The Earth’s Atmosphere
What holds the Earth’s
atmosphere to the planet?
Development of the
Earth’s Atmosphere
• Primordial atmosphere (4.6 to 4.0 bya)
• Evolutionary atmosphere (4.0 to 3.3 bya)
• The living atmosphere (3.3 bya to 500
mya)
• The modern atmosphere (500 mya to
present)
The Modern Atmosphere
(500 mya to the present)
• Nitrogen, N2 (78%)
• Oxygen, O2 (21%)
• Argon, Ar (0.9%)
99.9%
Trace gases
– water vapor (0-4%)
– carbon dioxide (.036%), methane (greenhouse
gases)
– nitrogen oxides, sulfur oxides (acid rain and
more)
– many other trace gases
– particulate (dust)
Atmospheric Pressure
Atmospheric Pressure
Atmospheric Pressure
Pressure can be
thought of as the
weight of all
overlying air
(though, in reality,
pressure exerts
force in all
directions).
Average Sea Level
Atmospheric Pressure:
29.92” of Mercury
76 cm of Mercury
1013 millibars (mb)
Mercury Barometer
- Invented by Toricelli, 1643
Aneroid Barometer
- also altimeter
Average Sea Level
Atmospheric
Pressure:
29.92” of Mercury
76 cm of Mercury
1013 millibars (mb)
Atmospheric Pressure Is
Related to Weather Conditions
Less-Dense, Low
Pressure Rises:
Clouds and Stormy
Weather
More-Dense, High
Pressure Air Sinks:
Fair Weather
The Vertical Thermal
Structure of the
Atmosphere
IONOSPHERE
OZONOSPHERE
Temperature, Precipitation, and Elevation
Temperature decreases with increasing elevation.
Precipitation increases with increasing elevation.
Temperature Inversions
When warmer air overlies cooler air, pollutants
and fog are trapped beneath the inversion.
Common Winter Radiation Inversion in Valleys
Temperature Inversions
Common Summer Inversion in Los Angeles
Troposphere
[tropopause at 8-18 km, or 5-11 miles]
• Troposphere
– contains 90% of the mass
of the atmosphere
– decrease of mass with altitude
– mostly mixed gases (not layered)
– clouds / weather layer
– temperatures decrease with
altitude - WHY?
mesosphere
Stratosphere
[stratopause at 50 km, about 30 miles]
– decrease in amount of gases with
altitude
– mixed gases (not stratified) except for
ozone layer
– temperatures increase with altitude
[tropopause at 8-18 km, or 5-11 miles]
Aurora borealis / australis
• The northern / southern lights
•
(click for video)
(click for photos and legends)
• Thermosphere and uppermost Mesosphere
– solar wind (clouds of electrically charged particles)
– Earth’s magnetic field directs them towards poles
– excite oxygen (O) and nitrogen (N2) ions in ionosphere
 emit light
The Importance of
Stratospheric Ozone
• Ozone forms naturally in stratosphere
light

O2  2 O
then O + O2  O3
• UV radiation (sun) --> mutations
– plankton reduced (food chain base), crops decline
– weaker immune systems, skin cancer
• Stratospheric ozone (O3) absorbs UV rays
UV rays

O3  O2 + O
The Importance of
Stratospheric Ozone
CFC’s
– link to ozone hole established in
1970s
– Chloroflourocarbons (refrigerants,
aerosols)
– one Cl can decompose more than
100,000 O3
– Montreal Protocol, 1987: U.N.
agreement on ban
– up to 10 years for rising CFC gases
to reach stratosphere; once in the
stratosphere, CFC’s can last up to
50-100 years
RECENT YEARS
RECENT YEARS
Key Points
• Development of Earth’s
atmosphere
– 4 periods
• Vertical structure of the
atmosphere
– 4 temperature layers
– changes in pressure
– Aurora borealis / australis
– the ozone layer