Geography 140
Introduction to Physical Geography

Lecture: Vertical Thermal Structure of the Atmosphere

 IV. The atmosphere has a vertical thermal structure as well as a vertical 
     pressure structure.
     A. The atmosphere has been divided into layers according to the behavior 
        of temperatures in their relationship to altitude.  In this lecture, 
        I'll start from the ground and move up through the atmosphere.

        [ vertical temperature structure of the atmosphere ]

     B. The lowest layer is the troposphere, the layer in which we live and in 
        which our weather is experienced.  In fact, "troposphere" means the 
        realm of mixing, because air is vigorously mixed and stirred here by 
        storms, convection, and wind systems.
        1. It extends up roughly 10 km (oh, about 6 miles-ish).
        2. It is characterized by an inverse relationship between air 
           temperatures and altitude:  Temperatures drop as you climb up in 
           the troposphere.
           a. In still air, it cools by an average of 6.5 C for every 
              kilometer (1,000 meters) gain in elevation.  So, if it's a 
              gorgeous, sunny, toasty day at Beach, 30 C (~86 F), 
              it'll be about 23.5 C (room temperature) on top of the 
              Santa Monicas out near the Ventura County line.  
           b. This drop in temperatures with a rise in elevation is called the 
              "normal lapse rate."  It represents an average, not the actual 
              lapse rate at a particular place and time.
        3. The tropopause is the top of the troposphere: The troposphere stops 
           a. It is situated about 10 km up.
                i. It's more like 8 km (5 mi.) over the polar regions in 
                   winter (cold air tends to settle downward) and 18 km (11 
                   mi.) over the equatorial regions, due to greater convection 
                   there (heating caused by the direct rays of the sun).
               ii. In the mid-latitudes, it's lower in winter and higher in 
                   summer, for the same sorts of reasons.

                   [ cross section showing tropopause from pole to 

           b. At the tropopause, temperatures stop dropping with gains in 
                i. The normal lapse rate no longer applies.
               ii. In fact, temperatures do nothing as altitude increases:  
                   The tropopause could be described as an "isothermal" belt 
                   (a zone having the  same temperatures as you move up).
              iii. Temperatures at the tropopause are pretty cold:  about 
                   -50 C (roughly -120 F).  
           c. The tropopause, then, separates the intensely mixed air of the 
              troposphere from the much quieter air of the stratosphere (home 
              of the ozone layer, which you met before).
     C. The stratosphere is the next major division.
        1. It extends from the tropopause up to about 50 km.
        2. It is characterized by a direct relationship between temperatures 
           and altitude:  Temperatures climb as you climb.        
           a. The top of the stratosphere is called, yes, you guessed it, the 
              stratopause, another isothermal belt.
           b. By the time you get to the stratopause, temperatures have warmed 
              up to freezing or close to it! 
           c. This warming with altitude has to do with ... ... the presence 
              of the ozone layer in the stratosphere.  Remember the ozone 
              layer, up there from, oh, 20 to 50 km?  Ozone absorbs high 
              energy, shortwave, ultraviolet radiation from the sun.  
              Absorption of energy heats the absorbing object, and so it is 
              here:  Ozone heats up by absorbing UV radiation, and that 
              accounts for the climb in temperatures with a climb in altitude 
              here in the stratosphere.
     D. The mesosphere is the layer above the stratopause.
        1. It extends up from the stratopause to about 80 km.
        2. It is characterized by resumption of an inverse relationship 
           between temperature and altitude: Temperatures go back to dropping 
           as you climb.
        3. And do they ever drop!  They get down to nearly -100 C 
           (~200 F).  This is the coldest layer in the atmosphere.
        4. That low temperature is attained at the mesopause, which tops the 
        5. In a manner of thinking, the mesosphere is a resumption of the 
           troposphere after the rude interruption of the ozone layer in the 
        6. The difference is that the mesosphere is not in contact with the 
           forces mixing up the air in the troposphere, so it's much quieter 
           up there.
     E. The thermosphere is the last thermally defined layer of the 
        1. It is characterized by a direct relationship between temperature 
           and altitude.
        2. Temperatures get up to 725 - 1,225 C.
        3. This sounds terribly impressive, but you have to remember that 
           there are so few molecules and ions up there to get excited to 
           motion levels corresponding to 725 - 1,225 C.  If you 
           were up there, it would feel pretty cold as you got yourself a 
           nasty dose of radiation, because there are so very few molecules to 
           transfer energy to your skin.  If you were up there, bare-skinned, 
           you'd have bigger things to worry about than the measured air 
        4. The thermosphere can be further subdivided on the basis of physico-
           chemistry, kind of like when we noted that the (thermally-defined) 
           stratosphere contains the (chemically-defined) ozone layer.
           a. The lower thermosphere is called the ionosphere.
                i. The ionosphere extends from roughly 80 km (50 mi.) to 
                   somewhere around 300 to 600 km out (~185 - 375 mi.).
               ii. It is the first line of defense for Earth against extremely 
                   short wave radiation (e.g., UV-B and UV-C) and, to a lesser 
                   extent, high energy particles from the sun and cosmic 
                   "rays." These particles are ionized atoms, that is, atoms 
                   with missing electrons, including isolated protons and 
                   alpha-particles (two protons with two neutrons and no 
                   electrons).  Cosmic "rays" are deadly to life on Earth.  
              iii. These rays and really high energy, fast-moving particles 
                   smash into the few molecules of the ionosphere with such 
                   force that they strip them of electrons, turning them into 
                   ions, or electrically-imbalanced atoms, too.
               iv. The ions, with their electrical imbalances, are drawn by 
                   the earth's magnetic field and align themselves with that 
                   field's lines of force, rather the way iron filings align 
                   themselves with the magnetic field lines on a sheet of 
                   paper lying above a magnet.  This means that they are 
                   really concentrated in great abundance where those magnetic 
                   field lines converge, at the north and south magnetic 
               iv. One side effect of this process and of further interactions 
                   between Earth's ions and those from the sun or outer space 
                   is a cascade of lower energy particles, including visible 
                   light photons.  This produces the aurorae, which are often 
                   visible in high latitude regions.  They look like glows, 
                   rays, arcs, and even curtain-like veils (which even move, 
                   kind of the way a curtain will move in an open window).  
                   They're usually greenish, but other colors are sometimes 

                   [ Aurora over Circle, Alaska, NASA Goddard SFC, 

                   Dick Hutchinson of Circle, Alaska, took this picture, and 
                   you can see more of his work with aurorae there at: 

                   a. These light displays are called the aurora borealis or 
                      Northern Lights in the Northern Hemisphere and the 
                      aurora australis or Southern Lights in the Southern 
                   b. Normally, you have to be near the polar regions to see 
                      them but, every once in a while, especially during the 
                      peaks of the sunspot cycle (when the sun is emitting a 
                      stronger solar wind of charged particles), they have 
                      been seen at lower latitudes, including even in the 
                      tropics!  Even if they were apparent at our latitude of 
                      34 N, we would probably miss the show, though, 
                      because of light pollution from our city lights:  You 
                      would have to be out in the mountains or desert to see 
                      them if they were down in our low latitudes.
                v. Another side effect is that the ionosphere absorbs and 
                   reflects radio waves.  A layer of ions at the very bottom 
                   of the ionosphere (largely oxygen) forms in the daytime and 
                   can absorb a lot of radio waves.  It disappears at night, 
                   but dense ion layers higher up last longer into the night 
                   and reflect radio waves, espeially short wave radio.  This 
                   allows radio stations to be picked up way beyond the 
                   horizon from the broadcast tower, which is why you can pick 
                   up radio broadcasts from Nevada, Nebraska, and Iowa at 
                   night sometimes.

           b. The exosphere is the second, outer layer of the thermosphere                      

                   [ image of the exosphere, courtesy of NASA's 

                i. The exosphere lies beyond about 500-1,000 km and is 
                   characterized by increasing hydrogen and helium content, 
                   because the oxygen and nitrogen that dominate the lower 
                   atmosphere have been dissociated into ions in the 
                   ionosphere.  We're not talking too many molecules and ions 
                   way the heck up here.   Density shades into the levels seen 
                   in interplanetary space up around, oh, 10,000 km.
               ii. Hydrogen and helium molecules can and do easily leave Earth 
                   orbit for outer space from the top of the exosphere, 
                   because at this altitude, when their trajectories are 
                   changed by collision with another molecule to a generally 
                   upward direction, they have less and less chance of 
                   colliding with another one and being bounced back 
                   earthward.  So, there is a constant loss of H and He to 
                   outer space from the upper exosphere, and Earth's gravity 
                   is not strong enough to hold onto these lightest of all 
                   gasses (unlike such big bruisers as Jupiter, Saturn, 
                   Uranus, and Neptune, the gas giant planets in the outer 
                   solar system, which retain hydrogen and helium atmospheres 
                   with their tremendous size and gravity).
  V. So, you've seen that the atmosphere has a distinct vertical structure, 
     which can be described in terms of a pressure gradient, as in the last 
     lecture, and in terms of its thermal structure and its physico-chemical 
     structure.  I organized this particular lecture around the thermal 
     structure of the atmosphere but included discussion of the physico-
     chemical layers within the thermal layers where they're found.  So, the 
     ozone layer was discussed as part of the stratosphere, while the 
     ionosphere and exosphere were discussed under the thermosphere.  There is 
     another distinction often made, too: We can differentiate the vertical 
     structure of the atmosphere by the balance of chemical mixing and 
     A. The homosphere is that portion of the lower atmosphere with almost no 
        ionization and in which there are mixing mechanisms moving gases from 
        one part of the atmosphere to another, resulting in a fairly uniform 
        chemical composition:  This is the lower 80-100 km or thereabouts 
        (troposphere through mesosphere).  
        1. The major variation you get in here is a greater concentration of 
           ozone in the stratosphere, but ozone is a non-ionized gas, even so.
        2. Throughout the homosphere, then, nitrogen and oxygen gasses 
     B. The heterosphere is the area in which you get varying mixes of gas 
        molecules and ions, and in which the nitrogen and oxygen that dominate 
        the lower atmosphere give way, through ionization, to hydrogen and 
        helium at the top of the atmosphere.  That would be somewhere from 80 
        km to 10,000 km, where the hydrogen and helium gasses are so feebly 
        held to Earth that they are constantly being Lost in Space.

Come away from this lecture knowing the four different thermally-defined 
layers of the atmosphere (troposphere, statosphere, mesosphere, and 
thermosphere) and what sort of relation (direct or inverse) between 
temperature and altitude is found in each of them.  Know the three isothermic 
belts (tropopause, stratopause, and mesopause) and what the dickens an 
"isothermic belt" is.  

Learn which physico-chemical feature is found in the four layers (weather in 
the troposphere, ozone layer in the stratosphere, and the ionosphere and 
exosphere in the thermosphere), and be able to explain the aurorae (Northern 
Lights or aurora borealis and Southern Lights or aurora australis).  Also, 
know the difference between the homosphere and the heterosphere and which 
thermally-defined layers belong to which.