The Atmosphere
Weather is a matter of perspective
Unit Objectives
• To describe the composition of the atmosphere
and to illustrate some of the issues involved
with some of the trace elements that may affect
human health.
• To describe and illustrate the structure of the
atmosphere
• To explain the characteristics and contents of
the atmospheric layers
Satellite
image.
Note how
thin the
atmosphere
appears.
View from the space shuttle
Evolution of the Atmosphere
The importance of the atmosphere
• The atmosphere is a necessary component of
life on earth. It provides oxygen for living
organisms; it provides carbon dioxide for
photosynthesis thereby allowing for the
propagation of plant life; and it filters out
harmful electromagnetic energy that would
otherwise be harmful to life on the planet
• The atmosphere is the theatre within which the
various processes that create weather occur.
Dust off
the West
Coast of
the Sahara
Desert
Dust is included in a category
generally labelled as
atmospheric aerosols.
Suspended particulate matter
that circulates through the
atmosphere. The source could
be salt from ocean waves, dust
from forest fires, volcanic
eruptions, or industrial activity.
Often a nuisance or even a
pollutant these microscopic
particles may serve to block
solar radiation from hitting the
earth or provide a necessary
nucleus for raindrops to
coalesce.
Atmospheric Dust
Chile
Arabian
Sea
Volcanic dust if extensive can interfere with airline traffic (as happened
a couple of years ago with eruptions in Iceland) and block solar
radiation from hitting the earth.
The ozone issue
• Several meteorologists from Penn State alerted the scientific community
in the 1970s about the possible depletion of the ozone layer but were
generally disregarded.
• Satellite imagery finally convinced the scientific community that there
was a problem and several international conferences (Montreal, 1987
and London, 1992) addressed the issue. The main culprit was CFCs
(chlorofluorocarbons) used as a refrigerant, most notably in air
conditioning. Since then, a replacement was found – HFCs
(hydrofluorocarbons) which worked and progress is being made in
repairing the ozone hole.
• However, HFCs are a green house gas many times more potent than
carbon dioxide. In Rwanda, this week, an international conference
added an amendment to the Paris accord of 2015 (it was a compromise
regulation) that mandates a transition from HFCs to an alternative gas
for air conditioning.
Ozone hole at the poles in 1994 and 1997
Ozone hole
1985 and 2006
Because the overall pattern of global circulation is from the
tropics to the polar regions, it is not illogical that it becomes
a dump for pollutants
Ozone concentration by altitude
• Ozone in the stratosphere
absorbs ultraviolet radiation and
is therefore beneficial.
• The ozone in the lower
troposphere is a photochemical
pollutant and is often an element
in smog.
Climate change item: Increase in carbon
dioxide - one of the green house gases
Structure of
the
Atmosphere
Distinct atmospheric layers
separated by temperature.
Density and barometric
pressure decrease with
altitude.
Barometric pressure is simply the weight of the air caused by
the pull of gravity. There are fewer molecules of air with
increased height. A square inch of a column of air at the surface
weighs about 15 pounds.
Types of barometers
1647
1844
Radiosonde Ballon
Instrument used to gather
information regarding the
atmospheric conditions. Still in use,
usually set off twice a day. An
expensive instrument often
damaged when it lands. Slowly
being replaced by satellites.
9
Structure of
the
Atmosphere
Distinct atmospheric layers
separated by temperature.
Density and barometric
pressure decreases with
altitude. We’ll start with
the troposphere, the area
with the most weather.
The layers of the Atmosphere based on
temperature profiles
• Troposphere – fairly consistent rate of decrease of temperature (the
environmental lapse rate) which is 6.5°C for every 1,000 meters (a
km) or 3.6°F for every 1,000 feet. If you were in a commercial jet
flying at about 30,000 feet (about 9 km) the temperature would be
about -50°C outside the plane (more than 60°C colder than at the
surface). The troposphere extends upward to a height of about 11
km and is where weather as we know it occurs.
• Sufficient turbulence and circulation (horizontal and vertical) takes
place to keep the mixture of molecules homogeneous throughout
the troposphere.
• The top of the troposphere is delineated by the tropopause, an area
with a stable temperature.
Structure of
the
Atmosphere
Next we’ll look at the
stratosphere.
The Stratosphere
• Begins at about 12 miles (20 km) and extends upward to about 30
miles (50 km).
• Temperature increases with altitude. The increase in temperature is
due to the presence of ozone which absorbs incoming ultraviolet
radiation. There is ozone throughout the stratosphere though it is
concentrated in an area between about 20 to 30 km (a layer within
the stratosphere often referred to as the ozone layer). This is a
very important component in the atmosphere and protects the
earth from excessive ultraviolet radiation.
• A key aspect of the stratosphere is that it is “stratified.” There is
much less turbulence and mixing of molecules, hence heavier
molecules settle to the bottom of the stratosphere.
• At the top of the stratosphere is another area of stable temperature
called the stratopause (at about 50 km). The air is extremely thin
at the stratopause, averaging at about 1 millibar, only about one
thousandth of all atmospheric molecules are above this level.
Structure of
the
Atmosphere
Now we’ll examine the
middle layer, the
mesophere.
The Mesophere
• The mesophere is the “middle” layer extending from
about 60 km to 90 km, where temperatures fall with
altitude due in part to the lack of ozone. Near the top of
the mesophere temperatures reach the atmosphere’s
lowest average values (- 90°C /- 130°F).
• The top of the mesophere is delineated by another area of
stable temperature called the mesopause.
Structure of
the
Atmosphere
Finally we’ll look at the
thermosphere and the
exosphere.
The Thermospere
• Extends from about 90 km (60 miles) to a height of about 500
km (300 miles).
• Temperature increases due to heat absorption by oxygen
molecules. However the gap between air molecules is vast.
The “mean free path” is over one km. Some molecules may
escape the earth’s gravitational pull.
• Even though the air is thin, there are enough molecules to have
an effect on satellites, causing friction on spacecraft re-entry
into the earth’s atmosphere.
• Above the thermosphere and extending to thousands of miles is
the exosphere, the upper limit of the atmosphere containing
very few air molecules.
the atmosphere based on composition
The Ionosphere
• An area in which atoms have lost or gained electrons
and are “electrified” (not neutrally charged). The
base of the ionosphere is about 60 km above the
surface and extends to the upper atmosphere. It is a
significant component in radio communications.
The Ionosphere
Summary
• The atmosphere contains mostly nitrogen and oxygen and a
number of trace elements
• Oxygen and carbon dioxide are important to sustain organic
life on the planet.
• The atmosphere is a layered environment that is kept in place
by gravity.
• The earth’s weather is confined to the troposphere.
• The atmosphere is a layered environment that is kept in place
by gravity.
• Ozone is important as a filter against ultraviolet radiation and
occurs mostly in the stratosphere.
• Note: We’ll have more to say about the atmosphere when we
discuss atmospheric pressure, atmospheric moisture and the
interaction of solar radiation with the atmosphere.