Chap. 1 - Part I Composition of the Atmosphere

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Chap. 1 - Part IComposition of the Atmosphere

• WX 201
• Dr. Chris Herbster

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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 Earths atmosphere
• Comparisons with Mars and Venus
• Unique features of Earths atmosphere compared to
  the other planets

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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

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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.
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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)

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Composition of Earths Atmosphere
Important gases in the Earths Atmosphere (Note
Influence not necessarily proportional to by
volume!)
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Permanent Gases

• 78 Nitrogen (N2)
• 21 Oxygen (O2)
• lt1 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)

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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

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Variable Gases in the Earths 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 Earths atmosphere after nitrogen and
oxygen)
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Variable Gases - Water Vapor

• Water vapor is invisible dont confuse it with
  cloud droplets
• Less than 0.25 of total atmosphere
• Surface percentages vary between ltlt1 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
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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)
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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.
Earths greenhouse gases contribute to a 30C
warmer surface temperature than would otherwise
exist. More on this phenomenon in Ch. 2.
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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.
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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)
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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.

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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)

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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

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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

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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.
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Earths 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 Earths 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 O2the H2 escaped.
• Life formed, plants grew adding additional O2
  through photosynthesis leading to todays
  atmosphere.

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Unique Features of Earths 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, Earths surface temp would
  only be approximately 0F!
• Water in all three phases solid, liquid, gas.
• Patchy cloud fields extensive up and down
  convective motions in atmosphere.
• Circular motions with storms.

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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, Earths mean
sea-level pressure is approximately 1,013 mbar
Venus surface pressure is 90x greater!) Temperat
ure at surface 900oF (by comparison, Earths
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).
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Earth and Venus nearly same size velocity
required to escape gravitational pull similar for
both.
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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.
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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
  Earths.
• Venus has CO2 in atmosphere, while Earth has CO2
  in limestone.

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Mars

• About half the size of the earth (less gravity)
• Atmosphere primarily CO2 -- too heavy to escape
  gravitational pull
• Surface pressure 1/100 of earths (10 mbar)
• Average surface T213K (-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

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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
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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.

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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.

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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.

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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
  Earths 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

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Chap. 1 - Part II Fundamental QuantitiesVertica
l Structure of the AtmosphereWeather Basics

• WX 201
• Dr. Chris Herbster

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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

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Fundamental Physical Quantities Units 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 0C 32F
• 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 ma Newton (N) 1 N 1 kgm/s2
• Weight Wt mgo Newton (N) 1 N 0.225 lb go
  9.8 m/s2

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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 FL 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). Well 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

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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 (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) 1/1,000,000 1/1,000,000,000 (10?6) (10-9)
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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
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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.

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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.

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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)

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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

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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 dont have as many molecules on top of you,
  the air pressure also decreases with height.

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

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Temperature Scales
K C 273.16
F 9/5C 32 C 5/9(F 32)
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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)

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Air Temperature Change w/ Changes in Parcel
Altitude

• Rising ? Expansion ? Cooling
• Sinking ? Compression ? Warming

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Relating State VariablesEquation of State or
Ideal Gas Law

• Temperature, pressure and density related
• Pressure densitygas constanttemperature
• 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

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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

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Temperature Layers of the Atmosphere Troposphere

• Lower part of the atmosphere
• Energy source is heating of the earths 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).

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Temperature Layers of the Atmosphere Stratosphere

• Suns 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.

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Temperature Layers of the Atmosphere Mesosphere

• 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
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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.

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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.

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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).

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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

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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

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Weather BasicsWeather Map
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Weather BasicsStation Plot
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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)

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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

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A little more on pressure

• Net Forces0
• 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.
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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.

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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.

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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 Earths rotation
• Movement of gas molecules is the wind.
• Pressure differences cause wind. (will cover in
  more detail in chapter 9)

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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 doesnt the
air flow straight up given that the pressure
decreases rapidly with height?
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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.

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Pressure Altimeter

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

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