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Atmosphere
A planet's climate is decided by its mass, its distance from the sun and the
composition of its atmosphere. Earth's atmosphere is 78% nitrogen, 21% oxygen,
and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%. Water vapour,
varying in amount from 0 to 2%, carbon dioxide and some other minor gases
present in the atmosphere absorb some of the thermal radiation leaving the
surface and emit radiation from much higher and colder levels out to space.
These radiatively active gases are known as greenhouse gases because they act as
a partial blanket for the thermal radiation from the surface and enable it to be
substantially warmer than it would otherwise be, analogous to the effect of a
greenhouse. This blanketing is known as the natural greenhouse effect. Without
the greenhouse gases, Earth's average temperature would be roughly -20°C. T
The atmosphere is a cloud
of gas and suspended solids extending from the Earth's surface out many
thousands of miles, becoming increasingly thinner with distance but always
held by the Earth's gravitational pull.
The atmosphere is made up
of layers. it surrounds the Earth and holds the air we breathe; it
protects us from outer space; and holds moisture (clouds), gases, and tiny
particles. In short, the atmosphere is the protective bubble we live in.
This protective bubble
consists of several gases (listed in the table to the right) with the top
four making up 99.998% of all gases. Of the dry composition of the
atmosphere nitrogen, by far, is the most common. Nitrogen dilutes oxygen
and prevents rapid burning at the Earth's surface. Living things need it
to make proteins. Oxygen is used by all living things and is essential for
respiration.
It is also necessary for
combustion or burning.
Argon is used in light
bulbs, in double-pane windows, and to preserve the original Declaration of
Independence and the Constitution. Plants use carbon dioxide to make
oxygen. Carbon dioxide also acts as a blanket that prevents the escape of
heat into outer space.
These percentages of
atmospheric gases are for a completely dry atmosphere. The atmosphere is
rarely, if ever, dry. Water vapor (water in a 'gas' state) is nearly
always present up to about 4% of the total volume. In the Earth's desert
regions (30°N/S) when dry winds are blowing, the water vapor contribution
to the composition of the atmosphere will be near zero.
Water vapor contribution
climbs to near 3% on extremely hot/humid days. The upper limit,
approaching 4%, is found in tropical climates. The table below shows the
changes in atmospheric composition with the inclusion of different amounts
of water vapor.
The earth's atmosphere
is a very thin layer wrapped around a very large planet.
Two gases make up the
bulk of the earth's atmosphere: nitrogen (),
which comprises 78% of the atmosphere, and oxygen (),
which accounts for 21%. Various trace gases make up the remainder.
Based on temperature,
the atmosphere is divided into four layers: the troposphere,
stratosphere, mesosphere, and thermosphere.
Energy is transferred
between the earth's surface and the atmosphere via conduction,
convection, and radiation.
Ocean currents play a
significant role in transferring this heat poleward. Major currents,
such as the northward flowing Gulf Stream, transport tremendous amounts
of heat poleward and contribute to the development of many types of
weather phenomena.
Cambridge University Graphic
The atmosphere is divided into five
layers. It is thickest near the surface and thins out with height until it
eventually merges with space.
1) The troposphere is the
first layer above the surface and contains 70 to 80 per cent of the
Earth's atmosphere. Weather occurs in this layer.
2) Many jet aircrafts fly in the
stratosphere
because it is very stable. Also, the ozone layer absorbs harmful rays from the
Sun.
3) Meteors or rock fragments burn up
in the mesosphere.
4) The thermosphere is a
layer with auroras. It is also where the space shuttle orbits.
5) The atmosphere merges into space
in the extremely thin exosphere. This is the upper limit of our
atmosphere.
National Geographic Video
U.S. Weather Service Graphic
The inhabitants of our planet live in the Troposphere.
Earth's atmosphere varies in density and composition as the altitude
increases above the surface. The lowest part of the atmosphere is called
the troposphere (and it extends from the surface up to about 10 km (6
miles). The gases in this region are predominantly molecular Oxygen
( O2
) and molecular Nitrogen (
). All weather is confined to this lower region and it contains 70
to 80 per cent of the Earth's atmosphere and 99% of the water vapor.
The highest mountains are still within the troposphere and all of our
normal day-to-day activities occur here. The high altitude jet stream is
found near the tropopause at the the upper end of this region.
The layer above this is the Stratosphere, this is where the Ozone Layer is
formed. The atmosphere above 10 km is called the stratosphere. The gas is
still dense enough that hot air balloons can ascend to altitudes of 15 -
20 km and Helium balloons to nearly 35 km, but the air thins rapidly and
the gas composition changes slightly as the altitude increases. Within the
stratosphere, incoming solar radiation at wavelengths below 240 nm. is
able to break up (or dissociate) molecular Oxygen (
) into individual Oxygen atoms, each of which, in turn, may combine with
an Oxygen molecule (
), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms (
). This gas reaches a peak density of a few parts per million at an
altitude of about 25 km (16 miles).
The Ozone Layer absorbs ultra-violet radiation from the Sun. Without the
Ozone Layer life as we know would cease to exist on our planet. Ozone is
important because it is the only atmospheric gas which absorbs light in
the B region of UVB rays.
The Ozone layer extends from a height of 20 kilometers to 60 kilometers
above the Earth's surface. The air is very thin at these altitudes.
If all of the Ozone in the Earth's atmosphere were compressed into a
single layer at the Earth's surface, it would only be 3 millimeters
thick-basically two stacked pennies!
The gas becomes increasingly rarefied at higher altitudes. At heights of
80 km (50 miles), the gas is so thin that free electrons can exist for
short periods of time before they are captured by a nearby positive ion.
The existence of charged particles at this altitude and above, signals the
beginning of the ionosphere a region having the properties of a gas and of
a plasma.
US Navy Graphic
The Ozone
Hole increases in size during the months of September and October
Standardized Temperature Profile
An average temperature profile through the lower layers of the atmosphere.
Height (in miles and kilometers) is indicated along each side. Temperatures in
the thermosphere continue to climb, reaching as high as 2000°C.
credit: National Weather Service
Troposphere
The troposphere begins at the
Earth's surface and extends up to 4-12 miles (6-20 km) high. This is where we
live. As the gases in this layer decrease with height, the air become thinner.
Therefore, the temperature in the troposphere also decreases with height. As
you climb higher, the temperature drops from about 62°F (17°C) to -60°F
(-51°C). Almost all weather occurs in this region.
The height of the troposphere varies from the equator to the poles. At the
equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles
and at the poles just under four miles high. The transition boundary between
the troposphere and the layer above is called the tropopause. Both the
tropopause and the troposphere are known as the lower atmosphere.
EPA Graphic
Stratosphere
The Stratosphere extends from
the tropopause up to 31 miles above the Earth's surface. This layer holds 19
percent of the atmosphere's gases and but very little water vapor.
Temperature increases with height as radiation is increasingly absorbed by
oxygen molecules which leads to the formation of Ozone. The temperature rises
from an average -76°F (-60°C) at tropopause to a maximum of about 5°F (-15°C)
at the stratopause due to this absorption of ultraviolet radiation. The
increasing temperature also makes it a calm layer with movements of the gases
slow.
The regions of the stratosphere and the mesosphere, along with the
stratopause and mesopause, are called the middle atmosphere by scientists. The
transition boundary which separates the stratosphere from the mesosphere is
called the stratopause.
Mesosphere
The mesosphere extends from
the stratopause to about 53 miles (85 km) above the earth. The gases,
including the oxygen molecules, continue to become thinner and thinner with
height. As such, the effect of the warming by ultraviolet radiation also
becomes less and less leading to a decrease in temperature with height. On
average, temperature decreases from about 5°F (-15°C) to as low as -184°F
(-120°C) at the mesopause. However, the gases in the mesosphere are thick
enough to slow down meteorites hurtling into the atmosphere, where they burn
up, leaving fiery trails in the night sky.
Thermosphere
The Thermosphere extends from
the mesopause to 430 miles (690 km) above the earth. This layer is known as
the upper atmosphere.
The gases of the thermosphere are increasingly thinner than in the
mesosphere. As such, only the higher energy ultraviolet and x-ray radiation
from the sun is absorbed. But because of this absorption, the temperature
increases with height and can reach as high as 3,600°F (2000°C) near the top
of this layer.
However, despite the high temperature, this layer of the atmosphere would
still feel very cold to our skin because of the extremely thin air. The total
amount of energy from the very few molecules in this layer is not sufficient
enough to heat our skin.
Exosphere
The Exosphere is the outermost
layer of the atmosphere and extends from the thermopause to 6200 miles (10,000
km) above the earth. In this layer, atoms and molecules escape into space and
satellites orbit the earth. The transition boundary which separates the
exosphere from the thermosphere below it is called the thermopause.
credits: NOAA, NASA, EPA, National
Weather Service, Cambridge University, U.S. Navy, The Franklin institute, UK MET
Office
),
which comprises 78% of the atmosphere, and oxygen (
),
which accounts for 21%. Various trace gases make up the remainder.
