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

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1 Chap. 1 - Part I Composition 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 Important gases in the Earth’s Atmosphere (Note: Influence not necessarily proportional to % by volume!) Composition of Earth’s Atmosphere

7 Permanent Gases 78% Nitrogen (N 2 ) 21% Oxygen (O 2 ) <1% Argon (Ar) Relative percentages of the permanent gases remain constant up to km 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 ~ km (i.e., 78% N 2, 21% O 2 ) Heterosphere: Above ~ km, much lower density, molecular collisions much less, heavier molecules (e.g., N 2, O 2 ) settle lower, lighter molecules (e.g., H 2, He) float to top

9 Variable Gases in the Earth’s Atmosphere VARIABLE gases in the atmosphere and typical percentage values (by volume): Water vapor (H 2 O)0 to 4% Carbon Dioxide (CO 2 ) 0.038% Methane(CH 4 ) % Ozone(O 3 ) % (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 ImageVisible Image 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

11 Mauna Loa Observatory CO 2 trace (annual variations embedded in the long-term record) Variable Gases - Carbon Dioxide (CO 2 ) Small percentage of total atmosphere (380 ppm) But, very important green house gas

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 (O 3 ) Near the surface, ozone concentrations about 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 (CH 4 ) 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 H 2, He as well as some NH 3, and CH 4. Free H 2 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 H 2 O, CO 2 as well as lesser amounts of N 2 (outgassing) Clouds rained forming oceans, which dissolved much of CO 2 locking it in sedimentary rocks through chemical and biological processes (e.g., seashell formation) allowing concentrations of N 2 to increase. O 2 increased through phododissociation of H 2 O into H 2 and O 2 —the H 2 escaped. Life formed, plants grew adding additional O 2 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% CO 2, 3% N 2 (compare to Earth—.04% CO 2, 78% N 2 ) 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: ~ 900 o F (by comparison, Earth’s mean sfc temperature is about 59 o F) Extreme atmospheric pressures on Venus due large amount of gaseous CO 2. No mechanisms to remove CO 2 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 CO 2 and N 2 Earth actually has more CO 2 than Venus (as fraction of total planet mass). Earth and Venus have similar amounts of N 2. CO 2 is 96% of Venus atmosphere and only.04% of Earth’s. Venus has CO 2 in atmosphere, while Earth has CO 2 in limestone.

26 Mars About half the size of the earth (less gravity) Atmosphere primarily CO 2 -- 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 CO 2 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 CO 2 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 CO 2 and H 2 O. 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 CO 2 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 QuantitySymbolSI UnitEquivalent Units LengthLMeter (m)1 m ≈ 3.28 ft MassmKilogram (kg)1 kg ≈ lb TimetSecond (s)60 s = 1 min TemperatureTKelvin (K)273.15K ≈ 0°C = 32°F Derived Quantities AreaA = L 2 Sq meter (m 2 )1 m 2 ≈ ft 2 VolumeV = L 3 Cu meter (m 3 )1 m 3 ≈ 35.3 ft 3 Density  m/VKg/m 3 1 kg/m 3 ≈ 0.06 lb/ft 3 VelocityV = L/tm/s1 m/s ≈ 2.24 mph ≈ 1.94 kt Accelerationa = V/tm/s 2 ForceF = m·aNewton (N)1 N = 1 kg·m/s 2 WeightWt = m·g o Newton (N)1 N ≈ lb; g o ≈ 9.8 m/s 2 Fundamental Physical Quantities Units of Measure Needed for this Course

35 Derived Quantities (cont.) QuantitySymbolSI UnitEquivalent Units Pressurep = F/aPascal (Pa)*1Pa = mb = 100 N/m 2 1hPa = 1 mb 1013 hPa ≈ in Hg Energy/Heat/E = F · LJoule (J)1 J = 1 N-m Work1 cal ≈ J (note: 1 cal is the amount of heat needed to raise 1 g of water 1 K) PowerP = E/tWatt (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 ≈ m/s 1 inch Mercury (in Hg) ≈ mb Centigrade (Celsius) to Kelvin: Add 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 Prefix # of Base Units Scientific Notation Terra (T) Giga (G) Mega(M) Kilo (k) 1,000,000,000,000 1,000,000,000 1,000,000 1,000 (10 12 ) (10 9 ) (10 6 ) (10³) Hecto (h)100(10²) Deca (da)10(10¹) Base1(10°) Deci (d)1/10(10 ‾ ¹) Centi (c)1/100(10 ‾ ²) Milli (m)1/1,000(10 ‾ ³) Micro (µ) Nano (n) 1/1,000,000 1/1,000,000,000 (10‾ 6 ) (10 -9 )

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 mb

42 State Variables Density 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 Variables Temperature 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 F = 9/5°C + 32 C = 5/9(°F – 32) K = °C

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).

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 Cold FrontWarm Front

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 Basics Weather Map

59 Weather Basics Station 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 ~ 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. Pressure outside balloon equals the pressure inside plus the tension of the balloon, so no air moves.

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. High Pressure Low Pressure

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) From previous slide, we saw that air will flow from higher to lower pressure. Why doesn’t the air flow straight up given that the pressure decreases rapidly with height?

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.

69 Altimeter indicates 2000’ (equiv. to a 70 mb pressure drop!) Airspeed indicates 120 kt The mysterious cockpit picture from the ERAU tornado – confirmed and re-confirmed by our faculty These readings would confirm the NWS estimate of F2 damage from this tornado


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