Chap. 1 - Part IComposition of the

Atmosphere

WX 201Dr. Chris Herbster

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

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

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.

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)

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

Composition of Earth’s Atmosphere

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)

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

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)

Variable Gases - Water Vapor

Water Vapor Image Visible 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

Mauna Loa Observatory CO2 trace (annual variations embedded in the long-term record)

Variable Gases - Carbon Dioxide (CO2)

Small percentage of total atmosphere

(380 ppm)

But, very important green house gas

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.

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.

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)

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.

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)

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

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

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.

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.

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.

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).

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

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.

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.

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

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

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.

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.

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.

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

Chap. 1 - Part II Fundamental

Quantities~

Vertical Structure of the Atmosphere

~Weather Basics

WX 201Dr. Chris Herbster

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

Basic QuantitiesQuantity Symbol SI Unit Equivalent UnitsLength L Meter (m) 1 m ≈ 3.28 ftMass m Kilogram (kg) 1 kg ≈ 2.205 lbTime t Second (s) 60 s = 1 minTemperature T Kelvin (K) 273.15K ≈ 0°C = 32°F

Derived QuantitiesArea A = L2 Sq meter (m2) 1 m2 ≈ 10.76 ft2

Volume V = L3 Cu meter (m3) 1 m3 ≈ 35.3 ft3

Density 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 ktAcceleration 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

Derived Quantities (cont.)Quantity Symbol SI Unit Equivalent UnitsPressure p = F/a Pascal (Pa)* 1Pa = 10-2 mb = 100 N/m2

1hPa = 1 mb1013 hPa ≈ 29.92 in Hg

Energy/Heat/ E = F·L Joule (J) 1 J = 1 N-mWork 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 Conversions1 knot (kt) ≈ 1.15 mph ≈ 0.514 m/s1 inch Mercury (in Hg) ≈ 33.865 mbCentigrade (Celsius) to Kelvin: Add 273.15 to deg CCentigrade to Fahrenheit: Multiply by 1.8, then add 32Fahrenheit to Centigrade: Subtract 32, then multiply by 5/9

Fundamental Physical Quantities (cont.)

Scientific Notation

Prefix# of

Base UnitsScientific 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)

Base 1 (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)

Scientific Measurements

Significant Digits:

Nearest reportable values for common measurements

Upper 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: Decameter

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.

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.

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)

Pressure as Measured by BarometerWeight 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

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.

3m

kg

Density and Pressure with Height

Because of compression, the atmosphere is more dense near the surface.

Density decreases with altitude

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

Temperature Scales

F = 9/5°C + 32

C = 5/9(°F – 32)

K = °C +273.16

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)

Air Temperature Change w/ Changes in Parcel Altitude

Rising Expansion Cooling

Sinking Compression Warming

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

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

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).

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.

• 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

Temperature Layers of the Atmosphere: Mesosphere

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.

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.

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).

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 Front Warm Front

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

Weather BasicsWeather Map

Weather BasicsStation Plot

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)

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

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.

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”.

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

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)

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 giventhat the pressure decreases rapidly with height?

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.

Pressure Altimeter

• Change of pressure with height can be used to measure altitude of aircraft.

Altimeter indicates 2000’Altimeter indicates 2000’(equiv. to a 70 mb pressure drop!)(equiv. to a 70 mb pressure drop!)

Airspeed indicates 120 ktAirspeed indicates 120 kt

The mysterious cockpit picture from the ERAU tornado – confirmed The mysterious cockpit picture from the ERAU tornado – confirmed and re-confirmed by our facultyand re-confirmed by our faculty

These readings would confirm the NWSThese readings would confirm the NWSestimate of F2 damage from this tornadoestimate of F2 damage from this tornado