone levels have increased in recent years. 

 

The Mesosphere

The mesosphere is located between the altitudes of about 48 kilometers and 80 kilometers. The temperature in the lower mesosphere is approximately 0 degrees Celsius, while in the upper reaches of the mesosphere, the temperature has dropped to nearly -100 degrees Celsius! If any water vapor is present in this layer, thin clouds of ice are formed. You can observe these thin, feathery clouds of ice after sunset if the sun hits them at the right angle. They will appear as thin, silvery scratch-like marks in the sky.

A very important function of the mesosphere is that it is the first layer of our atmosphere that is thick and dense enough to protect us from large falling objects from space, known as meteorites. Friction is the force at work here. As the meteorite falls through our atmosphere, it bumps into air molecules. In the mesosphere, these air molecules are finally close enough together to cause the meteorite first to get warm, then to burn, then eventually to melt away entirely. The mesosphere is responsible for destroying all but the largest meteorites that attempt to strike the Earth. If any DO get through, we know that they were extremely large and dense to begin with. Even then, the friction that they will encounter during this trip, will certainly reduce them in size significantly.

The thermosphere extends from an altitude of about 80 kilometers and projects clear into space. The thermosphere is actually divided into two smaller, sub-layers, which we will now take a look at individually.

• The Ionosphere: The ionosphere is located between the altitudes of about 80 and 550 kilometers. It is known as the lower part of the thermosphere. The precise size of the ionosphere varies with he amount of ultra-violet radiation given off by the sun. Nitrous Oxide, oxygen and other gas particles in the ionosphere absorb the radiation from the sun. These particles become electrically charged in a process called ionization. These particles are now called "ions". Ions are really nothing more than an atom which carries an electrical charge. The phenomenon known as the Auroras (shown below) finds its home in the ionosphere. Ions from the sun get tangled in the Earth's magnetic field in this area and bounce around and bash into one another. These impacts release energy in the form of beautiful lights. The brightness of the Auroras varies with the quantity of ions coming from the sun. The ionosphere is also noteworthy because of its importance in two way radio communication here on Earth. Radio waves are bounced off of the ions and back to the Earth's surface. This allows radio messages to be sent over great distances. Solar phenomena such as solar flares can cause the number of ions in the ionosphere to increase greatly.
• The Exosphere: The exposphere extends from about 550 km above the Earth's surface, clear in to interplanetary space. The exact outer limit of this layer is open to individual scientific interpretation. The air simply gets thinner and thinner and thinner, until finally, there's no air at all. This is the upper level of the Thermosphere. The air is so thin here, that a particle of matter could travel large distances without hitting another particle of matter. This is the layer in which all of our artificial satellites orbit the Earth. The reason that they orbit in this layer is fairly simple. There isn't enough air up there to cause enough friction to burn up your satellites, yet the satellite will still be close enough to Earth to be held in a stable orbit by Earth's gravity.

The Aurora Borealis, the Northern Lights

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