Chap. 1 - Part I Composition of the Atmosphere

1. Chap. 1 - Part IComposition of the Atmosphere WX 201 Dr. Chris Herbster

2. Outline • Meteorology Defined • The atmosphere as a gas • Permanent and Variable Gases • Influence by planet size and distance from the Sun on atmospheric composition • Composition of Earth’s atmosphere • Comparisons with Mars and Venus • Unique features of Earth’s atmosphere compared to the other planets

3. What is Meteorology? • The study of the atmosphere and the processes that cause “weather” (cloud formation, lightning, wind movement) • Weather deals with the short term state of the atmosphere • Climate deals with the long-term patterns • More than simple long-term averages • Involves complex interactions and variability

4. 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. But what is the atmosphere? • Comprised of a mixture of invisible permanent and variable gases as well as suspended microscopic particles (both liquid and solid) • Permanent Gases – Form a constant proportion of the total atmospheric mass • Variable Gases – Distribution and concentration varies in space and time • Aerosols – Suspended particles and liquid droplets (excluding cloud droplets)

6. Composition of Earth’s Atmosphere Important gases in the Earth’s Atmosphere (Note: Influence not necessarily proportional to % by volume!)

7. Permanent Gases • 78% Nitrogen (N2) • 21% Oxygen (O2) • <1% Argon (Ar) • Relative percentages of the permanent gases remain constant up to 80-100km high (~ 60 miles!) • This layer is referred to as the Homosphere (implies gases are relatively homogeneous)

8. Homosphere: Turbulent mixing causes atmospheric composition to be fairly homogenous from surface to ~80-100 km (i.e., 78% N2, 21% O2) Heterosphere: Above ~80-100km, much lower density, molecular collisions much less, heavier molecules (e.g., N2, O2) settle lower, lighter molecules (e.g., H2, He) float to top Homosphere and Heterosphere

9. Variable Gases in the Earth’s Atmosphere • VARIABLE gases in the atmosphere and typical percentage • values (by volume): • Water vapor (H2O) 0 to 4% • Carbon Dioxide (CO2) 0.038% • Methane(CH4)         0.00017% • Ozone(O3)            0.000004% • (Note that water vapor is the third most common molecule in Earth’s atmosphere after nitrogen and oxygen)

10. 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

11. Variable Gases - Carbon Dioxide (CO2) Small percentage of total atmosphere (380 ppm) But, very important green house gas Mauna Loa Observatory CO2 trace (annual variations embedded in the long-term record)

12. Atmospheric CO2 cycle. Global climate models used to examine greenhouse warming must be able to account for multiple, complex processes in atmosphere, over land, and in ocean. Earth’s greenhouse gases contribute to a ~30C warmer surface temperature than would otherwise exist. More on this phenomenon in Ch. 2.

13. 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.

14. 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)

15. 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.

16. What Determines Atmospheric Composition? • Composition of gases on a planet is determined largely by how easily gases can escape to space • Also depends on the existence of life or geologic processes • For a gas to escape to space, it must reach its “escape velocity.” • Escape velocity is the speed required to overcome the gravitational pull of the planet • Molecular velocity is determined by the gas temperature (or average kinetic energy)

17. Escape Velocity • Gas is made up of free molecules in constant motion. • Speed of the gas molecules is determined by the temperature • Temperature determined largely by proximity to the Sun • Escape velocity depends on the gases’ molecular weight and the planets size • Lighter molecules require less speed to escape • Larger planets have stronger gravitational pull

18. Relative Planet Size and Distance from Sun • Size comparison of planets – larger planets have stronger gravitational pull • Planets closer to the Sun receive more radiant energy

19. The required “escape velocity” is determined planet size Temperature of gas determined by distance from sun. Molecular speed determined by molecular weight and temperature Gas lines above the planet will escape to space. Gas lines below the planet will remain in the atmosphere. i.e. Earth will lose hydrogen but hold water. Mars will lose water but hold carbon dioxide.

20. Earth’s Early Atmosphere • 5 Billion years ago when Earth formed, atmosphere consisted of mostly H2 , He as well as some NH3 , and CH4. • Free H2 and He molecules have low molecular weight (so move very fast), and were able to escape Earth’s gravitational pull. • Volcanoes spewed large amounts of H2O, CO2 as well as lesser amounts of N2 (outgassing) • Clouds rained forming oceans, which dissolved much of CO2 locking it insedimentary rocks through chemical and biological processes (e.g., seashell formation) allowing concentrations of N2 to increase. • O2 increased through phododissociation of H2O into H2 and O2—the H2 escaped. • Life formed, plants grew adding additional O2 through photosynthesis leading to today’s atmosphere.

21. Unique Features of Earth’s Atmosphere • Atmospheric composition – high Oxygen content, low Carbon Dioxide content. • Greenhouse gases contribute to livable surface temperatures • Most important greenhouse gas is water vapor! • Without an atmosphere, Earth’s surface temp would only be approximately 0°F! • Water in all three phases: solid, liquid, gas. • Patchy cloud fields – extensive up and down convective motions in atmosphere. • Circular motions with storms.

22. Comparison with Venus • Composition of Venus Atmosphere: 96% CO2, 3% N2 (compare to Earth—.04% CO2, 78% N2) • Pressure at surface: 90,000 mbar (by comparison, Earth’s mean sea-level pressure is approximately 1,013 mbar — Venus’ surface pressure is 90x greater!) • Temperature at surface: ~ 900oF (by comparison, Earth’s mean sfc temperature is about 59oF) • Extreme atmospheric pressures on Venus due large amount of gaseous CO2. • No mechanisms to remove CO2 from atmosphere (e.g., photosynthesis, dissolution in water).

23. Earth and Venus nearly same size – velocity required to escape gravitational pull similar for both.

24. Why the drastic difference? Venus is closer to Sun Warmer temperatures prevented liquid water from forming. With no liquid water, no means to dissolve the carbon dioxide. Result is a rich carbon dioxide atmosphere.

25. Earth and Venus CO2 and N2 • Earth actually has more CO2 than Venus (as fraction of total planet mass). • Earth and Venus have similar amounts of N2. • CO2 is 96% of Venus atmosphere and only .04% of Earth’s. • Venus has CO2 in atmosphere, while Earth has CO2 in limestone.

26. Mars • About half the size of the earth (less gravity) • Atmosphere primarily CO2 -- too heavy to escape gravitational pull • Surface pressure 1/100 of earth’s (~10 mbar) • Average surface T~213K (-76F) • Temperature between equator and poles 130C. • Temperature change of 60C between day and night (low thermal inertia) • Ice caps at poles composed of frozen CO2 • Small size of planet allowed most of atmosphere to escape

27. Weather on Earth in relation to orbital characteristics • Rotation once per 24 hrs. • Primary weather systems are moving storms with clouds, circular winds, and precipitation http://www.ssec.wisc.edu/data/globe/cldspin.html

28. Weather on Venus in relation to orbital characteristics • Rotation once per 243 (earth) days (Venus day is longer than year) • Thick atmosphere of CO2 causes greenhouse “pressure cooker.” Surface temperatures ~ 900 deg. F. • Uniform temperatures all over globe, little surface winds but strong upper level winds.

29. Weather on Mars in relation to orbital characteristics • Rotation once per 24.6 hours. • Surface temperature from –200 to +80 F. • Has frequent dust storms. • Has polar caps of CO2 and H2O. • Seasonal change causes caps to melt and reform. • Has very few clouds.

30. Summary • Composition of gases on a planet is a function of the planet size (strength of gravity holding gases onto the planet), planet temperature, and life • Primary permanent gases on Earth are Nitrogen, Oxygen, Argon • Variable gases include Water Vapor, Carbon Dioxide, Ozone, Methane, CFCs, etc. • The importance of variable trace gases is notalways proportional to the amount.

31. Summary (cont.) • Water vapor is the most important greenhouse gas, others include Carbon Dioxide, Methane and Ozone • Gases on other planets are quite different from Earth’s because of differing planet characteristics (Venus & Mars have primarily CO2 atmospheres) • Weather on Earth different from weather on other planets because of gas composition, planet size, oceans and planet rotation speed

32. Chap. 1 - Part II Fundamental Quantities~Vertical Structure of the Atmosphere~Weather Basics WX 201 Dr. Chris Herbster

33. Outline • Fundamental physical quantities covered in this course • Atmospheric state variables • Density, Pressure, temperature • Structure of the atmosphere • Troposphere • Stratosphere • Mesosphere • Thermosphere • Importance of the stratosphere and thermosphere

34. Fundamental Physical QuantitiesUnits of Measure Needed for this Course Basic Quantities Quantity Symbol SI Unit Equivalent Units Length L Meter (m) 1 m ≈ 3.28 ft Mass m Kilogram (kg) 1 kg ≈ 2.205 lb Time t Second (s) 60 s = 1 min Temperature T Kelvin (K) 273.15K ≈ 0°C = 32°F Derived Quantities Area A = L2 Sq meter (m2) 1 m2 ≈ 10.76 ft2 Volume V = L3 Cu meter (m3) 1 m3 ≈ 35.3 ft3 Density r=m/V Kg/m3 1 kg/m3 ≈ 0.06 lb/ft3 Velocity V = L/t m/s 1 m/s ≈ 2.24 mph ≈ 1.94 kt Acceleration a = V/t m/s2 Force F = m·a Newton (N) 1 N = 1 kg·m/s2 Weight Wt = m·go Newton (N) 1 N ≈ 0.225 lb; go≈ 9.8 m/s2

35. Fundamental Physical Quantities (cont.) Derived Quantities (cont.) Quantity Symbol SI Unit Equivalent Units Pressure p = F/a Pascal (Pa)* 1Pa = 10-2 mb = 100 N/m2 1hPa = 1 mb 1013 hPa ≈ 29.92 in Hg Energy/Heat/ E = F·L Joule (J) 1 J = 1 N-m Work 1 cal ≈ 4.184 J (note: 1 cal is the amount of heat needed to raise 1 g of water 1 K) Power P = E/t Watt (W) 1 W = 1 J/s * Meteorologists tend to use milli-bars (mb), which are identical equivalent to hecto-Pascals (hPa). We’ll use mb and hPa interchangeably in this course. Some Useful Conversions 1 knot (kt) ≈ 1.15 mph ≈ 0.514 m/s 1 inch Mercury (in Hg) ≈ 33.865 mb Centigrade (Celsius) to Kelvin: Add 273.15 to deg C Centigrade to Fahrenheit: Multiply by 1.8, then add 32 Fahrenheit to Centigrade: Subtract 32, then multiply by 5/9

36. Scientific Notation

37. Scientific Measurements Significant Digits: Nearest reportable values for common measurements Upper Air Wind Speeds: 5 Knots Surface Wind Speeds: Whole Knot Upper Air Pressure: Whole Millibar (mb) Surface Pressure: 1/10 (.1) mb Skew-T Temperatures: 1/10 (.1) Degree Temperatures: Whole Degree Relative Humidity: Whole Percent Upper Air Heights: Decameter

38. Atmospheric State Variables • State variables include: • Pressure • Temperature • Density • State variables are related to one another by the Ideal Gas Law (IDL) • IDL often referred to as the “Equation of State” • The state variables will be detailed throughout the course.

39. State Variables Pressure • Air is mostly made up of free molecules in constant motion (gases). • Air molecules have mass. • You can feel the mass of the air when the wind is blowing hard. • Weight (a vertical force) = Mass x Gravity • Air has mass therefore weight; pressure (weight/area) is measured by a barometer.

40. Surface Pressure • The pressure at the surface is caused by the weight of all the air molecules in the column above the surface. • Add more air molecules to the column and the pressure goes up. (High Pressure areas) • Take away air molecules from the column and the pressure goes down. (Low Pressure areas)

41. Pressure as Measured by Barometer • Weight of mercury in column equals weight of atmosphere • Average sea level pressure is: • 14.7 pounds per square inch, • 760 mm or 29.92” mercury or • 1013.25 mb

42. State VariablesDensity • Air density is the mass of the air divided by the volume of measurement. • As one goes higher in the atmosphere the number of molecules in a given volume decreases, so like pressure, density also decreases monotonically with height. • Since don’t have as many molecules on top of you, the air pressure also decreases with height.

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

44. 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

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

46. 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)

47. Air Temperature Change w/ Changes in Parcel Altitude Rising  Expansion  Cooling Sinking  Compression  Warming

48. Relating State Variables:“Equation of State” or “Ideal Gas Law” • Temperature, pressure and density related • Pressure = density*gas constant*temperature P = ρRT • If the pressure decreases, the density will decrease for constant Temp. • If the pressure decreases, the temperature will decrease for constant density, etc. • It is possible for all three state variables to change at the same time! • More in later chapters

49. Vertical Structure of the Atmosphere • Vertical Structure of the Atmosphere commonly broken into layers • Layers are most often defined by the vertical change of temperature within the layer since this is related to the presence of vertical motions (or lack of) in the layer

50. 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).