LECTURE 6 - THE EARTH'S ATMOSPHERE

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1 LECTURE 6 - THE EARTH'S ATMOSPHERE Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Earth s Atmosphere, slide 1 here INTRODUCTION Earth s Atmosphere, slide 2 here Photos of earth from space do not show that atmosphere. Yet this invisible region is extremely important. The atmosphere of the earth is complex. It consists of numerous layers and sublayers, each undergoing constant change. Earth s Atmosphere, slide 3 here The atmosphere is heated from above by sunlight, and loses heat to space. The heating and cooling are uneven, which insures that energy will constantly be transferred from region to region and from layer to layer. Chemical differences exist between layers, and often work with the physical processes to make the atmosphere the complex, dynamic system that it is. Earth s Atmosphere, slide 4 here Many environmental concerns center on the atmosphere. Earth s Atmosphere, slide 5 here Air pollution, often as smog, influences many areas. Global warming, due to the increase of greenhouse gases in the atmosphere, raises the threat of increasing numbers of very large hurricanes, which are one way that the atmosphere transfers energy from one region to another. Global warming also poses the threat of sea-level rise. Since that topic is covered in another graduate course, it will not be discussed here. Another concern is the depletion of ozone in the upper part of the earth's atmosphere. In this course we will examine the latter problem. A SUMMARY OF THE EARTH'S ATMOSPHERE Earth s Atmosphere, slide 6 here Earth's atmosphere is subdivided into five layers, based on their respective thermal properties. Layer names end in the suffix -sphere, and are separated by boundaries named with words ending in the suffix -pause. Table 6-1 lists the atmospheric layers and their altitudes. Note that the average height of the tropopause is higher at the equator than at the pole. Thermal properties and the temperatures associated with the boundaries are also noted in table

2 TABLE 6-1 ATMOSPHERIC LAYERS Name of Layer or Boundary Altitude (Pole) in km Altitude (equator) in km Thermal Properties Troposphere Temperature decreases with altitude Tropopause C Stratosphere Temperature increases with altitude Stratopause C Mesosphere Temperature decreases with altitude Mesopause C to -120 C Thermosphere C at base to 500 C at sunspot minima; to 1500 C at sunspot maxima Thermopause C Exosphere space space C After Emiliani, 1992, p. 260 Earth s Atmosphere, slide 7 here In the troposphere, the temperature decrease is due to increasing distance from the heat source, which is the ground. The troposphere is transparent to most incoming radiation. Little incoming radiation is absorbed within the layer, and thus no heating occurs within the layer. Earth s Atmosphere, slides 8-9 here The troposphere is well-mixed, but it may be unstable. Earth s Atmosphere, slide 10 here Instability can lead to the formation of storms, including large supercell storms. The troposphere is thus the layer in which most of the earth s weather occurs. Clouds and storms form when pockets of air rise and cool because they expand in the lower pressure of the upper atmosphere. The air pockets become saturated and the water vapor condenses to form clouds. 2

3 Earth s Atmosphere, slide 11 here In the stratosphere some radiation is absorbed. Ozone absorbs strongly in the ultraviolet part of the spectrum, and is most abundant in the lower stratosphere (up to about 22 kilometers). The ultraviolet radiation that produces the ozone (see next section) is, of course, strongest at the top of the atmosphere. The resulting heating is a combination of the UV energy intensity and the ozone concentration. Ozone forms a kind of layer in the stratosphere, where it is more concentrated than anywhere else, but even there it is relatively scarce. Its concentrations in the ozone layer are typically only 1 to 10 parts of ozone per 1 million parts of air. Earth s Atmosphere, slide 12 here In the mesosphere temperature again decreases with increasing altitude, as the mesosphere is transparent to incoming radiation and no in situ heating occurs. Mixing in this layer is good. The mesosphere is also the layer in which many meteoroids burn up. Earth s Atmosphere, slide 13 here The thermosphere stretches above the mesopause. The space shuttle flies in the thermosphere. Earth s Atmosphere, slide 14 here The space shuttle flies in the thermosphere. Earth s Atmosphere, slide 15 here Incoming radiation is strong enough to ionize some gas particles. Ionization processes release energy which heat up the upper atmosphere. So temperature increases with height in the ionosphere region to the extent that by km, the Earth's atmosphere is extremely hot compared to surface temperatures. Above 150 km, the region is approximately isothermal because the particle densities are low, so particles have long free paths, enabling the entire region to equilibrate. Temperatures increase to about 500 C in the region 150 to kilometers altitude during sunspot minima. During sunspot maxima, the temperature may reach 1500 C. Earth s Atmosphere, slide 16 here It should be noted that the layer from km is normally designated as the ionosphere, because it is within this layer that most of the ionization takes place. The ionosphere is contained wholly within the thermosphere. It is responsible for the reflection of radio waves around the earth, thus enabling long distance communication. (Emiliani, 1992) The ionosphere is also the region in which aurora occur. The video shows an aurora taped on January 22,

4 Earth s Atmosphere, slide 17 here Invisible layers of ions and electrons are suspended in the Earth's atmosphere above about 60 kilometers in altitude. The main source of these layers is the Sun's ultraviolet light which ionizes atoms and molecules in the Earth's upper atmosphere. During this process, called photoionization, an electron is knocked free from a neutral atmospheric particle, which then becomes an ion. Because the Sun's light is responsible for most of the ionization, the ionosphere reaches maximum densities just after local noon. In this region, at altitudes where the highest densities occur, about one in every 1000 air particles is ionized. Resulting ionospheric densities are about a million ions and electrons per cubic centimeter. (Bergman et al., 2004) Flares and other energetic events on the Sun produce increased ultraviolet, x-ray and gamma-ray photons that arrive at the Earth just 8 minutes later and dramatically increase the density of the ionosphere on the dayside. These solar events also can produce high velocity protons and electrons (arriving at Earth hours to days later) that precipitate into the ionosphere in the polar regions producing large increases in the density of the ionosphere at low altitudes. Earth s Atmosphere, slide 18 here Above the thermosphere is the exosphere, which extends into space. In this region atoms and molecules escape into space. This is the true upper limit of the Earth's atmosphere. PROPERTIES OF OZONE Earth s Atmosphere, slide 19 here Ozone is an unusual form of oxygen. It contains three oxygen atoms and may be written chemically as O 3. Ozone is a colorless, unstable gas with a slightly sweet odor (Botkin and Keller, 1987). Ozone is an extremely strong oxidizing agent that normally combines quickly with other substances. Earth s Atmosphere, slide 20 here In the lower portions of the atmosphere, it is regarded as a serious pollutant. In the troposphere, high-energy electrical discharges we know as lightening can produce ozone. The electrical discharge can break apart an ordinary oxygen atom into two free radicals (eq. 6-1), Earth s Atmosphere, slide 21 here 6-1 4

5 where O * represents an oxygen free-radical. The free radicals are extremely reactive. They may react in several ways, two of which are shown in equations 6-2 and Equation 6-2 destroys the free radicals with no net change. Equation 6-3 produces ozone, which can then undergo further reactions. In the stratosphere, at levels of about 16 to 50 kilometers above the surface, free oxygen radicals may be produced by equation 6-1, this time with ultraviolet radiation instead of lightening serving to split the diatomic oxygen molecule. Equation 6-3 then produces ozone with the aid of other molecules that serve as catalysts. Ozone can absorb incoming ultraviolet radiation. Earth s Atmosphere, slide 22 here 6-4 The ultraviolet light splits the ozone, destroying it temporarily. However, the ozone is immediately recreated. M represents an additional molecule of any substance. It is needed simply to conserve momentum, and thus functions only as a catalyst. Normally, a steady state exists in which the amount of ozone destroyed and the amount created are equal. Atmospheric Changes Earth s Atmosphere, slide 23 here Since the discovery of ozone depletion in the atmosphere, much scientific research, and even more attention in the popular press, has focused on ozone and the stratosphere. Other studies have focused on climate change, but these have received much less attention in the popular press. Earth s Atmosphere, slide 24 here 6-5 5

6 Polvani et al. (2011) studied the importance of stratospheric ozone depletion on the atmospheric circulation of the troposphere in the Southern Hemisphere, in contrast with changes caused by increased greenhouse gases and accompanying sea surface temperature changes. They showed that the impacts of ozone depletion are roughly 2 3 times larger than those associated with increased greenhouse gases, for the Southern Hemisphere tropospheric summer circulation Earth s Atmosphere, slide 25 here Gonzalez et al. (2014), based on climate modeling studies, have concluded that, All 6 climate models consistently reveal that stratospheric ozone depletion results in a significant wetting of SESA over the period Taken as a whole, these model results strongly suggest that the impact of ozone depletion on SESA precipitation has been as large as, and quite possibly larger than, the one caused by increasing greenhouse gases over the same period. (SESA means southeastern South America) These are just a couple of examples of studies linking the effects of ozone depletion in the stratosphere to observed climate change at the earth s surface. References Jennifer Bergman and the Windows on the Universe Team, Layers of the Earth's Atmosphere, November 7, 2010, (last seen August 19, 2014). Daniel B. Botkin and Edward A. Keller, Environmental Studies, Earth as a Living Planet, Second edition, Merrill, Columbus, Ohio, Cesare Emiliani, Planet Earth: Cosmology, Geology, and the Evolution of Life and the Environment, Cambridge University Press, Cambridge, United Kingdom, Paula L.M. Gonzalez, Loreenzo M. Polvani, Richard Seager, and Gustavo J.P. Correa, 2014, Stratospheric ozone depletion: a key driver of recent precipitation trends in South Eastern South America, 42, , available at (last seen August 21, 2014) Lorenzo M. Polvani, Darryn W. Waugh, Gustavo J.P. Correa, and Seok-Woo Son, 2011, Stratospheric Ozone Depletion: The Main Driver of Twentieth-Century Atmospheric Circulation Changes in the Southern Hemisphere, Journal of Climate, 24, , available at (last seen August 21, 2014) \4241LN06_PP_F16.pdf September 06,