Chapter 1: The Earth and its atmosphere
Overview of the Earth’s Atmosphere
Stars are hot, glowing balls of gas that generate energy by converting hydrogen into helium near their
centers. Our sun is an average size star situated near the edge of the Milky Way galaxy.
At an average distance from the sun of nearly 150 million kilometers (km) or 93 million miles (mi), the
earth intercepts only a very small fraction of the sun’s total energy output. However, it is this radiant
energy (or radiation)* that drives the atmosphere into the patterns of everyday wind and weather and
allows the earth to maintain an average surface temperature of about 15°C (59°F).
The earth’s atmosphere is a thin, gaseous envelope comprised mostly of nitrogen and oxygen, with
small amounts of other gases, such as water vapor and carbon dioxide. Nestled in the atmosphere are
clouds of liquid water and ice crystals. Although our atmosphere extends upward for many hundreds of
kilometers, almost 99 percent of the atmosphere lies within a mere 30 km (19 mi) of the earth’s surface.
Atmosphere shields the surface and its inhabitants from the sun’s dangerous ultraviolet radiant energy,
as well as from the onslaught of material from interplanetary space. There is no definite upper limit to
the atmosphere; rather, it becomes thinner and thinner, eventually merging with empty space, which
surrounds all the planets.
COMPOSITION OF THE ATMOSPHERE
Notice that nitrogen (N2) occupies about 78 percent and oxygen (O2) about 21 percent of the total
volume of dry air. If all the other gases are removed, these percentages for nitrogen and oxygen hold
fairly constant up to an elevation of about 80 km (50 mi).
*Radiation is energy transferred in the form of waves that have electrical and magnetic properties. The
light that we see is radiation, as is ultraviolet light.
At the surface, there is a balance between destruction (output) and production (input) of these gases.
For example, nitrogen is removed from the atmosphere primarily by biological processes that involve
soil bacteria. In addition, nitrogen is taken from the air by tiny ocean-dwelling plankton that convert it
into nutrients that help fortify the ocean’s food chain. It is returned to the atmosphere mainly through
the decaying of plant and animal matter. Oxygen, on the other hand, is removed from the atmosphere
when organic matter decays and when oxygen combines with other substances, producing oxides. It is
also taken from the atmosphere during breathing, as the lungs take in oxygen and release carbon
dioxide (CO2). The addition of oxygen to the atmosphere occurs during photosynthesis, as plants, in the
presence of sunlight, combine carbon dioxide and water to produce sugar and oxygen.
The concentration of the invisible gas water vapor (H2O), however, varies greatly from place to place,
and from time to time. The falling rain and snow is called precipitation.
In the lower atmosphere, water is everywhere. It is the only substance that exists as a gas, a liquid, and
a solid at those temperatures and pressures normally found near the earth’s surface.
Water vapor is an extremely important gas in our atmosphere due to precipitation and also it releases
large amounts of heat — called latent heat. Latent heat is an important source of atmospheric energy,
especially for storms, such as thunderstorms and hurricanes. Moreover, water vapor is a potent
greenhouse gas because it strongly absorbs a portion of the earth’s outgoing radiant energy
Carbon dioxide (CO 2), a natural component of the atmosphere, occupies a small (but important)
percent of a volume of air, about 0.038 percent. Carbon dioxide enters the atmosphere mainly from the
decay of vegetation, but it also comes from volcanic eruptions, the exhalations of animal life, from the
burning of fossil fuels (such as coal, oil, and natural gas), and from deforestation. The removal of CO2 from the atmosphere takes place during photosynthesis, as plants consume CO 2to produce green
matter. The CO 2is then stored in roots, branches, and leaves. The oceans act as a huge reservoir
for CO 2, as phytoplankton (tiny drifting plants) in surface water fix CO 2into organic tissues. Carbon
dioxide that dissolves directly into surface water mixes downward and circulates through greater
depths. Estimates are that the oceans hold more than 50 times the total atmospheric CO 2content.
This increase means that CO 2is entering the atmosphere at a greater rate than it is being removed. The
increase appears to be due mainly to the burning of fossil fuels; however, deforestation also plays a role
as cut timber, burned or left to rot, releases CO 2directly into the air.
Carbon dioxide is another important greenhouse gas because, like water vapor, it traps a portion of the
earths outgoing energy. Other green house gases include methane (CH 4), nitrous oxide (N 2O), and
chlorofl uorocarbons (CFCs).*
*Because these gases (includ2) occupy only a small fraction of a percent in a volume of air near the surface, they are referred to
collectively as trace gases.
At the surface, ozone (O 3) is the primary ingredient of photochemical smog, the majority of
atmospheric ozone (about 97 percent) is found in the upper atmosphere — in the stratosphere — where it
is formed naturally, as oxygen atoms combine with oxygen molecules.
When CFCs enter the stratosphere, ultraviolet rays break them apart, and the CFCs release ozone-
destroying chlorine. Because of this effect, ozone concentration in the stratosphere has been decreasing
over parts of the Northern and Southern Hemispheres. The reduction in stratospheric ozone levels over
springtime Antarctica has plummeted at such an alarming rate that during September and October,
there is an ozone hole over the region.
Collectively, these tiny solid or liquid suspended particles of various composition are called aerosols.
The earths first atmosphere (some 4.6 billion years ago) was most likely hydrogen and helium — the
two most abundant gases found in the universe — as well as hydrogen compounds, such as methane
(CH 4) and ammonia (NH 3). A second, more dense atmosphere, however, gradually enveloped the earth
as gases from molten rock within its hot interior escaped through volcanoes and steam vents. We
assume that volcanoes spewed out the same gases then as they do today: mostly water vapor (about 80
percent), carbon dioxide (about 10 percent), and up to a few percent nitrogen. These gases (mostly
water vapor and carbon dioxide) probably created the earth’s second atmosphere. As millions of years
passed, the constant outpouring of gases from the hot interior — known as outgassing — provided a
rich supply of water vapor, which formed into clouds. Rain fell upon the earth for many thousands of
years, forming the rivers, lakes, and oceans of the world. It appears that oxygen (O 2), the second most abundant gas in today’s atmosphere, probably began an
extremely slow increase in concentration as energetic rays from the sun split water vapor (H 2O) into
hydrogen and oxygen during a process called photodissociation. The hydrogen, being lighter, probably
rose and escaped into space, while the oxygen remained in the atmosphere.
The volume of an average size breath ofair is about a liter.* Near sea level, there are roughly ten thousand million million
million (102)† air molecules in a liter. So, 1 breath of a22molecules. There are as many molecules in a single breath
as there are breaths in the atmosphere.
Ozone (O 3) in the stratosphere protects life from harmful ultraviolet (UV) radiation. At the surface, ozone is the
main ingredient of photochemical smog.
●The majority of water on our planet is believed to have come from its hot interior through outgassing.
A vertical profile of the atmosphere reveals that it can be divided into a series of layers. Each layer may
be defined in a number of ways: by the manner in which the air temperature varies through it, by the
gases that comprise it, or even by its electrical properties. At any rate, before we examine these various
atmospheric layers, we need to look at the vertical profile of two important variables: air pressure and air
Our atmosphere is crowded close to the earth’s surface, air molecules are held near the earth by gravity.
Air above squeezes (compresses) air molecules closer together, which causes their number in a given
volume to increase. Weight is the force acting on an object due to gravity. Weight is defined as the mass
of an object times the acceleration of gravity; Weight =mass x gravity.
An object’s mass is the quantity of matter in the object. The mass of air in a rigid container is the same
everywhere in the universe. However, if you were to instantly travel to the moon, where the acceleration
of gravity is much less than that of earth, the mass of air in the container would be the same, but its
weight would decrease.
Near sea level, air density is about 1.2 kilograms per cubic meter.
The density of air (or any substance) is determined by the masses of atoms and molecules and the amount
of space between them. In other words, density tells us how much matter is in a given space (that is,
volume). We can express density in a variety of ways. The molecular density of air is the number of
molecules in a given volume. Most commonly, however, density is given as the mass of air in a given
volume; thus Density= mass volume.
Because there are appreciably more molecules within the same size volume of air near the earth’s surface
than at higher levels, air density is greatest at the surface and decreases as we move up into the
Air near the surface is compressed; air density normally decreases rapidly at first, then more slowly as we
move farther away from the surface. Air molecules are in constant motion. Pressure= force/area.
hectopascal (hPa) is gradually replacing the millibar as the preferred unit of pressure on surface charts. At
sea level, the standard value for atmospheric pressure is 1013.25 mb =1013.25 hPa = 29.92 in. Hg.
The weight of the air molecules acts as a force upon the earth. The amount of force exerted over
an area of surface is called atmospheric pressure or, simply, air pressure
As we climb in elevation, fewer air molecules are above us; hence, atmospheric pressure always
decreases with increasing height.
Like air density, air pressure decreases rapidly at first, then more slowly at higher levels. The rate at which the air temperature decreases with height is called the temperature lapse rate.
On some days, the air becomes colder more quickly as we move upward. This would increase or steepen
the lapse rate. On other days, the air temperature would decrease more slowly with height, and the lapse
rate would be less. Occasionally, the air temperature may actually increase with height, producing a
condition known as a temperature inversion.
This region of circulating air extending upward from the earth’s surface to where the air stops becoming
colder with height is called the troposphere. Here, the lapse rate is zero. This region, where, on average,
the air temperature remains constant with height, is referred to as an isothermal (equal temperature)
zone.† The boundary separating the troposphere from the stratosphere is called the tropopause.
It is normally found at higher elevations over equatorial regions, and it decreases in elevation as we travel
poleward. troposphere air mixing with stratospheric air and vice versa, these breaks also mark the position
of jet streams.
in the stratosphere, the air temperature begins to increase with height, producing a temperature inversion.
The inversion region, along with the lower isothermal layer, tends to keep the vertical currents of the
troposphere from spreading into the stratosphere. The inversion also tends to reduce the amount of
vertical motion in the stratosphere itself; hence, it is a stratified layer.
The reason for the inversion in the stratosphere is that the gas ozone plays a major part in heating the air
at this altitude.
Above the stratosphere is the mesosphere (middle sphere). The boundary near 50 km, which separates
these layers, is called the stratopause. Without proper breathing equipment, the brain would soon become
oxygenstarved — a condition known as hypoxia.
The “hot layer” above the mesosphere is the thermosphere. The boundary that separates the lower, colder
mesosphere from the warmer thermosphere is the mesopause. In the thermosphere, oxygen molecules
(O 2) absorb energetic solar rays, warming the air. Because there are relatively few atoms and molecules in
the thermosphere, the absorption of a small amount of energetic solar energy can cause a large
increase in air temperature. This lower, well-mixed region is known as the homosphere. The region from
about the base of the thermosphere to the top of the atmosphere is often called the heterosphere.
THE IONOSPHERE is an electrified region within the upper atmosphere where fairly large
concentrations of ions and free electrons exist. Ions are atoms and molecules that have lost (or gained)
one or more electrons. Atoms lose electrons and become positively charged when they cannot absorb all
of the energy transferred to them by a colliding energetic particle or the sun’s energy. The lower region of the ionosphere is usually about 60 km above the earth’s surface. From here (60 km), the ionosphere
extends upward to the top of the atmosphere, the bulk of the ionosphere is in the thermosphere.
A knot is a nautical mile per hour. One knot is equal to 1.15 miles per hour (mi/hr), or 1.9 kilometers per hour (km/hr).
The vertical distribution of temperature, pressure, and humidity up to an altitude of abou 30 km can be obtained with an
instrument called a radiosonde.* The radiosonde is a small, lightweight box equipped with weather instruments
and a radio transmitter. It is attached to a cord that has a parachute and a gas-fi lled balloon tied tightly at the end . As the
balloon rises, the attached radiosonde measures air temperature with a small electrical thermometer — a thermistor —
located just outside the box. The radiosonde measures humidity electrical by sending an electric current across acarbon-
coated plate. Air pressure is obtained bya small barometer located inside the box. All of this information is transmitted to
the surface by radio. Here, a computer rapidly reconverts the various frequencies into values of temperature, pressure, and
moisture. Special tracking equipment at the surface may also be used to provide a vertical profi le of winds.* (When winds
are added, the observation is called a rawinsonde.)
When plotted on a graph, the vertical distribution of temp