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NATS 101Section 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

Water vapor (H2O) comprises 0 to 4% of the atmosphere and is highly dependent on the specific location. More on that later…

Chemically active

Inert gases(not reactive)

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 0.01-0.15

Chloroflourocarbons CFCs 0.0002

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.)

ppm = parts per million

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…

You’re probably more familiar with the measure of dewpoint…Model forecast output (from NCAR).

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:

LENGTHTIMEMASS

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…

What would Sir Isaac Newton say to that?Master Yoda, Jedi Knight

What say you, Sir Isaac?

Sir Isaac Newton

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 ACCELERATIONF = ma

SI Units: Newton (kg m s-2)…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 = mgWeight (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 Earth = 9.8 m s-2

g of Moon = 1.6 m s-2

(About 1/6 of Earth)

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):

SI Units: kg m-3 or g m-3

Vm

Changes in density

Density DECREASES when:

Mass decreases or volume increases

ab

Density INCREASES when:

Mass increases or volume decreases

Original box

Add more mass Decrease volume

What is pressure?Pressure (P) is the force per unit area (A)

SI Units: m-1 kg s-2= Pa (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

Blaise Pascal

AF

P

Air 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.

Top

Bottom

Higher elevationLess air aboveLower pressure

Lower elevationMore air aboveHigher pressure

Incr

easi

ng p

ress

ure

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..

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!

Top of Mount Everest Elevation 8,850 m (29,035 ft.)

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= Elevation in kilometersP = pressure in mb at locationPMSL = mean sea level pressure in mb = 1013 mb

km 10 X 16Z

MSLPP

km e X 8Z

MSLPPOR

e = exponential function = 2.71828….

Surface pressure Tucson vs. Humphrey’s Peak

Tucson:Elevation 728 m

Humphrey’s Peak:Elevation 3850 mHighest Point in AZ

Using equation for pressure variation

km km 0.728

10 X 16MSLPP

km km 0.728

10 Xmb) 1013( 16P

km km 3.850

10 X 16MSLPP

km km 3.850

10 Xmb) 1013( 16P

P = 912 mb P = 582 mb

TUCSON HUMPHREY’S PEAK

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…)

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.

inversion

isothermal

6.5oC/km

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.