Page 123 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 3 Earth’s Modern Atmosphere 87
concepts review
KEy LEARNING
■ Draw a diagram showing atmospheric structure based on three criteria for analysis—composition, tempera- ture, and function.
The principal substance of Earth’s atmosphere is air—the medium of life. Air is naturally odourless, colourless, tasteless, and formless.
Above an altitude of 480 km the atmosphere is rar- efied (nearly a vacuum) and is called the exosphere, which means “outer sphere.” The weight (force over a unit area) of the atmosphere, exerted on all surfaces, is air pressure. It decreases rapidly with altitude.
By composition, we divide the atmosphere into the heterosphere, extending from 480 km to 80 km, and the homosphere, extending from 80 km to Earth’s sur- face. Using temperature as a criterion, we identify the thermosphere as the outermost layer, corresponding roughly to the heterosphere in location. Its upper limit, the thermopause, is at an altitude of approximately 480 km. Kinetic energy, the energy of motion, is the vibrational energy that we measure as temperature. Heat is the flow of kinetic energy between molecules and from one body to another because of a tempera- ture difference between them. The amount of heat ac- tually produced in the thermosphere is very small be- cause the density of molecules is so low there. Nearer Earth’s surface, the greater number of molecules in the denser atmosphere transmits their kinetic energy as sensible heat, meaning that we can feel it as a change in temperature.
In the homosphere, temperature criteria define the mesosphere, stratosphere, and troposphere. Within the mesosphere, cosmic or meteoric dust particles act as nu- clei around which fine ice crystals form to produce rare and unusual night clouds, the noctilucent clouds.
The top of the troposphere is wherever a temper- ature of –57°C is recorded, a transition known as the tropopause. The normal temperature profile within the troposphere during the daytime decreases rapidly with increasing altitude at an average of 6.4 C° per km, a rate known as the normal lapse rate. The actual lapse rate at any particular time and place may deviate consider- ably because of local weather conditions and is called the environmental lapse rate.
The outermost region we distinguish by function is the ionosphere, extending through the heterosphere and part- way into the homosphere. It absorbs cosmic rays, gamma rays, X-rays, and shorter wavelengths of ultraviolet radia- tion and converts them into kinetic energy. A functional region within the stratosphere is the ozonosphere, or ozone layer, which absorbs life-threatening ultraviolet radiation, subsequently raising the temperature of the stratosphere.
air (p. 66)
exosphere (p. 66)
air pressure (p. 66) heterosphere (p. 67) homosphere (p. 67) thermosphere (p. 69) thermopause (p. 69)
kinetic energy (p. 69) mesosphere (p. 70)
noctilucent clouds (p. 70) stratosphere (p. 71)
troposphere (p. 71)
tropopause (p. 71)
normal lapse rate (p. 71) environmental lapse rate (p. 71) ionosphere (p. 71)
ozonosphere, ozone layer (p. 72)
1. What is air? Where generally did the components in Earth’s present atmosphere originate?
2. Characterise the various functions the atmosphere performs that protect the surface environment.
3. What three distinct criteria are employed in dividing the atmosphere for study?
4. Describetheoveralltemperatureprofileoftheatmo- sphere and list the four layers defined by temperature.
5. Describe the two divisions of the atmosphere on the basis of composition.
6. What are the two primary functional layers of the atmosphere and what does each do?
■ List and describe the components of the modern atmosphere, giving their relative percentage contribu- tions by volume.
Even though the atmosphere’s density decreases with in- creasing altitude in the homosphere, the blend of gases (by proportion) is nearly uniform. This mixture of gases has evolved slowly and includes constant gases, with concentrations that have remained stable over time, and variable gases, that change over space and time.
The homosphere is a vast reservoir of relatively inert nitrogen, originating principally from volcanic sources and from bacterial action in the soil; oxygen, a by-product of photosynthesis; argon, constituting about 1% of the homo- sphere and completely inert; and carbon dioxide, a natural by-product of life processes and fuel combustion.
7. Name the four most prevalent stable gases in the ho- mosphere. Where did each originate? Is the amount of any of these changing at this time?
■
Describe conditions within the stratosphere; specifi- cally, review the function and status of the ozono- sphere, or ozone layer.
The overall reduction of the stratospheric ozonosphere, or ozone layer, during the past several decades repre- sents a hazard for society and many natural systems and