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NATS 101
Section 4: Lecture 2
Atmospheric Composition and
Structure
Composition of the Dry Atmosphere
Gas
Chemical symbol
% of Atmosphere
Nitrogen
N2
78.08
Oxygen
O2
20.95
Argon
Ar
0.93
Neon
Ne
0.0018
Helium
He
0.0005
Hydrogen
H2
0.00006
Xenon
Xe
0.000009
Chemically
active
Inert gases
(not reactive)
Water vapor (H2O) comprises 0 to 4% of the
atmosphere and is highly dependent on the specific
location. More on that later…
Important Trace Gases
Gas
Chemical symbol
Concentration (ppm)
Carbon dioxide
CO2
380 (at present)
Methane
CH4
1.7
Nitrous Oxide
N2O
0.3
Ozone
O3
0.04
Particulates
Chloroflourocarbons
0.01-0.15
CFCs
0.0002
ppm = parts per million
These gases are “important” because they :
• Affect Earth’s energy budget and/or atmospheric chemistry.
• May be human influenced (e.g. global warming, ozone hole, etc.)
Carbon Dioxide (CO2)
The “Keeling curve” shows increase in atmospheric
carbon dioxide at Mauna Loa since 1950s
Stratospheric Ozone and CFCs
Stratospheric ozone
protects from Sun’s UV
rays.
An ozone hole occurs over
the polar regions because
of the combination of CFCs,
chemical reactions on polar
stratospheric clouds, and
the dynamics of the polar
vortex.
(NASA imagery)
Water Vapor
Water vapor is highly
dependent on the
given weather
conditions and local
climate.
It also plays a big
role in the Earth’s
energy budget, as
we’ll see later…
Model forecast output (from NCAR).
You’re probably
more familiar with
the measure of
dewpoint…
Mass, Force, Weight, Density,
and Pressure
What is mass?
Mass is an intrinsic property based on the molecular
composition of matter. As long as the amount of
matter does not change, it’s mass remains
constant—regardless of location.
SI units of mass: grams (g) or kilograms (kg)
SI Units = Système Internationale Units
Just to clarify this IS NOT a course in French!
Basically think SI  metric system.
Some SI Units we’ll use in next few lectures
Quantity
Name (Symbol)
SI Units
Length
meter
m
Time
second
s
Mass
grams or kilograms
g or kg
Area
meter squared
m2
Volume
meter cubed
m3
Density
kilograms per meter
cubed
kg m-3
Frequency
hertz
s-1
Velocity or speed
meter per second
m s-1
Acceleration
meter per second squared
m s-2
Force
newton (N)
m kg s-2
Pressure
pascal (Pa)
m-1 kg s-2
Energy
joule (J)
m2 kg s-2
ALL quantities
are derived from:
LENGTH
TIME
MASS
Note:
Temperature is
actually a
measure of
energy, and
we’ll talk about
that next
week…
Now let’s use our basic units of length,
time, and mass to start deriving some
more complex units of measures…
What is force? Let’s ask some experts…
The force is all
around us and binds
all things, young
Skywalker…
Master Yoda, Jedi Knight
What would Sir Isaac Newton say to that?
What say you, Sir Isaac?
Now see here Master
Yoda! In my galaxy
force is the mass of
an object multiplied
the change in its
velocity over time, or
acceleration!
FORCE = MASS X ACCELERATION
F = ma
SI Units: Newton (kg m s-2)
Sir Isaac Newton
…looking QUITE disgruntled!
What is weight?
NOT the same as mass!
The concept of weight is a specific application of the concept
of force:
Weight (W) is the force on an object to the gravitational
acceleration (g):
W = mg
Weight (kg m s-2) = mass (kg) X gravitational acceleration (m s-2)
We Americans typically think in terms of pounds (English system):
FYI  1 pound force = 4.44 Newtons
Weight is dependent on the
size of the attracting body…
Newton’s law of gravitation indicates that the gravitation
acceleration is dependent on the size of the body. The more
massive, the bigger g.
g of Moon = 1.6 m s-2
(About 1/6 of Earth)
g of Earth = 9.8 m s-2
Alan Shepard
showed us that
playing in lunar
gravity certainly
might improve your
golf game!
Just how much is
that shot worth?!
What is density?
Density (ρ) is the mass (m) per unit volume (V):
m

V
SI Units: kg m-3 or g m-3
Changes in density
Density DECREASES when:
Density INCREASES when:
Mass decreases or volume increases
Mass increases or volume decreases
a
b
Original box
Add more mass
Decrease volume
What is pressure?
Pressure (P) is the force per unit area (A)
F
P 
A
SI Units: m-1 kg s-2= Pa (Pascal)
Blaise Pascal
The typical unit of atmospheric pressure is millibars
1 mb = 100 Pa
The air pressure at the surface of the Earth at sea level is
defined as 1 Atmosphere (Atm):
“Atmosphere”  1 Atm = 1013 mb = 29.92 in Hg
Air pressure
Top
Higher elevation
Less air above
Lower pressure
Bottom
Lower elevation
More air above
Higher pressure
Increasing pressure
Given the mathematical definitions we’ve already
discussed, air pressure can be thought of as the
weight of a column of air above you.
Mercury Barometer
One atmosphere
Change in density and pressure with height
Density and pressure decrease exponentially with
height. For each 16 km in altitude, the pressure
decreases by a factor of 10..
Top of Mount Everest
Elevation 8,850 m (29,035 ft.)
So if you ever decide
to be ambitious and
brave the top of Mt.
Everest, make sure
to bring oxygen!
You won’t get very far
with an air pressure
of 300 mb!
Equation for pressure variation
Given that we know atmospheric pressure changes
exponentially with height, we can apply this relationship
to derive the air pressure at various altitudes above sea
level:
 Z 


 16 km 
P  PMSL X 10
OR
P  PMSL X e
 Z 


8
km


e = exponential function = 2.71828….
Z= Elevation in kilometers
P = pressure in mb at location
PMSL = mean sea level pressure in mb = 1013 mb
Surface pressure
Tucson vs. Humphrey’s Peak
Humphrey’s Peak:
Elevation 3850 m
Highest Point in AZ
Tucson:
Elevation 728 m
Using equation for pressure variation
HUMPHREY’S PEAK
TUCSON
 0.728 km 


16
km


P  PMSL X 10
 0.728 km 


 16 km 
P  (1013 mb) X 10
P = 912 mb
 3.850 km 


16
km


P  PMSL X 10
 3.850 km 


 16 km 
P  (1013 mb) X 10
P = 582 mb
Change in Temperature with Height
Changes in temperature are more
complicated and have to do with the
radiative processes in different parts of
the atmosphere (more on that later…)
inversion
isothermal
6.5oC/km
The rate of change of temperature with
height is called the lapse rate. Positive
lapse rate mean temperature decrease
with height.
The places where the sign of the lapse
rate changes defines the different levels of
the atmosphere.
Atmospheric Layers
• Troposphere (surface – 11 km): Nearly all of what we think of as
“weather” happens here. Lapse rate of 6.5 °C per km.
“Tropo” = Greek for “overturning”
• Stratosphere (11 km – 50 km): Where the ozone layer is located
and Sun’s UV rays are absorbed by photodissociation.
“Strato” = Greek for “layered”
• Mesosphere (50 km – 90 km)
• Thermosphere: (90 km – 500 km): Ionization of atmospheric gases
• Exosphere (500 km): Basically outer space…
Reading Assignment
Ahrens: Second half of Chapter 1, pp. 16-24 (8th ed.)
pp. 18-25 (9th ed.)
Appendices B and C: Weather Station Models
Chapter 1 Questions:
Questions for Review: 2,5,10,11,12,13,14,15,16,17,19
Questions for Thought: 2,3
Summary of Lecture 2
The atmosphere is composed of chemically active and inert gases.
The “important” gases affect the Earth’s energy budget and/or
atmospheric chemistry. Carbon dioxide, water vapor, and ozone
are good examples.
We defined mass, force, weight, density, and pressure. Know how
each of these are derived, what they physically mean, and their SI
units of measurement.
Pressure can be thought of as the weight of a column of air above
you, and it decreases exponentially with height. A simple equation
was presented with relates the variation in pressure with height.
Temperature changes with height are more complicated and have to
do with radiative processes in different parts of the atmosphere.
Places where the lapse rate changes define the various
atmospheric layers.