What is the atmosphere?

It's a thin layer of gases surrounding our planet.  Many of the planets in this solar system have atmospheres, but none that we know of have an atmosphere quite like ours - one that can support life.

As we learned last lecture, the Earth's primitive atmosphere was much different from today's and consisted primarily of ammonia, methane, and trace amounts of carbon dioxide and water vapor.  There was little, if any, free oxygen present.

The atmosphere decrease in concentration, and hence pressure, as you rise above the surface of the earth.
 

Altitude (km)	Percent sea level pressure
0	100
5.6	50
16.2	10
31.2	1
48.1	0.1
65.1	0.01
79.2	0.001
100	0.00003

(Figure 1-17, page 18 in Lutgens and Tarbuck's The Atmosphere, 2001)  Pressure variations caused by altitude

The earth's outermost atmosphere, the part above a few hundred kilometers, is a region of extremely low density.  Near sea level, the number of atoms and molecules in a cubic centimeter of air is about 2x10¹⁹; near 600 km it is only about 2x10⁷, which is the sea level value divided by a million million.  At sea level, an atom or molecule can be expected, on the average, to move about 7x10-6 cm before colliding with another particle; at the 600 km level this distance, called the "mean free path," is about 10 km.  Near sea level, an atom or molecule, on the average, undergoes about 7x10⁹ such collisions each second; near 600 km, this number is about 1 each minute.

Not only does the pressure change with altitude, but temperature does as well.

This is easily seen by looking at the snow covered Rocky Mountains or Alps in the summer time.  In Denver, it can be 95°F while on Pikes Peak there is still snow.

Originally, scientists thought that temperature decreased continuously with increasing height until reaching absolute zero (-273.16°C).  This decrease of temperature with increasing altitude is known as the environmental lapse rate and is approximately 6.5°C/1000 m (3.5°F/1000').  In the figure below, the environmental lapse rate can be seen graphically as the decrease in temperature with increasing height.  This environmental lapse rate is observed until the tropopause is reached.

(Figure 1-20, page 21 in Lutgens and Tarbuck's The Atmosphere, 2001)  Observed lapse rate and the tropopause
 

Vertical structure of the atmosphere

(Figure 1-19, p. 20 in Lutgens and Tarbuck's The Atmosphere, 2001)  Thermal structure of the atmosphere

But after many balloons flights and temperature measurements, we know that's not the case.  The atmosphere can be divided into four regions.

The bottom layer, where temperature decreases with altitude, is known as the troposphere (from the Greek for "turning layer").  The troposphere is approximately 12 kilometers thick, but there are slight variations.  If the temperature increases with increasing altitude in the troposphere, then a temperature inversion exists.  All the weather that we are primarily interested in, occurs in the troposphere.  The top of the troposphere is marked by the tropopause.

Above the tropopause lies the stratosphere.  It gets it's name from the Greek meaning "stratified layer."  The layer is stratified with the denser, cooler air below the warmer, lighter air.  This leads to an increase in temperature with height.  Since the stratosphere isn't turbulent this is where most planes like to fly.  The temperature increases with height until it reaches about 10°C at an altitude of 48 km.  The primary reason that there is a temperature increase with altitude is that most of the ozone is contained in the stratosphere.  Ultraviolet light interacting with the ozone causes the temperature increase.  The boundary between the stratosphere and the next layer is called the stratopause.

Above the stratopause, the temperature again decreases with altitude.  This layer is called the mesosphere, or "middle layer."  The temperature drops to ~-90°C near the top of the mesosphere where the mesopause is located.

Above the mesopause is the thermosphere, or "warm layer."  In the thermosphere the temperature does increase with height (to >1000°C), but as we have already seen, the number of molecules present are so few that even thought they are very energetic, they have such a low density, that temperature as we call it means very little.

Above the thermosphere lies the exosphere ("outer layer").  The boundary between the two is very diffuse.  Molecules in the exosphere have enough kinetic energy to escape the earth's gravity and thus fly off into space.  This is where helium "disappears."

The outer part of the mesosphere and the thermosphere are sometimes called the ionosphere since most of the molecules and atoms are ionized by the ultraviolet light and other high energy particles at this height.  The ionosphere is what radio signals bounce off.

Vertical Composition

The atmosphere also change composition with height and can be divided into two layers.  The lower layer is called the homosphere and has the composition we talked about earlier.  It's top is approximately the mesopause.

Above the homosphere lies the heterosphere, a layer in which the gases are stratified into four shells.  The lowermost shell is dominated by molecular nitrogen (N₂); next, a layer of atomic oxygen (O) is encountered, followed by a layer dominated by helium atoms (He), and finally, a layer consisting of hydrogen atoms (H).

The Ionosphere

The ionosphere lies from about 80-400 km in height and is electrically charged as short wave solar radiation ionizes the gas molecules.  The electrical structure of the atmosphere is not uniform and is arranged into three layers, D, E, and F.  Since the production of charged particles requires solar radiation, the thickness of each layer, particularly the D and E layers, changes from night to day.  The layers weaken and disappear at night and reappear during the day.  The F layer is present during both day and night.  This change in height of the various electrically charged layers doesn't effect the weather, but does effect radio signals.

The auroras also take place in the ionosphere since this is the electrically charged layer.  The aurora borealis (northern lights) and aurora australis (southern lights) is closely correlated to solar flare activity.