THE EARTH’S ATMOSPHERE
• The atmosphere of Earth consists of several regions or layers
having different properties (temperature, pressure, and composition)
which vary with altitude in different ways.
• The lowest portion of the atmosphere, the troposphere, contains
most of the atmospheric mass, is the part of most direct importance
to life on Earth, and is the subject of the field of atmospheric science
known as meteorology.
• The next highest layer of the atmosphere, the stratosphere, is
important because it contains the ozone layer which shields life on
Earth from harmful solar ultraviolet radiation.
• The higher layers of the atmosphere, the mesosphere and the
thermosphere, differ significantly in composition from the lower
regions, and also contain a significant proportion of ionized gas
which constitutes the ionosphere.
• The ionosphere is of practical importance because it facilitates longdistance radio communications.
• The field of atmospheric science concerning the mesosphere and
upper atmosphere is aeronomy.
EARLY HISTORY OF EARTH’S ATMOSPHERE
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Earth’s original atmosphere was quite different from the present
atmosphere; it probably consisted mostly of nitrogen (N2) and carbon
dioxide (CO2) with much smaller amounts of reduced gases such as
carbon monoxide (CO), methane (CH4), ammonia (NH3) and hydrogen
sulfide (H2S).
It is also possible that very early in Earth’s history, significant amounts of
hydrogen (H2), the most abundant part of the material from which the solar
system was formed, could have been a major part of the atmosphere.
Water (H2O), currently mostly liquid in Earth’s oceans, has about 300 times
the mass of Earth’s current atmosphere. If temperatures on the early Earth
were sufficiently high, H2O would have been the primary constituent of the
atmosphere (about 300 bars), followed by CO2 (70 to 90 bars).
Most of the original CO2 is now locked up in carbonate rocks, such as
limestone (calcium carbonate, CaCO3), as a result of water erosion (as
carbonic acid, H2CO3) of the original silicate rocks:
(Ca, Mg)SiO3 + H2CO3
(Ca, Mg)CO3 + H2O + SiO2
The “noble” gases, such as helium, neon, and argon in the atmosphere,
are primarily produced by the decay of radioactive elements.
EARLY HISTORY OF EARTH’S ATMOSPHERE
• Both fossil and mineral evidence indicate that oxygen (O2) was
only a minor constituent of the atmosphere for the first half
of Earth’s existence (the Archean eon), and did not reach
near-current levels until the Phanerozoic eon (beginning 600
million years ago).
• The primary source of Earth’s atmospheric oxygen is green
plant photosynthesis, in which carbon dioxide and water are
combined to produce carbohydrates (and molecular oxygen is a
“waste product”).
• In the first half of Earth’s early history, only photosynthetic
bacteria (cyanobacteria, or “blue-green algae”), which did not
require oxygen themselves (also known as anaerobic bacteria),
were available for photosynthesis.
Evolution of Atmospheric Oxygen in Geologic Time
INVENTORY OF CARBON NEAR EARTH’S SURFACE
(Relative Units)
Biosphere
Atmosphere (as CO2)
Oceans (as dissolved CO2)
Fossil Fuels
Shales
Carbonate Rocks
2
70
4000
800
800,000
2,000,000
THE EARTH’S SEASONS
• The seasonal variations in temperature, and the ratio of day to
night hours, are due to the fact that Earth’s axis of rotation is
tilted about 23.5° away from perpendicular to the plane of its
orbit around the Sun (the ecliptic).
• On the first day of summer in the northern hemisphere, the north
pole of Earth’s axis is tilted 23.5° toward the Sun, days are
longer than nights, and the Sun illuminates the surface at a
higher angle from horizontal (both of which contribute to higher
average temperatures).
• On the first day of winter in the northern hemisphere, the north
pole of Earth’s axis is tilted 23.5° away from the Sun, nights are
longer than days, and the Sun illuminates the surface at a lower
angle from horizontal (both of which contribute to lower average
temperatures).
THE EARTH’S SEASONS
• The variation of the seasons is opposite in the southern
hemisphere than in the northern hemisphere; i.e. it is winter in
the southern hemisphere when it is summer in the northern
hemisphere, and vice versa.
• The first day of spring (vernal equinox) and the first day of fall
(autumnal equinox) occur when Earth’s rotation axis is parallel
to the Sun, and day and night are of equal length everywhere on
Earth.
• The slight variations in Earth’s distance from the Sun has very
little effect on Earth’s temperature and seasonal variations.
Distribution of Solar Radiation Over Sunlit Hemisphere of Earth
Seasonal Variations of Solar Radiation Intensity
ATMOSPHERIC STRUCTURE
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The pressure, temperature, and composition of the atmosphere vary with
height above Earth’s surface, as well as with location on Earth’s surface.
In the lower regions of the atmosphere, known as the troposphere, the
composition is relatively independent of altitude, and variations are primarily
of pressure and temperature, which decrease with altitude.
In the region known as the stratosphere, beginning at about 10 km altitude
(the tropopause), pressure continues to decrease with altitude, but
temperature increases with altitude.
In the next higher region, known as the mesosphere (middle atmosphere),
beginning at about 50 km altitude (the stratopause), temperature again
decreases with altitude.
At altitudes greater than about 80 km, known as the mesopause,
temperature again increases with altitude, in the region known as the
thermosphere.
This region of the atmosphere is also noteworthy in that its composition
changes with altitude, with lighter gases becoming more abundant relative
to heavier gases (the region known as the heterosphere, as distinct from
the lower regions where composition is nearly constant with altitude, known
as the homosphere).
ATMOSPHERIC THERMODYNAMICS
• The temperature and pressure of the atmosphere vary with
altitude in a complex manner, due to a variety of external, as
well as internal, influences which can be quantitatively
understood using the laws of thermodynamics.
• Important aspects include the specific heat of the gas (the
amount of energy required to increase the temperature by a
given amount), which is different for the cases of constant
pressure and of constant volume.
• The pressure of a given number, n, of gas atoms or molecules
per unit volume is given by: p = nkT, where k is Boltzmann’s
constant, and T is the temperature (in degrees Kelvin).
• Note also, the specific heat at constant pressure is higher than
the specific heat at constant volume, because work has to be
done to make the gas expand.
• In the case of atmospheric science, the force which compresses
the atmosphere, i.e. Earth’s gravity, decreases relatively slowly
(as 1/R2, with altitude, where R is distance from Earth’s center).
ATMOSPHERIC SCALE HEIGHT
Pressure in a planetary atmosphere decreases with height above
the surface, at a rate that depends on the acceleration of gravity, g;
the atomic or molecular weight, M, the temperature, T (°K); and
Boltzmann’s constant, k:
dp
Mg

dh
kT
which yields a pressure at altitude h, relative to the pressure p0 at
the surface or reference altitude, of
p  p0e

Mgh
kT
 p0e

h
H
where H is the scale height:
kT
H
Mg
The scale height is the distance over which the pressure decreases
by a factor of 1/e. Note, this relationship is complicated by the fact
that temperature, acceleration of gravity g, and, in some cases,
average atomic or molecular weight, M, vary with altitude.
THE EARTH’S ATMOSPHERE
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Earth’s weather and climate are dependent on the balance between heat
input and heat output in the lower atmosphere.
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Solar radiation is the primary source of heat energy; however, most of the
input to the atmosphere is not directly from the Sun, but is a secondary result
of heating Earth’s surface.
Heat transfer as a result of the evaporation and condensation of water is
also an important part of atmospheric thermodynamics.
The atmospheric circulation pattern is also influenced by Earth’s rotation, by
way of the Coriolis effect.
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Weather and climate are also affected by the detailed distribution of land vs.
sea surface areas.
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The global climate can be influenced by changing concentrations of infraredtrapping “greenhouse gases”, such as CO2 and CH4, in the atmosphere.
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Water vapor (H2O) is also an important greenhouse gas, whose
concentration in the atmosphere increases with temperature.
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Therefore, in principle, increasing amounts of other greenhouse gases could,
in principle, cause a catastrophic “run-away greenhouse” event, by
increasing the amount of water vapor in the atmosphere.
Typical Temperature Variations with Altitude in Earth’s Atmosphere
Attenuation of Incoming Sunlight by Earth’s Atmosphere
Spectral Intensity of Sunlight in Near-Earth Space
Spectral Intensity of Sunlight at Earth’s Surface
Attenuation of Incoming Sunlight by Earth’s Atmosphere
Incoming
Outgoing
METEOROLOGY
• The field of atmospheric science which is most well-known and
of practical importance to the general public is meteorology, the
study of weather.
• Meteorology is usually concerned only with the lowest region of
the atmosphere, the troposphere.
• Weather is influenced not only by vertical, diurnal, and seasonal
variations of atmospheric density and temperature, and of solar
heating, but also by horizontal variations over Earth’s surface.
• Atmospheric winds and circulation are influenced by Earth’s
rotation, and by surface conditions (i.e. whether land or sea,
topography, and surface temperature).
• The advent of weather-monitoring satellites, and of
supercomputers, have greatly facilitated the science and
application of meteorology in recent years.
Atmospheric Circulation on Non-Rotating vs.
Rotating Earth
Typical Atmospheric Circulation Patterns in the
Northern Hemisphere
H
L
Atmospheric Isobaric Contours and Air Flow Directions
1024
1020
1016
Pressure in Millibars
H
1028
1012
1008
1000
1004
L
996
Cloud Types
ACTIVE WEATHER PHENOMENA
• The topics discussed up to now are typical conditions and
motions within Earth’s troposphere. However, as is well known,
meteorology also deals with weather conditions which are not
typical, but can occur when conditions, such as temperature,
humidity, and wind motions, are conducive. In order of
increasing strength (but decreasing probability, at any particular
location) are the following:
o
o
o
o
Rainstorms and Snowstorms
Windstorms and Lightning (often associated with rainstorms)
Hurricanes
Tornadoes
• Weather conditions of these types depend on the local climate
(typical average temperatures, altitudes, and windiness),
proximity to oceans or mountains, and time of year.
• Although tornadoes can have higher wind velocities than
hurricanes, they are much smaller in size and shorter in effective
lifetime than are hurricanes.
LIGHTNING PRODUCTION PROCESS
Typical Electric Charge Separation in a Thundercloud
Lightning Discharge Processes
Cloud to Ground Lightning Discharges
Tornado Alert!
LandSat View of Tornado Path
La Plata, MD - April 28, 2002
Hurricane Viewed from a Space Shuttle
Air Flow Directions in a Typical Hurricane
Structure of a Hurricane
Viewing Earth’s Atmosphere From Space
• One of the most important practical benefits from the U.S. space
program has been the development and operational use of
space-based observations of Earth’s atmosphere, to detect,
measure, and track weather patterns in the troposphere and
lower stratosphere.
• The benefits of space-based operations include the ability of a
satellite, in orbit around Earth, to view (depending on its orbit)
the entire Earth, at regular intervals for each location, and/or
nearly half of the Earth’s surface at one time, continuously, for
long periods of time.
• Satellites in low-altitude, polar orbits can view nearly the entire
Earth, over long periods of time, but only a small portion of the
Earth at one time.
• Satellites in high-altitude, equatorial “geosynchronous” orbits
can view nearly half of Earth’s surface continuously, over long
periods of time, but at lower resolution than achievable with
satellites in low-altitude orbits.
Earth Observing Satellites
• Satellites in near-polar “sun synchronous” orbits at relatively low
altitudes (> 500 km) can view, over a period of about 1 day, nearly
the entire Earth with high resolution (but only a small area at any
one time).
• Satellites in equatorial “geosynchronous” orbits, at about 35,800
km altitude, revolve around Earth once every 24 hours, the same
time period that Earth rotates on its axis, and hence appear (to an
observer on the Earth) to be always in the same position in the sky
(geostationary).
View of Earth and Hurricane Andrew from
Geosynchronous Orbit
Sequence of 3 Images of Hurricane Andrew (1992)
The Aura Earth Observing System Satellite
• The Aura satellite, launched into near-polar Earth orbit July 15, 2004,
is the last of the series of Earth Observing System (EOS) research
satellites launched by NASA, whose predecessors are the Terra landobserving satellite, and the Aqua water-observing satellite.
• The Aura mission is one of scientific research, as distinct from
“operational” meteorological satellites, most of which are under the
National Oceanic and Atmospheric Administration (NOAA).
Aura EOS Satellite Instrumentation
•High Resolution Dynamics Limb Sounder (HIRDLS)
o Stratospheric and upper tropospheric trace gases and aerosols measured by
infrared limb emission
•Microwave Limb Sounder (MLS)
o Stratospheric and upper tropospheric trace gases measured by microwave limb
emission
•Ozone Monitoring Instrument (OMI)
o Tropospheric and stratospheric ozone, aerosols, air quality, and surface ultraviolet
radiation measured by backscatter ultraviolet and visible radiation
•Tropospheric Emission Spectrometer (TES)
o Tropospheric and lower stratospheric trace gases and aerosols measured by nadir
and limb infrared emission.
The various gases in the atmosphere absorb or emit radiation at specific
wavelengths, depending on their molecular structure. Aura’s instruments measure
atmospheric constituents by observing Earth over a large range of the
electromagnetic spectrum, including solar backscatter in the UV and visible
ranges, and thermal emission in the infrared and microwave ranges.
CLIMATOLOGY
• Climatology, the study of climate, differs from meteorology in that
climate is the long-term pattern of temperature, precipitation, wind
patterns, etc. at a particular location, over periods of a year or
more, whereas weather is the current (or very near-term) state of
affairs at the location or region of interest.
• For example, the climate in Antarctica is quite different from that
in the Sahara Desert, or the Amazon river basin.
• The latter two are also quite different from each other, despite
being at nearly the same latitudes on Earth.
• Climate can, however, change over long periods of time, and the
topic of climate change is currently of practical importance,
because of the known or potential effects of human activity on
local, regional, or even world-wide climates.
Mean Temperatures (°C) for January
-40
-30
-10
10
15
5
-20
0
20
25
30
20
15
10
5
Mean Temperatures (°C) for July
0
5
10
15
20
30
25
35
30
25
20
0
5
-10
-20
10
15
Annual Average Rainfall vs. Location on Earth
World Climate Classifications
Wet /Moderate
Steppe
Desert
Human Induced Air Pollution
THE EARTH’S MIDDLE ATMOSPHERE
• The middle atmosphere (stratosphere and mesosphere) are
regions in which the composition and temperature are affected
by the influence of solar ultraviolet radiation.
• The ozone layer results from the dissociation of O2 by solar UV,
followed by the reaction
O + O2
O3
• Because ozone absorbs solar UV more strongly than O2, the
stratosphere is heated to higher temperatures than the upper
layers of the troposphere below it.
• Temperature increases with altitude from the tropopause to the
stratopause.
• In the mesosphere, atomic oxygen replaces both O2 and O3 as
the dominant form of oxygen, and temperature decreases with
altitude up to the mesopause.
Reduction in Antarctic Ozone Due to Chlorine Pollution of the Stratosphere
Minor Constituent Variations with Altitude in
the Lower and Middle Atmosphere
AERONOMY
• The regions of Earth’s atmosphere above the troposphere and
stratosphere, the regions of primary concern for the subject
areas of Meteorology and Climatology, constitute the regions of
concern for the subject area of aeronomy.
• These regions, in order of increasing altitude, are the
mesosphere, the thermosphere, and the exosphere.
• Another feature that distinguishes the different regions of the
upper atmosphere, is that in the regions below about 100 km
altitude, the different atoms and molecules are well-mixed (i.e.
do not change their relative percentages of composition with
altitude), the region known as the homosphere.
• At higher altitudes, however, the different atoms and molecules
diffusively separate, due to the reduced frequency of collisions
with each other, and vary with altitude according to their atomic
or molecular weights (the region known as the heterosphere).
THE EARTH’S UPPER ATMOSPHERE
• In the thermosphere, atomic oxygen is the dominant
constituent, followed by molecular nitrogen, and temperature
again increases with altitude to the thermopause.
• In the exosphere, the temperature remains relatively constant
with altitude, at a level determined largely by solar activity.
• In the exosphere, collisions between gas atoms and molecules
are rare, and an atom traveling upward at a sufficiently high
velocity can escape from the atmosphere altogether.
• The Sun’s far-ultraviolet and X-ray radiations are primarily
responsible for heating of the thermosphere and exosphere.
• These highly energetic radiations constitute only a small fraction
of the Sun’s total radiation output, but are far more variable with
solar activity than is the visible radiation that reaches Earth’s
surface.
Typical Density vs. Altitude of Atmospheric
Constituents in the Upper Atmosphere
Apollo-16 Far-UV Images of Earth’s Upper Atmosphere
105-160 nm
Primary Emission: H (Lyman-) 121.6 nm
(Diagonal Streaks are Instrumental Artifacts)
123-160 nm
Primary Emissions: O 130.4, 135.6 nm
EFFECTS OF SOLAR ACTIVITY ON THE UPPER ATMOSPHERE
• Solar ultraviolet and X-ray radiation can heat Earth’s upper
neutral atmosphere, and can cause significant variations in the
scale height and atmospheric densities at high altitudes.
• This is of practical importance because of the effects of
atmospheric drag on satellites in low Earth orbit.
• The temperature, and corresponding scale height and highaltitude atmospheric density, can vary by a very large factor
between times of solar minimum activity (characterized by few
sunspots) and solar maximum (large numbers of sunspots).
• Solar activity varies over a relatively regular cycle of about 11
years (between maxima of sunspot number and solar activity).
• High solar activity also results in increased auroral activity,
which is usually localized to Earth’s polar regions and is a result
of charged particles outflowing from the Sun (the “solar wind”)
and their interaction with Earth’s magnetic field.
THE EARTH’S IONOSPHERE
• The mesosphere and thermosphere are also the region of the
atmosphere where a significant number of atoms and molecules
can be ionized, constituting the region we call the ionosphere.
• The ionosphere is produced primarily by solar far-ultraviolet and
X-ray radiation, having photon energies sufficient to remove one
or more electrons from atmospheric molecules and atoms.
• The composition and electron density of the ionosphere are
strongly dependent on both solar activity and altitude, and are quite
variable from day to night.
• In the regions near Earth’s geomagnetic poles, charged particles
from the solar wind and Earth’s magnetosphere can enter the
upper atmosphere, creating the aurora, and also can produce local
enhancements of the ionosphere.
• The ionosphere is of great practical importance, because it makes
possible long-distance radio communications, due to its ability
to reflect radio waves around the Earth’s curvature.
THE EARTH’S IONOSPHERE
• The ionosphere is, nominally, the region of the atmosphere
above about 60 km altitude, above which the electron densities
are sufficiently high to affect radio wave propagation.
• The ionosphere is divided, nominally, into three main regions,
based on their electron densities and radio wave propagation
effects:
o D-Region: 60-90 km altitude
o E-Region: 90-140 km altitude
o F-Region: 140-1000 km altitude
• The ionosphere also contains electric fields and currents, which
are highly variable with solar activity and other influences.
o The ionospheric wind dynamo results from winds in the
thermosphere (90-500 km altitude) which move the conducting
(ionized) medium relative to the geomagnetic field.
o Above 140 km (F-region), solar far-ultraviolet radiation is the
primary driver during the day, but particle precipitation from the
magnetosphere is important at night.
Atmospheric Ionization by Solar Far UV and X-Ray Radiation
(Typical Values)
(Typical Values)