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

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. Water vapor: extremely important due to its being found in all three phases in Earth’s atmosphere, latent heat release, etc. Carbon dioxide: important gas due to its role as “greenhouse gas” (not totally correct term as we’ll see later) Ozone: dual importance (pollutant in troposphere, but very important absorber of UV in stratosphere) CFCs: Influence on ozone holeWater vapor: extremely important due to its being found in all three phases in Earth’s atmosphere, latent heat release, etc. Carbon dioxide: important gas due to its role as “greenhouse gas” (not totally correct term as we’ll see later) Ozone: dual importance (pollutant in troposphere, but very important absorber of UV in stratosphere) CFCs: Influence on ozone hole

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 and Heterosphere 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

9. Variable Gases in the Earth’s Atmosphere

10. Variable Gases - Water Vapor

11. How many have seen the movie “An Inconvenient Truth” or read the book?How many have seen the movie “An Inconvenient Truth” or read the book?

13. Variable Gases – Ozone (O3)

14. Variable Gases – Methane (CH4)

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

19.

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 in sedimentary 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

24. Why the drastic difference?

25. Earth and Venus CO2 and N2

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

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 not always 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. 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 Fundamental Physical Quantities Units of Measure Needed for this Course Mass (m) Amount of matter in an object. Length (L) A measurement of distance. Time (t) A period over which an action takes placeMass (m) Amount of matter in an object. Length (L) A measurement of distance. Time (t) A period over which an action takes place

35. 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 Fundamental Physical Quantities (cont.)

36. Scientific Notation Unit Prefixes are used to determine very large numbers from very small numbers. Placed ahead of the base units, (i.e. Kilogram, decameter, millisecond).Unit Prefixes are used to determine very large numbers from very small numbers. Placed ahead of the base units, (i.e. Kilogram, decameter, millisecond).

37. Scientific Measurements Important concept when dealing with numerical values in weather.Important concept when dealing with numerical values in weather.

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

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

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

46. Temperature Change w/Altitude

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 = ?RT 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).

51. 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.

52. 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 Temperature Layers of the Atmosphere: Mesosphere

53. 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.

54. 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.

55. Weather Basics Atmospheric Pressure Horizontal pressure differences cause the wind Air tends to blow, at an angle, from high pressure to low pressure near the surface Effect of rotating planet is that wind blows along a near constant pressure trajectory when friction is minimal Pressure is identified on weather maps using isobars (iso = constant, bar = pressure).

56. 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

57. 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

58. Weather BasicsWeather Map

59. Weather BasicsStation Plot

60. Summary Atmospheric pressure caused by weight of column of air above you. Pressure changes because of adding or taking away air from the column. Temperature is a measure of the average speed of the molecules per standard volume. Density is the mass per volume Pressure, Temperature, and Density all related by the Ideal Gas Law (a.k.a. the Equation of State)

61. Summary (cont.) Temperature decreases with height unless energy is added. Troposphere temperature decreases with height. Stratosphere temperature increases with height because of ozone absorption of dangerous UV radiation Mesosphere temperature decreases with height Thermosphere temperature increases with height because of absorption of solar particles, x-rays and gamma rays. Atmospheric composition remains fairly homogeneous up to ~80-100 km

62. A little more on pressure Net Forces=0 If all sides of an object are exposed to the air pressure, the net forces will cancel each other out.

63. Balance of Forces Not Equal to Zero Upward force of molecules balanced by downward force of weight of molecules above. Sideways force of molecules balanced by sideways force of molecules next to the air parcel. If some of the surrounding air is removed, then the molecules will be forced into the lower pressure region, causing “wind”.

64. Pressure Differences in the Horizontal Fluids will flow from regions of high pressure to low pressure. Consider the apparatus below The pressure at the surface is proportional to the weight (or height) of the fluid above. The fluid will flow from left to right until the surface pressures on both sides are equal.

65. Pressure Differences in the Horizontal Now consider the atmosphere If pressure is higher in one location than another at same elevation, gas molecules will move from high pressure towards lower pressure. In absence of influence by Earth’s rotation Movement of gas molecules is the wind. Pressure differences cause wind. (will cover in more detail in chapter 9)

66. Pressure Differences in the Vertical Near sea level, pressure decreases about 1 mb for every 10 meter (33 ft) increase with height. At 700 mb, 30% of atmosphere is below you and 70% is still above you. 700 mb = 3 km = 10,000 ft. (approximately) At 500 mb, half the atmosphere is below you. 500 mb = 5.5 km = 18,000 ft (approximately) 250mb = 10.5 km = 34,400 ft. (approximately)

67. Pressure in the Vertical Pressure decreases “monotonically” with height. Pressure always decreases with increasing height. Often convenient to use pressure instead of height as our vertical coordinate. Meteorologists frequently refer to the temperature, moisture and winds at standard pressure levels, e.g., 925, 850, 700, 500, 300, 250mb pressure levels.

68. Pressure Altimeter Change of pressure with height can be used to measure altitude of aircraft.