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Earth’s Atmosphere
Structure, Components
and Temperature
Vocabulary
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Weather
Climate
Ozone
Troposphere
Stratosphere
Mesosphere
Thermosphere
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Summer solstice
Winter solstice
Autumnal equinox
Spring equinox
Weather vs. climate
• Weather is the state of the atmosphere at any
given time and place. Weather is constantly
changing.
• Climate is based on observations of weather that
have been collected over many years. Climate
helps describe a place or region.
Measurable properties of
weather and climate
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Air temperature
Humidity
Type and amount of precipitation
Air pressure
Speed and direction of the wind
History of the Atmosphere
• Most scientists believe
that Earth’s early
atmosphere was made of
gases emitted during
volcanic eruptions.
Composition of the Atmosphere
• Air is a mixture of gases and
particles each with its own
unique properties.
Nitrogen – 78%
Oxygen – 21%
Argon – 0.93%
Carbon dioxide – 0.01 to 0.1%
Water vapor – 0 to 4%
Air composition varies from time to time and place
to place (water vapor, dust, pollutants)
Variable Components
• If water vapor, dust, and other variable
components were removed from the
atmosphere, its makeup would be very
uniform up to an altitude of about 80
kilometers or about 40 miles.
Variable components
• CO2 (carbon dioxide) – plays an important role in
heating the atmosphere as it absorbs heat given off by
Earth.
• Water vapor – Varies from 0 to 4% by volume. It is
the source of all clouds and precipitation. It absorbs
heat from both the Earth and the Sun.
Variable components
Particulates – sea salts from breaking waves, fine
soil, smoke and soot from fires, pollen and
microorganisms, and ash and dust from volcanic
eruptions.
Ozone – A type of oxygen made of 3 atoms in each
molecule instead of the usual two. Ozone is
concentrated in a region from 10 to 50 km above the
surface with the maximum concentration at 20-30
km (stratosphere).
Ozone Formation
• Diatomic molecules of oxygen
gas (O2) disassociate when
they absorb UV radiation.
• Ozone is produced when a
single atom of oxygen and
another diatomic molecule
of oxygen collide in the
presence of a third, neutral
molecule that acts as a
catalyst (allows a reaction to
take place without being
consumed itself).
Ozone Layer
Ozone is critical to life on
Earth because it
absorbs harmful UV
(ultraviolet) radiation
from the Sun. It acts as
a filter, preventing most
UV radiation from
reaching Earth’s surface
and the living things
upon it.
Without ozone, our planet
would be inhospitable
to most living organisms!
‘Thanks, ozone!’ (signed) All life on Earth
Human Influence on the
Atmosphere
Primary pollutants are
emitted directly into the air
from identifiable sources.
Secondary pollutants form
in the air from reactions
among primary pollutants
and other substances.
They are NOT emitted
directly into the air. Acid
rain (sulfuric acid) forms
from the primary pollutant
sulfur dioxide.
Some reactions can be triggered by strong
sunlight. These are called PHOTOCHEMICAL
REACTIONS. These can form secondary
products that are reactive, irritating, and toxic –
such as smog.
Structure of the Atmosphere
There is no sharp
boundary between
the atmosphere
and space – it just
thins out as you
travel away from
Earth’s surface,
until there are too
few gas molecules
to detect.
Structure of the Atmosphere
• Atmospheric pressure (the
weight of the air above)
decreases with height.
• At sea level, the average
pressure is slightly more
than 1000 millibars, or
slightly more than 1
kilogram per square
centimeter.
• One half of the
atmosphere lies below an
altitude of 5.6 km.
Structure of the Atmosphere
The
atmosphere is
divided into 5
layers based
on
temperature.
Troposphere
• The bottom layer,
where temperature
decreases with an
increase in altitude, is
the troposphere.
• It is in this layer that
essentially all important
weather phenomena
occur.
• The troposphere varies in thickness from
season to season and with different latitudes,
but its average height is about 12 km.
• The outer boundary of the troposphere is called
the tropopause.
Stratosphere
• The stratosphere is the next
layer, from 12 km to 50 km.
• In the stratosphere, the
temperature remains constant to a
height of about 20 km, but then
temperatures increase with
height.
• The atmosphere’s ozone layer is
concentrated here (20 to 30 km)
because there is both absorption
of UV radiation (the source of
heating) and enough oxygen
molecules for the production of
ozone.
Mesosphere
• The mesosphere is the next
layer, which extends from 50 to
85 km.
• In the mesosphere,
temperatures again decrease
with height until the
mesopause, the upper
boundary of this layer, which is
the coldest part of the
atmosphere. It can get down to
-90 °C (-130 °F) there!
• It is not easy to study the mesosphere directly. Weather
balloons can't fly high enough and satellites can't orbit
low enough, so scientists use sounding rockets (a
suborbital rocket that carries scientific instruments).
Mesosphere
What happens in the mesosphere?
• Most meteors burn up there.
• A type of lightning called sprites
sometimes appears in the
mesosphere above
thunderstorms.
• Strange, high-altitude clouds
called noctilucent clouds
sometimes form in this layer
above the North and South
Poles.
Thermosphere
• The fourth layer is the
thermosphere. It extends from
the mesopause at about 90 km
to between 500 and 1000 km!
• Temperatures go up in the lower
thermosphere (200 to 300 km)
because oxygen and nitrogen
absorb high energy, short-wave
solar radiation. Above that,
temperatures hold steady with
height.
• Temperatures in the upper thermosphere can range from
about 500 °C (932 °F) to 2,000 °C (3,632 °F) or higher.
The top of the thermosphere is the thermopause.
Thermosphere
• Although the thermosphere is considered part of Earth's
atmosphere, the air density is so low that most people think
of it as outer space, especially since the space shuttle
orbited and the International Space Station orbits here!
• The aurora borealis and aurora australis occur in the
thermosphere.
Exosphere
• The exosphere is the fifth and uppermost
layer, where the atmosphere thins out and
merges with interplanetary space. It is
located directly above the thermosphere.
• The exosphere has no clearly defined
upper limit.
• Here gas molecules can escape to space
but the density is so low they do not collide
with each other. The exosphere contains
only a small fraction of the mass of the
atmosphere.
You probably
thought it always
got colder the
higher up you
go, but that’s not
true for two of
the layers of the
atmosphere!
‘Hotter’ means the
gas molecules
have more
energy and
move faster.
Seasonal changes in the
atmosphere
The vertical structure of
the atmosphere
changes with the
seasons. The
troposphere and
stratosphere get
warmer in summer,
but the mesosphere
gets colder!
Earth-Sun Relationships
• Nearly all the energy that drives Earth’s
weather and climate comes from the Sun, but it
is only a tiny fraction of the energy given off by
the Sun (less than one-two billionth).
• Solar energy is not distributed equally over the
Earth, varying with latitude, time of day, and
season of the year.
• This unequal heating is what causes winds
and ocean currents which transport heat from
the tropics toward the poles and result in
weather.
Earth’s Two Principal Motions
• Rotational motion about its axis
 Once every 24 hours, produces daily
cycle of daylight and darkness
 Axis is an imaginary line that runs
through North and South Poles
• Revolution about the Sun
Orbital motion, one revolution every
365.25 days. Earth travels at nearly
113,000 km/h!
The earth is tilted at an angle of 23.5
degrees from the vertical and this causes
seasonal variations in how much sunlight
reaches the surface and at what angle.
The angle of the
noon sun can
vary up to 47
degrees during
the year at
many locations.
Solstices
• On June 21 or 22 each year the axis is such that the
Northern Hemisphere is “leaning” 23.5 degrees
toward the sun. This is known as the summer solstice,
or the first “official” day of summer.
• This is the longest day of the year because the length
of daylight on the summer solstice in the Northern
Hemisphere is greater than the length of darkness.
Winter Solstice
• December 21 or 22 is the winter solstice,
the first day of winter.
• In the Northern Hemisphere, the Earth’s
axis is tilted 23.5 degrees away from the
sun. This is the shortest day of the year.
Equinoxes
• March 21 or 22 is the date of the spring (or
vernal) equinox for the Northern Hemisphere,
while September 21 or 22 is the date of the
autumnal equinox.
• All latitudes receive 12 hours of daylight during
the vernal and autumnal equinoxes (Latin for
equal night) because the Earth’s axial tilt is
neither toward nor away from the sun.