Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 1. Energy and Mass
Chapter 1.
Composition and Structure of the
Atmosphere
Introduction
The Atmosphere
A mixture of gas molecules, microscopically small
suspended particles of solid and liquid, and falling
precipitation
Meteorology
The study of the atmosphere and the processes that
cause what we refer to as “weather”
The Thickness of the Atmosphere
Although atmospheric density decreases rapidly with height, the
top of the atmosphere is undefined
Overall, the majority of atmospheric mass is contained in a rather
thin layer near the surface
• The atmosphere still contains an impressive sum of mass
(5.14 x 1015 kg)
Composition of the Atmosphere
The atmosphere is an aggregate of gases, suspended particles, and
liquid/solid water
Gases and particles are readily
exchanged between Earth’s
surface and the atmosphere
through physical (volcanic) and
biological (photosynthesis,
respiration) processes
Water Vapor
The most abundant variable gas
Added or removed from atmosphere through the hydrologic cycle
Concentrations exist from nearly 0% over desert and polar regions
to nearly 4% near tropics
Major contributor to Earth’s energy balance and many important
atmospheric processes
Water vapor image
showing
broader distribution of
moisture
than the image of actual
clouds
(below)
Carbon Dioxide
A trace gas accounting for only 0.036% of total atmospheric mass
Important to Earth’s energy balance
Added through biologic respiration, volcanic activity, decay, and
natural and human-related combustion removed through
photosynthesis
The steady increase of
atmospheric CO2 due to
human activities.
Seasonal variation related to
rates of photosynthesis, respiration,
and organic decay.
Ozone
The tri-atomic form of oxygen
Near the surface it is a pollutant. In the stratosphere it is an
essential absorber of ultraviolet radiation
Formed when atomic oxygen (O) bonds with molecular oxygen
(O2)
Chlorofluorocarbons (CFCs), specifically chlorine atoms, react
with O3 in the stratosphere. Produces chlorine monoxide.
Chlorine remains unaffected through chemical reactions but
oxygen is affected thereby destroying ozone. Ozone destruction
peaks over the southern hemisphere and persists through spring.
The Antarctic circumpolar vortex contributes to O3 depletion by
inhibiting latitudinal mixing which leads to an O3 “hole”
Images showing
reduction of
ozone over
Antarctica
over time.
Area in red
indicates the
“ozone hole.”
Methane
A variable gas in small but recently increasing concentrations
Released to the atmosphere through fossil fuel activities, livestock
digestion, and agriculture cultivation (esp. rice)
As a very effective absorber of terrestrial radiation it plays an
active role in near surface warming
Annual increases in
atmospheric methane
Aerosols
Any solid and/or liquid particle, other than water, which exists in
the atmosphere
Synonymous with the term particulate
Both natural (sea spray, dust, combustion) and human
(combustion) products
Due to small size, easily remain in suspension for long periods
Contribute to precipitation processes as condensation nuclei
Vertical Structure of the Atmosphere
Density
Mass (kg) per unit volume (m3)
Due to compressibility, near surface air is more dense than that
above
This may be expressed in terms of the mean free path, or average
distance a molecule travels before colliding with another molecule.
Due to compressibility,
atmospheric mass
gradually “thins out”
with height
Thermal Layers of the Atmosphere
Four distinct layers of the atmosphere emerge from identifiable
temperature characteristics with height
Troposphere
is the lowest layer, named as this region promotes atmospheric
overturning
Layer of virtually all weather processes, identified by a steady
temperature decrease with height
• decrease = 6.5oC/km (3.6oF/1000ft)
This thinnest layer contains 80% of the mass of the atmosphere
Due to thermal expansion, the tropopause is roughly 16 km (10
mi) over the tropics and about 1/2 that at the poles
Colder temperature aloft implies the atmosphere is essentially
transparent to solar radiation and is heated from terrestrial radiation
absorption
Most clouds exist in the
troposphere. Occasionally,
violent updrafts penetrate
cloud tops into the
stratosphere. The flattened
top of this cumulonimbus
cloud is in the stratosphere.
The Stratosphere
Area of little weather
A layer of constantly inverted temperature after an initial layer of
constant temperature with height
The layer warms with height to the freezing point of water near the
stratopause
Inversion caused by the absorption of ultraviolet radiation by
ozone
Although the ozone layer exists through an altitude between 20-30
km (12-18 mi), actual concentration of ozone can be as low as 10
ppm
Mesosphere and Thermosphere
Combined the two layers account for only 0.1% of total
atmospheric mass
Mesosphere, which extends to about 80 km (50 mi) is
characterized by decreasing temperatures with height and is the
coldest atmospheric layer
The thermosphere above slowly merges with interplanetary space
and is characterized by increasing temperatures with height
Temperatures approach 1500oC, however, this only measures
molecular kinetic energy as the sparse amount of mass precludes
actual heat content
Electrical Properties
The Ionosphere
Located within the meso- and thermospheres, this portion of the
atmosphere is replete with ions; electrically charged particles
Subdivided into the D-, E-, and F-layers with increasing height
D and E layers diminish with reduced solar radiation allowing the
F layer to reflect radio waves through the night
Interactions between the ionosphere and subatomic particles
emitted from the Sun excite atmospheric gases causing the aurora
borealis (northern lights) and the aurora australis (southern lights)
Evolution of the Atmosphere
The early atmosphere was likely composed of hydrogen and
helium
Light gases either exited the atmosphere to space through high
escape velocities or through collisions with large celestial bodies
A secondary atmosphere formed through volcanic outgassing and
by material, largely water, gained from comets
Precipitation removed, and continues to remove, excess water
vapor, while high concentrations of CO2 were replaced by oxygen
through photosynthesis and dissolution in water.
Due to its stability, nitrogen concentrations slowly grew to presentday levels
Planetary Atmospheres
Mercury
Virtually no atmosphere due to small size and high temperatures
Venus
Very thick atmosphere with a mass 90 times greater than Earth
Primarily CO2 and N2
A “runaway” greenhouse effect responsible for very high
temperatures
Venus and its
extensive
cloud cover
Mean temperature
profile for Venus
Mars
Similar in composition to Venus but because of low atmospheric
density, much terrestrial radiation is lost to space. This accounts
for low overall temperatures
Mean temperature
profile for Mars
Jovian Planets
Composed of lighter gases with either solid or liquid cores
Voyager image
of Jupiter
Mean temperature
profile for Jupiter
End of Chapter 1
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt