1.  Nitrogen (N₂): 79%
                    originated from outgassing (given off as gas from erupting lava)  in the early Earth
            2.  Oxygen (O₂): 20%
                    originated as a product of photosynthesis by plants in early Earth.  Much of the CO₂ that was so abundant in Earth's early atmosphere was converted to O₂
            3.  Argon (Ar): ~0.93%
                    A gas with properties similar to helium and neon.  Originated from radioactive decay and outgassing.
            4.  Carbon dioxide:  ~0.036%
                     originally present in Earth's early atmosphere in much higher concentration, but was converted to O₂ and other carbon compounds through photosynthesis by plants
    B.  Other components of the atmosphere are present in variable concentrations
            1.  water vapor (H₂O): 0-4%.  We will discuss the atmosphere's vapor composition in more detail in the next chapter.
            2.  aerosols
                    a.  collectively these are dust-sized solid and liquid particles from various sources (volcanic eruptions, dust storms, fires, pollution) .
                    b.  Aerosols are capable of blocking out a portion of the Sun's energy.
            3.  ozone (O ₃)
                    a.  ozone is a form of oxygen with three oxygen atoms per molecule, instead of the usual two atoms per molecule
                    b.  ozone is often concentrated in two regions of the atmosphere
                            i.  in the troposphere around pollution sources (cities), ozone is a pollutant, and is harmful (can cause respiratory problems)
                            ii.  in the stratosphere, it is a beneficial product resulting from the interaction of Sun's ultraviolet radiation w/ O₂ molecules.  The production of O₃ results in the further absorption of Sun's ultraviolet (UV) radiation.  A depletion of ozone in the stratosphere results in more UV radiation reaching the Earth, and an increase in health problems associated with excessive exposure to UV radiation.
                                    a.  chlorofluorocarbons (CFC's) are a family of chemicals that produces a breakdown of ozone in the stratosphere
                                    b.  CFC's have been used industrially as refrigerants and as propellants in aerosol cans
            4.  various "Greenhouse" gases, apart from CO₂

III.  What is the structure of Earth's atmosphere?  Just as the solid Earth can be identified as distinct layers, Earth's atmosphere also has identifiable layers, based on trends in temperature change and other characterisitics.  Air pressure decreases steadily with an increase in altitude.
    A.  The troposphere is the lowest layer of Earth's atmosphere (the lowest 10 km, 6-7 miles)
            1.  All weather occurs in this layer
            2.  The temperature of the atmosphere decreases with altitude.  This rate of decrease is also referred to as the environmental lapse rate.  We will discuss this phenomenon in more detail in the next chapter.
            3.  The tropospause is the boundary between the troposphere and the next higher layer, the stratosphere
    B.  The stratosphere is the next higher layer after the troposphere.
            1.  The stratosphere contains beneficial ozone which absorbs much of the sun's ultraviolet radiation.  
            2.  This UV absorption contributes to a general rise in temperature with altitude in the stratosphere.
    C.  The next higher layers in the atmosphere are the mesosphere and the thermosphere.  As with the lower layers, these layers are defined by temperature trends.

IV.  Electromagnetic energy from the Sun
     A.  Energy from the Sun can be categorized on the electromagnetic spectrum.  The electromagnetic spectrum includes various types of related energy, including X-rays, visible light, and radio waves, that differ from each other by the wave length of the energy.
     B.  Electromagnetic radiation can be categorized into three groups, based on wavelength
          1.  Short wavelength energy includes forms of radiation which have wavelengths shorter than visible light, including gamma rays, X-rays, and ultraviolet radiation.  It makes up about 7% of the energy from the Sun which reaches Earth.
          2.  Visible light includes all of the various colors of light energy ranging from the shortest wavelength (violet) to the longest (red).  Approximately 48% of the Sun's energy is visible light.
          3.  Long wavelength energy includes radiation with wavelengths longer than visible light, including infrared (which we feel as heat energy), microwaves, television, and radio waves.  45% of the Sun's radiation falls into this category.

V.  Heating the Earth.
    A.  Heat and temperature
          1.  Heat is a form of energy that is expressed at a molecular level by the vibrations of molecules.  The more a substances is heated, the more vigorously the molecules vibrate.  The cooler a substance is (the less heat that is present), the less vigorously the molecules vibrate.
          2.  Heat is measured units called calories
                a.  A calorie is the amount of heat needed to raise a gram of water by 1^o C
                b.  The nutrional definition of a calorie (as in, how many calories that food has) is 1000X the energy as our definition of calorie.
          3.  Temperature is the average vibrational intensity of the molecules.  (do you remember the example of heating the bathtub of water and the cup of water by candle flame?)
          4.  Today temperature is measured by two standard scales, Fahrenheit and Celsius (also centigrade)

	Freezing	Boiling	Degree range	Who uses it?
Fahrenheit	32o	212o	180o	Most of the American public
Celsius	0o	100o	100o	Most of the rest of the world and the science community

                a.  For reference, room temperature is ~72^o F, and ~23^o C.  A really hot day would be 100^o+ F, or 40^o+ C.
                b.  Wonder why the heck 32^o is freezing in Fahrenheit scale?  Why not 0^o as freezing point?  Well, turns out that 0^o Fahrenheit was defined as the freezing point of water that is completely saturated with salt.  (you may know that when you throw salt out on ice-covered sidewalks in winter, you can get the water to melt, usually)

      B.  Heat transfer -- in Earth processes, we often need to know how heat gets from one place to another.  There are three basic types of heat transfer that we can observe in weather/climate processes:
            1. Conduction -- heat energy is transferred by collisions from one molecule to another.  Heat is expressed at a molecular level by the intensity of molecular vibrations
            2.  Convection -- heat transfer by circulation or movement of mass within a substance.
            3.  Radiation -- heat transfer of energy through space by wave energy.  Unlike other two types of heat transfer, radiation can travel through a vacuum.

     C.  Albedo
             1.  On reaching Earth, the Sun's energy may be scattered, reflected, or absorbed.
             2.  Albedo is the percent reflectivity of a surface.
                   a.  On average, Earth's albedo is 30%.
                   b.  Some surfaces on Earth have a higher reflectivity, such as snow or ice.
                   c.  Other surface on Earth have a low reflectivity, such as darker surfaces.
             3.  Some gases are effective at absorbing certain wavelengths of electromagnetic spectrum. 
                   a.  Ozone is effective at absorbing critical wavelengths of UV (ultraviolet) radiation.
                   b.  Water vapor and CO₂ are effective at absorbing longer wavelengths of energy such as infrared.  We experience this phenomenon as the Greenhouse effect.