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 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 half 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
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
Weather Service Graphic
inhabitants of our planet live in the Troposphere.
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
) and molecular Nitrogen (
). All weather is confined to this lower region and it contains 90% 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.
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).
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.
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.
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!
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
Hole increases in size during the months of September and October
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.
National Weather Service
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.
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
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.
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.
The Thermosphere extends from the
mesopause to 430 miles (690 km) above the earth. This layer is known as the
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.
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