We live at the bottom of an ocean of air - several oceans, in fact, as the atmosphere can be divided into layers distinguished by temperature. Starting at the surface of the earth's crust, the layer which most apparently affects us is called the troposphere. It extends roughly 5 miles above the poles and 10 miles above the equator, and contains about 75% of the total mass of the atmosphere. Due to its closeness to the earth's surface, nearly all water vapor and solid particles (from forest fires, volcanoes and the burning of fossil fuel, for example) are found in this sphere. All of earth's plant and animal life exist within the troposphere or the waters beneath it. Climate and weather are conditions within the troposphere. It is also the layer where most of the heat exchange between the earth and the atmosphere takes place, and where heat is distributed through the overturning of air (tropo means "to turn"). Temperature in the troposphere decreases with height to -76 degrees Fahrenheit.
The stratosphere extends to about 30 miles above earth, and can itself be subdivided (strato means "layers") In the lower levels of the stratosphere the temperature remains the same, but in the upper levels the temperature actually increases to roughly the same as that at sea level. This is due to the ozone layer, where a high concentration of ozone, a molecular variation of oxygen, absorbs ultraviolet rays from the sun. Because of this temperature distribution the over-turning of air is much less than within the troposphere below, and the stratosphere becomes a kind of giant lid. Debris from violent volcanic eruptions, like Mt. St. Helens in 1980, sometimes enter and remain suspended in the stratosphere for years before settling back to the earth's surface. The troposphere and stratosphere combined contain 99% of the total mass of the atmosphere.
In the mesosphere, which extends to about 50 miles, temperature drops again to as low as -173 degrees F. Meteors, small pieces of matter drawn to the atmosphere by earth's gravity, become visible to the naked eye as they enter the mesosphere and are heated through friction caused by collisions with air molecules. These "falling stars" usually disintegrate before they reach the earth's surface. Spectacular meteor showers can be observed at certain times of the year when the earth, in its orbit, passes through a swarm of particles generated from the breakup of a comet. The troposphere, stratosphere and mesosphere make up what is called the lower atmosphere.
Above 50 miles is the upper atmosphere, where air density is extremely rarefied. The thermosphere extends to 400 miles and is characterized by large fluctuations of temperature (thermo means "heat"). At these heights there are relatively few molecules and heat retention should be low. However, within the thermosphere solar energy is absorbed and reradiates heat. At its upper limits the temperature reaches 441 degrees F.
The exosphere is the outermost layer of the atmosphere, and extends to 40,000 miles above the earth. It is here that molecules escape from the atmosphere without colliding with other molecules. Throughout the history of our planet most of the lighter molecules have escaped through the exosphere (exo means "out of"), while the heavier molecules, such as nitrogen and oxygen, have remained.
The upper atmosphere can also be divided into regions characterized by exchanges of energy. Extending through both the thermosphere and exosphere is the ionosphere, named for its concentrated layers of ions, electrically charged particles which are responsible for reflecting radio signals. How do we listen to a radio program originating in a city far from our own? The radio waves transmitted from one point on earth reflect off the ions and return to the surface - but because of the earth's curvature, when they return they are hundreds of miles away from the point of transmission. The waves once again bounce up to the ionosphere, return to the earth, and so on.
Above the ionosphere is the magnetosphere, an area in which charged particles are trapped and their behavior dominated by earth's magnetic field. A beautiful example is an aurora, a luminous display of various forms and colors in the night sky. Visible in the Arctic ("northern lights") and Antarctic ("southern lights"), they occur most often during the equinoxes and at times of great sunspot activity. An aurora is thought to be caused by high-speed particles from the sun excited to brightness after colliding with nitrogen and oxygen molecules. Also within the magnetosphere, zones of highly concentrated solar radiation were discovered in the 1980s by the Explorer 1, the first United States artificial satellite. The Van Allen belts, named after the physicist who developed the detectors, circulate along the earth's magnetic field.
Let's summarize what we've learned so far. The earth was formed nearly 5 billion years ago from a swirling mixture of gases and solid particles. Earth's atmosphere developed about a billion years later, as the interior heated up and expelled gases. Oxygen appeared about 2 billion years ago as a product of photosynthesis. The rest of the atmospheric gases consist of nitrogen, argon and carbon dioxide. The atmosphere is divided into upper and lower regions. 99% of the mass of the atmosphere is in the lower atmosphere, which has three levels. The troposphere extends to 10 miles above the earth's surface and its temperature decreases uniformly with height. Solar heat is exchanged and distributed here. The stratosphere rises to 30 miles and is characterized by an increase in temperature caused by ozone, which traps solar energy. The mesosphere extends to 50 miles and temperature drops again. Above 50 miles is the upper atmosphere, and its layers are characterized by increasing weightlessness as well as temperature changes. The thermosphere extends to 400 miles and temperature increases. The exosphere loses temperature as elements leave the atmosphere and enter space. The ionosphere and magnetosphere are two other layers superimposed on the upper atmosphere, each containing high concentrations of electrically charged matter. Past the magnetosphere the earth's gravity stops. It is at this point that space craft leave the atmosphere for other planets in our solar system, and beyond.
Considering the age of our species, much less that of the earth, exploration of the atmosphere by rocket, satellite and extra-terrestrial craft is extremely recent. New discoveries are being announced in rapid succession, and the facts of today will be discarded tomorrow. If nothing else, it should be clear at this point that our atmosphere is complex. This delicate balance of immense forces will become even more apparent as we reverse our course and examine conditions in our local neighborhood. In other words, we're going to answer the question asked more often than any other: How's the weather?
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