
Gases are mechanically combined:

“collection of different atoms, molecules or larger particles in physical proximity to one another but not bound chemically”

AVERAGE COMPOSITION OF DRY ATMOSPHERE below 25 km. (~15 mi) :
99.98%
Component
Nitrogen (N
2
)
Oxygen (O
2
)
Argon
Carbon dioxide
Neon
Helium
Ozone (O
3
)
Hydrogen
Krypton
Xenon
Methane
Volume %
78.08
20.94
0.93
0.03 (variable)
0.0018
0.0005
0.00006
0.00005
Trace
Trace
Trace


Water vapor

Aerosols (non-gas) : suspended particles of sea salt, dust, organic matter, smoke

all gases (and non-gases) are mixed in
CONSTANT PROPORTIONS up to 80 km (50 mi)
1. ozone
2. water vapor
3. carbon dioxide
4. aerosols variable

3
Concentrated at 15 – 35 km (9-22 mi)
.
UV
O
2
O
O
+ O
2
+ O
2
O
3
O
3
above 35 km , collisions less likely;
below 15 km , not much UV

4% at surface; almost absent >10 km (6.2 mi)

Surface is source
 carried upward by turbulence
 most effective <10 km

Varies temporally; increase in late 20 th Century due to anthropogenic production

Latest IPCC data

Vary with source regions
Sources include: factories, urban areas, volcanic eruptions

obeys mechanical laws because it has mass.
behaves like a single
”.


Follows kinetic molecular theory; gas is made up of many molecules in rapid, random motion.

Perfect elastic collisions; so no momentum is lost in collisions.

Small enough that attractive forces between them are negligible.

PRESSURE, DENSITY, TEMPERATURE AND
VOLUME ARE INTERRELATED.
Temperature is average speed at which molecules are moving in a gas.
Pressure is force per area.
Density is mass per volume.

GAS PRESSURE
(different than atmospheric pressure)
Balloon analogy: Gas pressure is caused by impacts of molecules on inside of balloon.
The greater the number of collisions, the greater the pressure
(force per unit area).

(IF temperature is constant)
If volume (of balloon) is increased, pressure __ .
(In atmosphere, usually consider density rather than volume). If volume increases, density __.
If density (in balloon) decreases, pressure __.
Pressure and density are directly proportional .


At the same temperature, air at a higher pressure is more dense than air at a lower pressure.

(IF density is constant)
If temperature increases
Molecular movement becomes more vigorous and pressure __ .
Pressure and temperature are directly proportional .

(If pressure is constant)
If temperature increases, gas expands, volume increases, density decreases.
Temperature is inversely proportional to density.
Summary of relationships: Pressure ~ Density x Temperature

P ~ ρ x T

At a given pressure, air that is cold is more dense than air that is warm.

At a given density, increasing temperature increases molecular movement, which increases pressure.

At a given temperature, increasing density increases pressure by increasing number of collisions.

: pressure
P = ρ R T
Gas constant , for dry air = 287 J kg -1 K -1

Temperature in Kelvins
1 Joule = kg m 2 s -2
Pascals
1 Pa = kg m -1 s -2
1 mb = 100 Pa


P = ρRT ; R = 287 J kg -1 K -1 ; P in Pa; T in K

If pressure is 85000 Pa and T is 300 K, what is density?
ρ = P / RT
= 85000 / (287) (300)
= 85000 / 86100
= 0.987 kg m -3

ρ = P / RT


If temperature is 250 K, density is 0.25kg m -3 what is the pressure?


Atmospheric molecules have mass.

Earth exerts gravitational force on gas molecules in atmosphere.
Results:

Air is compressible.

Mass, density and pressure change with height.

Atmospheric pressure vs. gas pressure
Atmospheric pressure on the outside of the balloon is due to impacts from molecules on outside.
Amount of pressure is due to density of atmosphere. Density decreases with height.

ATMOSPHERIC PRESSURE:
 force exerted by a column of air per unit area
 at sea level:



14.7 lb/in 2
1.034 kg/cm 2
1013 millibars


Mass (proportional to weight)
50% mass of atmosphere : below 5 km (3 mi)
90% below 16 km (10 mi)

Density (mass per volume):
1.2 kg m -3 at surface
0.7 kg m -3 at 5 km

Pressure (force per unit area):

Atmospheric pressure (vs. gas pressure)
5 km from surface, pressure drops by half

negative exponential decrease

From surface up to1 km: leave lots of dense air beneath

Overload (density and pressure) decrease quickly.


Air in next km already much less dense than below
Overload decreases more slowly
Height km
0
6
12
18
24
Pressure mb
1013
500
250
125
63
Fraction of
Surface P
1
½
¼
1/8
1/16

…atmosphere is stratified.
“vertical structure” of the atmosphere
“layers” of the atmosphere

Profile of temperature change with altitude in the atmosphere
Temperature
Layers of
Atmosphere





Temp decreases with height

(because source of heat is surface and greenhouse gases)
 average rate: 6.4°C per km (3.5°F per 1000 ft)
“NORMAL LAPSE RATE”
90% mass
Weather
 virtually all water vapor, clouds, air pollution turbulent, well-mixed

“tropos” means turning; changing

 thickness of the troposphere varies with latitude.
(Also varies with season, surface temp and pressure)



Equator : 11 miles thick (great heating, convective mixing)
Midlatitudes : 8 miles thick
Poles : 5 miles thick
TROPOPAUSE altitude at which temp stops decreasing with height inversion ceiling on troposphere (warm layer over cold)
Average temp at tropopause is -57°C (-70°F)



 from tropopause to 31 mi (50 km) ozone temp. of lower stratosphere is constant; then temp increases due to the absorption of UV light by O
3 at low density, even small absorption causes great temp changes
 stratified; very little mixing
STRATOPAUSE temperature = 0°C


50-85 km
 decrease in temp. with height
 dominated by single atom oxygen
 little solar energy absorbed here
 temperature falls to lowest in atmosphere

“Shooting stars”


At 85 km, oxygen and ozone absorb radiation and heat atmosphere

Noctilucent clouds

High latitudes, summer

Meteoritic dust acts as nuclei for ice crystals
MESOPAUSE; inversion layer
 lowest temp: -90°C (-130F)


>85 km.
 extremely low density

Temps rise to highest in atmosphere

Due to absorption of UV by oxygen

Cosmic radiation, solar X-rays, UV cause ionization

Aurora Borealis and Australis

Penetration of ionized particles through atmosphere
(80 – 300 km); 10 - 20º from magnetic poles


Earth has North and South magnetic poles and a magnetic field


Solar Wind

Sunspot activity associated with appearance of auroras

Solar Wind : Ionized particles streaming from sun towards earth

Solar wind has magnetic fields


Solar wind distorts earth’s magnetic field ( magnetosphere)



Magnetic fields in solar wind disturbs magnetosphere
Magnetic field funnels electrons into upper atmosphere

Particles in magnetosphere enter thermosphere and excite atoms to emit visible radiation

Line is region of most likely occurrence, Northern Hemisphere

Line is region of most likely occurrence, Southern Hemisphere

Stratosphere and mesosphere are sometimes referred to as “middle atmosphere”
Troposphere : lower atmosphere
>100 km : upper atmosphere


A few things to think about……

Atmospheric pressure is standardized to sea level.
(Otherwise, since Denver (for example) is a mile high, it would always have low pressure).


Boiling point lower. Why??
Heating gives energy to water molecules so they can break bonds and become steam; at higher altitude, less pressure on water and therefore less energy required to break bonds, therefore lower boiling point. Have to cook things longer at a lower temperature.

(NO HEAT EXCHANGE WITH SURROUNDING AIR)
As a parcel of air rises, it expands due to decreasing atmospheric pressure. This causes parcel’s temperature to drop, which leads to condensation and cloud formation.
Why does temperature drop? Because it takes ENERGY for the parcel to expand.
It gets this energy from the molecules in the air mass, thus causing the temperature to drop.