The atmosphere has both awed and puzzled scientists since science itself began on the Earth, but it hasn't been until recently that the contents of the atmosphere have been identified and the individual layers analyzed. The origin of the Earth's atmosphere is still shrouded in a bit of mystery, and usually depends on your acceptance of how and when the Earth was formed. Suffice it to say that the Earth's atmosphere has stayed relatively constant since humans have existed on the Earth.
    Despite any rumors, the Earth's atmosphere is not primarily composed of Oxygen...not even close. The main ingredient in our atmosphere is Nitrogen, about 78% worth. Oxygen accounts for about 21%, and after Argon (at .93%) all the other gasses and components we often hear talked about and worried about take the remaining .07%.

    Some gases in the Earth's atmosphere are constant, comprising the general make-up, and giving it the properties necessary to sustain life in all corners of the Earth equally. There are also other elements that are in the atmosphere but can be concentrated heavily in a localized area, or virtually non-existent in other regions. The most notable and important of those is water vapor. Necessary to bring precipitation, it masses in storms systems and is a rare sight over the deserts of Africa. Other notable variable gases are Ozone and Chloroflourocarbons, the two elements that wage war against each other in atmospheric environmental issue of our time.
    The atmosphere also has certain elements in certain parts of the atmosphere, meaning the heavier gases are found more towards the Earth's surface while the lighter gases towards the outer extremity. This is a good thing, because we like having the oxygen down here towards the surface, rather than the hydrogen gas that exists in the higher levels of the atmosphere. The composition of the lowest part of the Atmosphere does  not change, and it is called the "Homosphere".

The elements found in the "Heterosphere" change with height, up to the lightest gases at altitudes farthest away from the surface.

Atmospheric Pressure and Density

    Each molecule in the Earth's atmosphere roams around and collides with one another at various speeds, and each time a collision occurs a force is exerted. During a given day, molecules in the atmosphere are constantly colliding harmlessly with the human body, pushing a force onto us. Any force exerted on a given area is called a "pressure". The reason why we don't buckle under the pressure of the atmosphere, is because our bodies have molecules pushing back at a balanced level. Your body may detect a sudden change in pressure, for example if you rise rapidly on a plane and your ears pop, or perhaps diving in the water and sensing mounting water pressure. The pressure of the Earth's atmosphere changes slightly over a given spot during the course of time, and is often measured and representative of changes in the weather.
    An important force to both the atmosphere and life in general is gravity. It is no small wonder that the heaviest molecules in the Earth's atmosphere are located closer to Earth. It follows then, as a percentage by volume, the lion-share of the atmosphere exists closest to the Earth's surface, and drops off dramatically in density as one increases in altitude. This is why it is harder to breathe, easier to boil water, and much easier to get a bad sunburn in higher altitudes. This decrease in density with height is exponential, exhibiting quite a rapid decrease at first, then a gradual lessening with higher altitude.

The Movement of Air in the Atmosphere

    At any given moment, all molecules that comprise the atmosphere are moving and colliding with one another. In fact, the way we measure temperature is through averaging the speed of molecules. At room temperature, air molecules are dashing around at about 1000 miles per hour. Since there are so many molecules in the atmosphere, there isn't much room for travel, as each molecule collides about 10 billion times with surrounding air molecules each second. If the speed of air molecules increases, so will its temperature. There's a relationship that governs air movement (or any gas for that matter), and it is this:

    You can think of this as a spinning wheel, all participants reacting in kind to maintain a balance. If you take a small piece of air and separate it from the rest of air (folks in the know call them "parcels") you can evaluate its properties in relation to the air around it. If you find that the temperature of this parcel of air is a bit warmer than its surroundings, then it's pressure will decrease as it also expanding and becoming less dense. When a parcel of air finds itself in surroundings that are denser, cooler, and higher in pressure, this parcel of air will be forced upward. This proportionality can be started at any point in the chain. For example, as a parcel of air rises, assuming no interaction and exchange of heat with its surroundings, it will also expand (become less dense), thus the pressure dropping, etc. Now, this wheel of interrelation is whole, and the manipulation of one of the parts will gradually result in a balanced being re-attained. If an air parcel is forced to expand, that expansion has a cooling effect on the parcel, gradually halting it's vertical movement when the temperature of the parcel is the same as its surroundings. Notice how the relationship of temperature, pressure, density, and air movement is one of balance, where even though there are many changes taking place within a small amount of air, eventually all effects work together to restore a balance. This is why, in absence of any external forces and heat sources, there wouldn't be any dramatic changes in air movement, and thus weather.

Layers of the Atmosphere

    Because pressure and density decrease exponentially with altitude, it therefore follows that the vertical structure of temperature can't be constant either. Because we have a external heat source of the sun, and because interactions between different air molecules and parcels do exist, the vertical temperature structure of the Earth's atmosphere is a bit more complicated. Meteorologists have divided the Earths atmosphere into vertical segments, or layers, to help categorize the different swings in temperature in the atmosphere.
Troposhere
    The most important layer in Meteorology is the bottom-most layer, called the "Troposphere". This word comes from the Greek tropein, which means 'to turn', or 'change'. In it, temperature decreases with height, and the heating of the Earth's surface often permits air molecules to freely circulate. Just about all that effects our weather occurs in this level. On some days, you can see a powerful thunderstorm reach the height of the troposphere, and start to fan out along the ceiling. That ceiling is called the "Tropopause". Each atmospheric layer has a line of demarcation between the next, called a 'pause'.
    The Stratosphere is the next layer up, followed by the Mesosphere and the Thermosphere.