The Earth’s Atmosphere

1. The Earth’s Atmosphere • Overview of the Earth’s atmosphere • Other planetary atmospheres • Vertical structure of the atmosphere • Weather and climate

2. Overview of the Earth’s Atmosphere • The atmosphere, when scaled to the size of an apple, is no thicker than the skin on an apple. • The atmosphere is a gas. • The atmosphere is a fluid. • There is a surface but no “top” – the atmosphere • gradually thins out with increasing altitude

3. Thickness of the Atmosphere Approximately 80% of the atmosphere occurs in the lowest 20km above the Earth. Radius of the Earth is over 6,000 km Atmosphere is a thin shell covering the Earth.

5. Structure of the Atmosphere • Observed lapse rate. • The temperature decreases approximately 6.5 OC for each km of alitude (3.5 OF/1,000 ft) • Inversion • When a layer of the atmosphere increases with altitude. • Troposphere • The layer of the atmosphere from the surface of the Earth up to where it stops decreasing in temperature. • Up to a height of about 11 km (6.7 mi) • Air is constantly mixed due to denser air being above less dense air.

6. On the average, the temperature decreases about 6.5OC/1,000 km, which is known as the observed lapse rate. An inversion is a layer of air in which the temperature increases with height.

7. Tropopause • The upper boundary of the Troposphere • The temperature remains constant with increasing altitude • Stratosphere • Temperature begins to increase with height. • Very stable as denser air is below less dense air. • Up to about 48 km (30 mi) • Temperature increases as a result of interactions between high energy UV radiation and ozone (O3)

8. Temperature Layers of the Atmosphere: Troposphere • Lower part of the atmosphere • Energy source is heating of the earth’s surface by the sun. • Temperature generally decreases with height. • Air circulations (weather) take place mainly here. • Troposphere goes from surface to about 30,000 ft. (10 km).

9. Temperature Layers of the Atmosphere: Mesosphere • Above 50 km, very little ozone, so no solar heating • Air continues to cool with height in mesosphere • Mesosphere extends from about 50 km to 90 km above the surface http://www.bath.ac.uk/pr/releases/images/antarctic/noctilucent-clouds.jpg

10. Temperature Layers of the Atmosphere: Thermosphere • Above 90 km, residual atmospheric molecules absorb solar wind of nuclear particles, x-rays and gamma rays. • Absorbed energy causes increase of temperature with height. • Air molecules are moving fast, but the pressure is very low at these heights.

11. Importance of Stratosphere, Mesosphere and Thermosphere • Solar nuclear particles, x-rays, gamma rays, and ultraviolet light can damage living cells. • Thermosphere, mesosphere and stratosphere shield life on Earth from these damaging rays.

12. Exosphere • Outermost layer of the atmosphere where molecules merge with the vacuum of space. • The high kinetic energy of the molecules at this height are significant enough to cause them to be able to escape into space. • Ionosphere • Alternative name for the thermosphere and upper mesosphere. • Due to the occurrence of free electrons and ions. • It is the electrons and ions in this layer that cause radio waves to be able to be reflected around the world.

13. The structure of the atmosphere based on temperature differences. Note that the "pauses" are actually not lines, but are broad regions that merge.

14. Fig. 1-4, p. 5

15. Temperature Layers of the Atmosphere: Stratosphere • Sun’s ultraviolet light is absorbed by ozone, heating the air. • Heating causes increase of temperature with height. • Boundary between troposphere and stratosphere is the tropopause. • Stratosphere goes from about 10 to 50 km above the surface.

16. 5.2 The ozone layer • In the 1970s, scientists noticed that the ozone layer in the stratosphere above Antarctica was thinning.

17. Variable Gases – Ozone (O3) • Near the surface, ozone concentrations about 0.04-0.15 ppm • In the upper atmosphere ozone concentration can reach ~15 ppm • Upper atmospheric ozone is vital to blocking harmful radiation • Ozone near the surface, however, harmful to life • Chlorofluorocarbons (CFCs) are believed to be depleting upper atmospheric ozone Satellite images showing depletion of ozone.

18. 5.2 Chlorofluorocarbons and the ozone layer • A group of chemicals called chlorofluorocarbons (or CFCs) were once commonly used in air conditioners, in aerosol spray cans, and for cleaning machine parts. • In the London Agreement of 1991, more than 90 countries banned the production and use of CFCs except for limited medical uses.

19. 5.2 Chlorofluorocarbons and the ozone layer • The ozone layer absorbs the Sun’s high-energy ultraviolet (UV) radiation and protects the Earth.

20. 5.2 Chlorofluorocarbons and the ozone layer • In the stratosphere, the CFCs break down and release chlorine. • The chlorine reacts with ozone molecules, which normally block incoming ultraviolet radiation.

22. Stratopause • Where the temperature reaches a maximum of 10 OC (50 OF) • Ozone shield • A layer of ozone that absorbs much of the ultraviolet radiation that enter the atmosphere. • Provides a significant shield to the Earth below from damging UV radiation

23. Composition of the Atmosphere • permanent gases • variable gases • trace gases • aerosols • roles of nitrogen, oxygen and argon • role of water vapor • carbon dioxide, methane, ozone, CFCs, et al.

24. Composition of the Atmosphere • The “dry atmosphere”: 78% N2, 21% O2, 1% Ar • N2 is primordial – it’s been part of the atmosphere as long as there’s been an atmosphere • O2 has been rising from none at all about 2.2 Gya – comes from photosynthesis • Ar40/Ar36 tells us that the atmosphere has been outgassed from volcanoes

25. Composition of the Atmosphere • Water Vapor: H2O 0-4% • H20 can exist in all three phases at the surface of the Earth – solid, liquid and gas • Liquid or solid H2O can be suspended by atmospheric winds (clouds) or fall to the surface (precipitation) • VERY powerful greenhouse gas (both in vapor form and as clouds)

26. Composition of the Atmosphere • The Hydrological Cycle

27. Table 1-1, p. 3

28. Composition of the Atmosphere • Carbon dioxide • 390 ppm (by mass) and counting… • Natural and anthropogenic sources/sinks • Strong greenhouse gas (GHG) CO2 is neither the strongest atmospheric GHG pound-for-pound nor molecule-for-molecule… Why the fuss? CO2 is a product of the reaction that allows modern civilization to exist: combustion.

29. Composition of the Atmosphere • The Global Carbon Cycle

30. Composition of the Atmosphere • Methane • CH4 concentration: 1.8 ppmv • anthropogenic and natural sources/sinks too • powerful greenhouse gas • oxidizes rapidly, hence low concentrations • Large concentrations proposed to explain greenhouse warming of early Earth

31. Composition of the Atmosphere • Ozone, CFCs and NOx • Ozone (O3) • shields the surface from UV rays • produced by reaction with NOx and sunlight near the surface • CFC’s (Chlorofluorocarbons) • destroy stratospheric ozone • chlorine is a catalyst: it destroys one O3 molecule and then is free to find another • Ozone at high altitudes (stratosphere) is “good”;ozone at low altitudes (troposphere) is “bad.”

32. Composition of the Atmosphere • Aerosols • Dust • Sea-spray • Microbes Suspended particles in the atmosphere are responsible for cloud formation: water drops nucleate on them Cloud Condensation Nuclei (CCN)

33. Earth's atmosphere has a unique composition of gases when compared to that of the other planets in the solar system.

34. Atmospheric Pressure • The atmosphere exerts pressure on the Earth that decreases with increasing altitude • This is due to the fact that with increasing altitude, there is a decrease in the column of gases above the Earth’s surface • Hydrostatics considers the pressure that is exerted by a fluid that is at rest. • Using this as a frame of reference the atmospheric pressure is viewed as a result of the mass of the column of gases above the Earth. • Using a molecular frame of reference, the atmospheric pressure is viewed as a result of the kinetic energy of molecules and the force with which they strike an object. • Atmospheric pressure is actually a result of the interaction between these two factors.

35. At greater altitudes, the same volume contains fewer molecules of the gases that make up the air. This means that the density of air decreases with increasing altitude.

36. Variable Gases - Water Vapor • Water vapor is invisible – don’t confuse it with cloud droplets • Less than 0.25% of total atmosphere • Surface percentages vary between <<1% in desserts to 4% in tropics • Typical mid-latitude value is about 1-2% • Some satellites sensors can detect actual water vapor in atmosphere Water Vapor Image Visible Image

37. Variable Gases – Methane (CH4) • Concentrations of about 1.7 ppm • Extremely potent green house gas - 21 times more powerful by weight than carbon dioxide • Has varied cyclically on a 23,000 year cycle • Pattern broken in past 5,000 years with unexpected increase – more abundant now than in last 400,000 years • Increase attributed to agriculture, bio-mass burning, fossil fuel extraction, some industry and ruminant out-gassing (cow/sheep burps) Methane growth and sources (From EPA)

38. Aerosols (or Particulates) • Small (or “tiny”) solid particles or liquid droplets (excluding clouds and rain) • Aerosols can be man-made (anthropogenic) or naturally occurring (like ocean salt, dust, plant emissions) • Aerosols are not synonymous with pollution • Some aerosols are very beneficial and, in fact, are required for precipitation processes to occur.

39. Density and Pressure with Height Because of compression, the atmosphere is more dense near the surface. Density decreases with altitude

40. State VariablesTemperature • Air molecules are moving all around us, bouncing off each other and us. • When the air molecules have greater kinetic energy (energy of motion), they are moving faster. • The temperature of the air molecules is a measure of the average speed of the molecules per standard volume

41. Temperature Scales K = °C +273.16 F = 9/5°C + 32 C = 5/9(°F – 32)

42. Temperature Change w/Altitude • As a parcel of air rises, it expands due to lower pressure. • Work done by molecules to expand causes temperature to decrease (cools) • As air sinks, the parcel experiences compression due to higher pressure • Air molecules have work done on them, temperature increases (warms)

43. Cold Front Warm Front Weather Basics • Atmospheric Temperature • Areas separating colder and warmer air on a weather map are represented by fronts • Cold Fronts (blue – pointed barbs) indicate the movement of a cold air mass into a warmer region • Warm Fronts (red – rounded barbs) indicate movement a warm air mass into a colder region

44. Weather Basics • Atmospheric Humidity • Relative Humidity provides a measure of the amount of water vapor in the air relative the maximum possible for a given temperature • Dew Point Temperature is the temperature the air must be cooled to for condensation to occur. • Much more on these concepts in later chapters

45. The earth's atmosphere thins rapidly with increasing altitude and is much closer to the earth than most people realize.

46. The mercury barometer measures the atmospheric pressure from the balance between the pressure exerted by the weight of the mercury in a tube and the pressure exerted by the atmosphere. As atmospheric pressure increases and decreases, the mercury rises and falls. This sketch shows the average height of the column at sea level.

47. Warming the Atmosphere • The temperature of an object is actually a measure of the kinetic energy of the molecules that make up the object. • Any object that contains any kinetic energy at all (i.e. has a temperature above absolute 0K gives off radiant energy. • Solar constant • When the sunlight is perpendicular to the outer edge and the Earth is at an average distance from the Sun it produces about 1,370 watts per m2. • This quantity is believed to remain constant.

48. On the average, the earth's surface absorbs only 51 percent of the incoming solar radiation after it is filtered, absorbed, and reflected. This does not include the radiation emitted back to the surface from the greenhouse effect, which is equivalent to 93 units if the percentages in this figure are considered as units of energy.

49. Mesosphere • Temperature again begins to decrease due to a decrease in gas molecules to absorb radiation • Thermosphere • Temperature again begins to rise due to the presence of molecular fragments which absorb radiation from space. • Temperature is extremely high here due to the average kinetic energy of the molecules. • Very little energy transfers, however, due to the lack of molecules (very few molecules to collide with objects)

50. The Early Atmosphere • reduced primitive atmosphere(H, He, CH4, NH3) • outgassing and the second atmosphere (N2, Ar – still no oxygen!) • The evolution of life and the atmosphere are closely linked – life produced the oxygen (photosynthesis) and cycles the carbon (e.g. limestone) • Oxidized modern atmosphere (N2, O2, CO2, etc.)