Presentation on theme: "Chap. 1 - Part I Composition of the Atmosphere"— Presentation transcript:
1Chap. 1 - Part I Composition of the Atmosphere WX 201Dr. Chris Herbster
2Outline Meteorology Defined The atmosphere as a gas Permanent and Variable GasesInfluence by planet size and distance from the Sun on atmospheric compositionComposition of Earth’s atmosphereComparisons with Mars and VenusUnique features of Earth’s atmosphere compared to the other planets
3What 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 atmosphereClimate deals with the long-term patternsMore than simple long-term averagesInvolves complex interactions and variability
4Thickness of the Atmosphere Approximately 80% of the atmosphere occurs in the lowest 20km above the Earth.Radius of the Earth is over 6,000 kmAtmosphere is a thin shell covering the Earth.
5But 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 massVariable Gases – Distribution and concentration varies in space and timeAerosols – Suspended particles and liquid droplets (excluding cloud droplets)
6Composition of Earth’s Atmosphere Important gases in the Earth’s Atmosphere(Note: Influence not necessarily proportional to % by volume!)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 hole
7Permanent Gases 78% Nitrogen (N2) 21% Oxygen (O2) <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)
8Homosphere and Heterosphere Homosphere: Turbulent mixing causes atmospheric composition to be fairly homogenous from surface to ~ 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
9Variable Gases in the Earth’s Atmosphere VARIABLE gases in the atmosphere and typical percentagevalues (by volume):Water vapor (H2O) 0 to 4%Carbon Dioxide (CO2) %Methane(CH4) %Ozone(O3) %(Note that water vapor is the third most common molecule in Earth’s atmosphere after nitrogen and oxygen)
10Variable Gases - Water Vapor Water vapor is invisible – don’t confuse it with cloud dropletsLess than 0.25% of total atmosphereSurface percentages vary between <<1% in desserts to 4% in tropicsTypical mid-latitude value is about 1-2%Some satellites sensors can detect actual water vapor in atmosphereWater Vapor ImageVisible Image
11Variable Gases - Carbon Dioxide (CO2) Small percentage of total atmosphere (380 ppm)But, very important green house gasHow many have seen the movie “An Inconvenient Truth” or read the book?Mauna Loa Observatory CO2 trace(annual variations embedded in the long-term record)
12Atmospheric 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.
13Variable Gases – Ozone (O3) Near the surface, ozone concentrations about ppmIn the upper atmosphere ozone concentration can reach ~15 ppmUpper atmospheric ozone is vital to blocking harmful radiationOzone near the surface, however, harmful to lifeChlorofluorocarbons (CFCs) are believed to be depleting upper atmospheric ozoneSatellite images showing depletion of ozone.
14Variable Gases – Methane (CH4) Concentrations of about 1.7 ppmExtremely potent green house gas - 21 times more powerful by weight than carbon dioxideHas varied cyclically on a 23,000 year cyclePattern broken in past 5,000 years with unexpected increase – more abundant now than in last 400,000 yearsIncrease attributed to agriculture, bio-mass burning, fossil fuel extraction, some industry and ruminant out-gassing (cow/sheep burps)Methane growth and sources (From EPA)
15Aerosols (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 pollutionSome aerosols are very beneficial and, in fact, are required for precipitation processes to occur.
16What Determines Atmospheric Composition? Composition of gases on a planet is determined largely by how easily gases can escape to spaceAlso depends on the existence of life or geologic processesFor 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 planetMolecular velocity is determined by the gas temperature (or average kinetic energy)
17Escape Velocity Gas is made up of free molecules in constant motion. Speed of the gas molecules is determined by the temperatureTemperature determined largely by proximity to the SunEscape velocity depends on the gases’ molecular weight and the planets sizeLighter molecules require less speed to escapeLarger planets have stronger gravitational pull
18Relative Planet Size and Distance from Sun Size comparison of planets – larger planets have stronger gravitational pullPlanets closer to the Sun receive more radiant energy
19The required “escape velocity” is determined planet size Temperature of gas determinedby distance from sun.Molecular speed determined by molecular weight and temperatureGas lines above the planetwill escape to space.Gas lines below the planet willremain in the atmosphere.i.e. Earth will lose hydrogenbut hold water. Mars willlose water but holdcarbon dioxide.
20Earth’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.
21Unique Features of Earth’s Atmosphere Atmospheric composition – high Oxygen content, low Carbon Dioxide content.Greenhouse gases contribute to livable surface temperaturesMost 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.
22Comparison with VenusComposition 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).
23Earth and Venus nearly same size – velocity required to escape gravitational pull similar for both.
24Why the drastic difference? Venus is closer to SunWarmer temperatures prevented liquid water from forming.With no liquid water, no means to dissolve the carbon dioxide.Result is a rich carbon dioxide atmosphere.
25Earth and Venus CO2 and N2Earth 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.
26Mars About half the size of the earth (less gravity) Atmosphere primarily CO2 -- too heavy to escape gravitational pullSurface 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 CO2Small size of planet allowed most of atmosphere to escape
27Weather on Earth in relation to orbital characteristics Rotation once per 24 hrs.Primary weather systems are moving storms with clouds, circular winds, and precipitation
28Weather 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.
29Weather 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 meltand reform.Has very few clouds.
30SummaryComposition of gases on a planet is a function of the planet size (strength of gravity holding gases onto the planet), planet temperature, and lifePrimary permanent gases on Earth are Nitrogen, Oxygen, ArgonVariable gases include Water Vapor, Carbon Dioxide, Ozone, Methane, CFCs, etc.The importance of variable trace gases is not always proportional to the amount.
31Summary (cont.)Water vapor is the most important greenhouse gas, others include Carbon Dioxide, Methane and OzoneGases 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
32Chap. 1 - Part II Fundamental Quantities ~ Vertical Structure of the Atmosphere ~ Weather Basics WX 201Dr. Chris Herbster
33Outline Fundamental physical quantities covered in this course Atmospheric state variablesDensity, Pressure, temperatureStructure of the atmosphereTroposphereStratosphereMesosphereThermosphereImportance of the stratosphere and thermosphere
34Fundamental Physical Quantities Units of Measure Needed for this Course Basic QuantitiesQuantity Symbol SI Unit Equivalent UnitsLength L Meter (m) 1 m ≈ 3.28 ftMass m Kilogram (kg) 1 kg ≈ lbTime t Second (s) 60 s = 1 minTemperature T Kelvin (K) K ≈ 0°C = 32°FDerived QuantitiesArea A = L2 Sq meter (m2) 1 m2 ≈ ft2Volume V = L3 Cu meter (m3) 1 m3 ≈ 35.3 ft3Density r=m/V Kg/m3 1 kg/m3 ≈ lb/ft3Velocity V = L/t m/s 1 m/s ≈ 2.24 mph ≈ 1.94 ktAcceleration a = V/t m/s2Force F = m·a Newton (N) 1 N = 1 kg·m/s2Weight Wt = m·go Newton (N) 1 N ≈ lb; go ≈ 9.8 m/s2Mass (m) Amount of matter in an object.Length (L) A measurement of distance.Time (t) A period over which an action takes place
35Fundamental Physical Quantities (cont.) Derived Quantities (cont.)Quantity Symbol SI Unit Equivalent UnitsPressure p = F/a Pascal (Pa)* 1Pa = 10-2 mb = 100 N/m21hPa = 1 mb1013 hPa ≈ in HgEnergy/Heat/ E = F·L Joule (J) 1 J = 1 N-mWork cal ≈ 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 Conversions1 knot (kt) ≈ 1.15 mph ≈ m/s1 inch Mercury (in Hg) ≈ mbCentigrade (Celsius) to Kelvin: Add to deg CCentigrade to Fahrenheit: Multiply by 1.8, then add 32Fahrenheit to Centigrade: Subtract 32, then multiply by 5/9
36Scientific Notation Prefix # of Base Units Scientific Notation Terra (T)Giga (G)Mega(M)Kilo (k)1,000,000,000,0001,000,000,0001,000,0001,000(1012)(109)(106)(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)1/1,000,0001/1,000,000,000(10‾6)(10-9)Unit Prefixes are used to determine very large numbers from very small numbers. Placed ahead of the base units, (i.e. Kilogram, decameter, millisecond).
37Scientific Measurements Significant Digits:Nearest reportable values for common measurementsUpper Air Wind Speeds: 5 KnotsSurface Wind Speeds: Whole KnotUpper Air Pressure: Whole Millibar (mb)Surface Pressure: 1/10 (.1) mbSkew-T Temperatures: 1/10 (.1) DegreeTemperatures: Whole DegreeRelative Humidity: Whole PercentUpper Air Heights: DecameterImportant concept when dealing with numerical values in weather.
38Atmospheric State Variables State variables include:PressureTemperatureDensityState 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.
39State 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 GravityAir has mass therefore weight; pressure (weight/area) is measured by a barometer.
40Surface PressureThe 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)
41Pressure as Measured by Barometer Weight of mercury in column equals weight of atmosphereAverage sea level pressure is:14.7 pounds per square inch,760 mm or 29.92” mercury ormb
42State 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.
43Density and Pressure with Height Because of compression, the atmosphere is more dense near the surface.Density decreases with altitude
44State 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
46Temperature 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 pressureAir molecules have work done on them, temperature increases (warms)
47Air Temperature Change w/ Changes in Parcel Altitude Rising Expansion CoolingSinking Compression Warming
48Relating State Variables: “Equation of State” or “Ideal Gas Law” Temperature, pressure and density relatedPressure = density*gas constant*temperatureP = ρRTIf 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
49Vertical Structure of the Atmosphere Vertical Structure of the Atmosphere commonly broken into layersLayers 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
50Temperature Layers of the Atmosphere: Troposphere Lower part of the atmosphereEnergy 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).
51Temperature 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.
52Temperature Layers of the Atmosphere: Mesosphere Above 50 km, very little ozone, so no solar heatingAir continues to cool with height in mesosphereMesosphere extends from about 50 km to 90 km above the surface
53Temperature 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.
54Importance 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.
55Weather Basics Atmospheric Pressure Horizontal pressure differences cause the windAir tends to blow, at an angle, from high pressure to low pressure near the surfaceEffect of rotating planet is that wind blows along a near constant pressure trajectory when friction is minimalPressure is identified on weather maps using isobars (iso = constant, bar = pressure).
56Weather Basics Atmospheric Temperature Areas separating colder and warmer air on a weather map are represented by frontsCold Fronts (blue – pointed barbs) indicate the movement of a cold air mass into a warmer regionWarm Fronts (red – rounded barbs) indicate movement a warm air mass into a colder regionCold FrontWarm Front
57Weather Basics Atmospheric Humidity Relative Humidity provides a measure of the amount of water vapor in the air relative the maximum possible for a given temperatureDew Point Temperature is the temperature the air must be cooled to for condensation to occur.Much more on these concepts in later chapters
60SummaryAtmospheric 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 volumePressure, Temperature, and Density all related by the Ideal Gas Law (a.k.a. the Equation of State)
61Summary (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 radiationMesosphere temperature decreases with heightThermosphere temperature increases with height because of absorption of solar particles, x-rays and gamma rays.Atmospheric composition remains fairly homogeneous up to ~ km
62A little more on pressure Net Forces=0If 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 tensionof the balloon, so no air moves.
63Balance 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”.
64Pressure Differences in the Horizontal Fluids will flow from regions of high pressure to low pressure.Consider the apparatus belowThe 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 PressureLow Pressure
65Pressure Differences in the Horizontal Now consider the atmosphereIf 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 rotationMovement of gas molecules is the wind.Pressure differences cause wind. (will cover in more detail in chapter 9)
66Pressure 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 higherto lower pressure. Why doesn’t the air flow straight up giventhat the pressure decreases rapidly with height?
67Pressure 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.
68Pressure AltimeterChange of pressure with height can be used to measure altitude of aircraft.
69The mysterious cockpit picture from the ERAU tornado – confirmed and re-confirmed by our faculty Airspeed indicates 120 ktAltimeter indicates 2000’(equiv. to a 70 mb pressure drop!)These readings would confirm the NWSestimate of F2 damage from this tornado