Introduction to the
Atmosphere
Structure and Composition
Atmosphere (air) is:
• a mixture of gases held to Earth by gravity.
• Earth’s primary means of transporting energy
and moisture over its surface.
• an insulator and shield (“ editing the sun”).
• relatively thin
Earth (including its
atmosphere) has a
radius of about 6500
km. The atmosphere
is only about 100 km
thick—less than 2%
of the Earth’s
thickness.
• Despite its relative shallowness, the
atmosphere contains considerable
mass: 5.14 X 10E15 kg
•  that’s a mass equivalent to 5.65
billion million tons, the amount of
water that would fill a lake the size
of California to a depth of 13 km (7.7
miles)
Thermopause: the top of Earth’s atmosphere
@ 480 km (or 300 miles) above the surface
Exosphere: H and He atoms weakly bound by
gravity up to about 32,000 km (or 20,000
miles) from the surface
Atmospheric density decreases with
altitude…
1.
About 97% of the
atmosphere is within
30 km (18 miles) of
the surface.
2. 75% is compressed
below 10,700 m or
35,100 feet (~7
miles)…which is to
say, within the
troposphere
Atmospheric density decreases with
altitude…
Density: the
amount of mass
of a substance
(expressed in
kilograms)
contained in a
unit of volume (1
cubic meter)
Atmospheric density decreases with
altitude…
In gases, individual molecules have
no attachment to each other and
move about randomly.
So…there is no definite limit to
the amount of mass that can exist
in a given volume—molecules can
always be added to or removed
from the volume…
Result  change in density
Atmospheric density decreases with
altitude…
Mean free path the average
distance a molecule travels before
colliding with another
Near Earth’s surfacemean free
path is a mere 0.0001 mm
150 km (93 miles) above sea level
(i.e., in the thermosphere) a
molecule travels about 10 m before
colliding with another
Atmospheric/Air Pressure
• Def. the weight (or force exerted over a
unit area) of atmosphere
• That is, under the influence of gravity, air
is compressed & thus denser (& to you,
heavier) at the Earth’s surface
• At sea level average air pressure is:
 1 kg/cm2 (14.7 lbs./in2 )
A Profile of the
Atmosphere:
Three Criteria
• 1. Composition
• 2. Temperature
(also pressure)
• 3. Function
Composition:
Heterosphere & Homosphere
• Heterosphere (ca. 80 km to 480 km
OR 50 miles to 300 miles)
• Not evenly mixed
• Less than 0.001% of the atmosphere’s
mass
Composition:
Heterosphere & Homosphere
• Homosphere (surface to ca. 80 km or 50
miles)
• Nearly uniform blend of gases
throughout…
• Exceptions?
• *Nitrogen, Oxygen (a byproduct of?),
*Argon (radioactive decay of a form of
potassium), Carbon dioxide (has been
increasing over last 200 years)
Temperature
Troposphere from
ground to 8 to 16
km
Stratosphere from
8 or 16 km to about
50 km
Mesosphere from
50 to 80 km
Thermosphere
from 80 to 480 km
(high temperature
but low heat)
Lapse Rates
Troposphere heated by longwave radiation from
Earth’s surface- further from surface colder temp
Lapse rate = rate of change
Normal lapse rate- average rate of temp change
6.4 °C/1000 m (ascent) (3.5 °F/1000 ft)
Environmental lapse rate- actual change in temp for
given location.
You are going hiking.. Your hike begins at 6000 feet
and the temperature is 80 degrees Fahrenheit.
The summit of this mountain is 12,100 feet. Using
the normal lapse rate, predict the temperature at
the summit. What is the change in temperature on
your hike?
1. Find change in elevation (12,100-6000 = 6100 feet)
2. Divide the change in elevation by the distance
portion of the lapse rate (6100 ft /1000 ft= 6.1)
3. Multiply the result by the temperature portion of
the lapse rate (6.1 x 3.5 degrees F = 21.35 degrees
F)
4. Subtract result from original temperature (80 –
21.35 = 58.65 degrees F)
Temperature
inversion
2. Ozonosphere- within stratosphere,
Composed of > reactive oxygen molecules (O3),
Filters solar radiation, absorbing UV light
Has been stable several 100 million years,
now greatly variable
The Ozone Layer
Why doesn’t Ozone continually build up?
1. OZONE (O3) recombines w/ atomic oxygen to form O2
+
+
2. OZONE (O3) absorption of UV radiation leads to break
down into atomic & molecular oxygen
UV radiation
+
Ionosphere:
Ion = electrically
charged particle
formed when
electrically neutral
atoms or molecules
lose or gain one or
more electrons
**due to
bombardment by
solar energy LOSE
electrons
Aurora borealis and Aurora australis
Subatomic particles captured by
Earth’s magnetic field
Captured energy excites gases
of atmosphere…meaning their
electrons jump to greater
orbital distances from their
nuclei
When electrons fall back to
lower orbital, radiation is
emitted.