Transcript Composition of the Atmosphere

Chap. 1 - Part I
Composition of the
Atmosphere
WX 201
Dr. 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)
Composition of Earth’s Atmosphere
Important gases in the Earth’s Atmosphere
(Note: Influence not necessarily proportional to % by volume!)
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 ~80100km, 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
•
•
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•
•
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
Water Vapor Image
Visible Image
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)
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
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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
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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 201
Dr. 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
Fundamental Physical Quantities
Units of Measure Needed for this Course
Basic Quantities
Quantity
Symbol
Length
L
Mass
m
Time
t
Temperature
T
SI Unit
Meter (m)
Kilogram (kg)
Second (s)
Kelvin (K)
Equivalent Units
1 m ≈ 3.28 ft
1 kg ≈ 2.205 lb
60 s = 1 min
273.15K ≈ 0°C = 32°F
Derived Quantities
Area
A = L2
Volume
V = L3
Density
r = m/V
Velocity
V = L/t
Acceleration
a = V/t
Force
F = m·a
Weight
Wt = m·go
Sq meter (m2)
Cu meter (m3)
Kg/m3
m/s
m/s2
Newton (N)
Newton (N)
1 m2 ≈ 10.76 ft2
1 m3 ≈ 35.3 ft3
1 kg/m3 ≈ 0.06 lb/ft3
1 m/s ≈ 2.24 mph ≈ 1.94 kt
1 N = 1 kg·m/s2
1 N ≈ 0.225 lb; go ≈ 9.8 m/s2
Fundamental Physical Quantities (cont.)
Derived Quantities (cont.)
Quantity
Symbol
SI Unit
Pressure
p = F/a
Pascal (Pa)*
Equivalent Units
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 hectoPascals (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
Scientific Notation
# of
Base Units
Prefix
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)
Scientific Measurements
Significant Digits:
Nearest reportable values for common measurements
Upper Air Wind Speeds:
Surface Wind Speeds:
Upper Air Pressure:
Surface Pressure:
Skew-T Temperatures:
Temperatures:
Relative Humidity:
Upper Air Heights:
5 Knots
Whole Knot
Whole Millibar (mb)
1/10 (.1) mb
1/10 (.1) Degree
Whole Degree
Whole Percent
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 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
State Variables
Density
• Air density is the mass of the air divided by the volume
of measurement.
kg
r= 3
m
• 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.
Density and Pressure with Height
Because of
compression, the
atmosphere is
more dense near
the surface.
Density decreases
with altitude
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
Temperature Scales
K = °C +273.16
F = 9/5°C + 32
C = 5/9(°F – 32)
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.
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
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 Basics
Weather Map
Weather Basics
Station 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 given
that 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.
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