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G E O G . 1710 E A R T H SC IE N C E
L E C T U R E 4. IN T R O D U C T IO N T O T H E A T M O SP H E R E
Harry Williams, Earth Science
1
The atmosphere is a mixture of gases, solids and liquids, held to
the earth by gravitational attraction. 98% of the mass of the
atmosphere lies within 16 miles of the surface. Very low-weight
hydrogen and helium atoms extend as far as 20,000 miles from the
earth’s surface, but the atmosphere at these distances is extremely
rarified and for practical purposes the top of the atmosphere is
considered to be about 300 miles (480 km) in altitude.
Harry Williams, Earth Science
2
The atmosphere can be
divided into layers
according to composition,
function and temperature
Harry Williams, Earth Science
3
Composition Profile
In terms of composition, the atmosphere is divided into the heterosphere and
homosphere.
In the heterosphere (above 80 km), gases are sorted into layers according to
their atomic weight – the lightest gases, such as hydrogen and helium, form
the higher layers; heavier gases, such as oxygen and nitrogen, form lower
layers. Less than one thousandth of 1 percent of the atmosphere’s mass is in
the heterosphere.
In the homosphere (below 80 km) gases are almost* completely mixed
together into what is commonly called “air” (* exceptions are ozone and some
variable gases – these are explained later).
Much of the homosphere is made up of uniform gases – meaning the
proportion of these gases remains constant from place to place and over time.
Harry Williams, Earth Science
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Harry Williams, Earth Science
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Major Components
Nitrogen (78.1%), oxygen (20.9%) and argon (0.9%) together make
up about 99.9% of gases in the atmosphere. These gases do not
greatly affect the weather.
Water vapor is the gaseous form of water and can vary from about 04% of the atmosphere's volume. The variation in the percentage of
water vapor can be from place to place (e.g. on a given day there may
be a lot more water vapor in the air over Louisiana than over Arizona)
or it can be at one location over time (e.g. there could lots of water
vapor in the air over Dallas after a spring thunderstorm, but little
water vapor after a long summer drought). Unlike nitrogen, oxygen
and argon, water vapor is very important for its effect on the weather it is the source of all clouds and precipitation, and is also involved in
the storage, transportation and release of heat energy.
Harry Williams, Earth Science
6
Carbon dioxide also has an important effect on weather, because it
has the ability to absorb infrared radiation (radiant heat) given off by
the surface of the earth, which helps to maintain warmth in the lower
levels of the atmosphere. Carbon dioxide is fairly uniformly
distributed, but the amount has been slowly increasing over the last
100 years or so, due to the burning of fossil fuels (a by-product of
burning is CO2). Many scientists believe that this will cause
GLOBAL WARMING, with unpredictable and serious consequences
for the earth's climates.
Ozone is concentrated in a layer 12 - 31 miles above the surface. It is
particularly important in that it absorbs ultraviolet radiation (UV)
from the sun. Absorption of UV heats this part of the atmosphere and
prevents most of this harmful radiation from reaching the surface.
Harry Williams, Earth Science
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Harry Williams, Earth Science
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Pollutant Gases make up most other minor gases from factories,
cars, etc. These include carbon monoxide, nitrogen oxides and
hydrocarbons. For the atmosphere as a whole these are present in
very small quantities, but these can be concentrated in certain
locations and can have significant impacts on life and climate.
Particles include solid and liquid particles, mainly of natural origin i.e. ice particles, volcanic ash, wind-blown dust (Figure 4) and
wildfire smoke particles. There are also man-made particles, again
mainly pollutants such as soot and smoke. Although the volume of
particles is very small compared to gases, they do have significant
effects on weather by:
1. many collect water and help to form clouds.
2. some absorb or reflect insolation - decreasing the amount that
reaches the surface.
Harry Williams, Earth Science
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DUST STORM
Harry Williams, Earth Science
10
Function Profile
In terms of function, the atmosphere is divided into two layers – the
ionosphere and the ozonosphere. Both layers function to remove
harmful radiation and charged particles from insolation. The
ionosphere, extending from 50 miles to 300 miles high, absorbs
cosmic rays, gamma rays, X-rays and shorter wavelengths of UV
radiation.
Harry Williams, Earth Science
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Absorption of
insolation by
the
ionosphere
and the
ozonosphere.
Harry Williams, Earth Science
12
Atoms in the ionosphere are changed into positively charged atoms
or ions by this absorption of energy – giving the ionosphere its
name.
The ozonosphere is named after the ozone layer (12-31 miles high).
The molecule ozone (O3) is concentrated in this layer and absorbs
UV radiation. The result of filtering of insolation by the ionosphere
and the ozonosphere is that it is mainly visible light and infrared
radiation that reaches the surface of the earth.
Harry Williams, Earth Science
13
Temperature Profile
The temperature profile
of the atmosphere results
from differences in how
different parts of the
atmosphere absorb
energy from the sun and
earth. The temperature
profile of the whole
atmosphere is shown by
the yellow line. Abrupt
changes in temperature
define four layers:
Harry Williams, Earth Science
14
Troposphere - this is the lowest layer where temperature decreases
with height. On average the troposphere is about 11 miles deep
over the equator and 5 miles deep over the poles. This layer is
warmed by the earth's surface, which in turn is warmed by
absorption of visible light and infrared radiation from the sun. The
farther from the surface, the colder the air becomes. The top of the
troposphere is the tropopause, where temperature no longer
decreases with height. Virtually all WEATHER occurs within the
troposphere. Weather is defined as atmospheric conditions
prevalent in a given place at a given time. Weather is essentially a
temporary phenomenon, changing from day to day or even from
hour to hour.
Harry Williams, Earth Science
15
Stratosphere – this layer lies between approximately 11 and 31
miles high and is generally a zone of increasing temperature: the
maximum temperature of about 0o C is reached at 31 miles, after
which temperature stops increasing with height - this marks the
upper boundary to the stratosphere - the stratopause. The
stratosphere is heated by the absorption of UV radiation by the
ozone layer it contains.
Harry Williams, Earth Science
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Mesosphere – this layer extends up to about 50 miles and is simply
the layer of the atmosphere that becomes colder with increasing
distance from the heated stratosphere. The top of the mesosphere,
where temperature again begins to increase, is the mesopause.
Harry Williams, Earth Science
17
Thermosphere - temperatures again rise in the thermosphere due to
the absorption of insolation by atmospheric gases. Temperatures
can reach 2200oF however, the air is extremely rarified (low
density), so the actual amount of heat present is low, despite the
high temperatures.
Harry Williams, Earth Science
18
Air Pressure
Air pressure can be thought of as the weight of the overlying air
(the atmosphere is composed of atoms with mass, and is acted upon
by gravity, so it does have weight). At the surface, the whole
atmosphere is above, and so pressure is greatest; higher up, there is
less overlying atmosphere and so pressure decreases.
Harry Williams, Earth Science
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Harry Williams, Earth Science
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However, the rate of pressure decrease with height is not
constant, because air is compressible. Air near the surface is
the most compressed because it bears the greatest weight;
because of its greater density, pressure decreases more rapidly
with height near the surface. Higher up, air is less dense and
therefore pressure decreases less rapidly. Air pressure is
measured in millibars (mb) – average sea-level pressure is
about 1,013 mb.
Harry Williams, Earth Science
21
Harry Williams, Earth Science
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