2. Definitions
■ Weather – day to day changes in
the state of the atmosphere.
■ Climate – average weather
conditions over a longer period of
time – 30 years.
3. What is the Atmosphere?
An atmosphere is defined as the gaseous
envelope that surrounds a celestial body.
Therefore, the Earth, like other planets in
the solar system, has an atmosphere, which
is retained by gravitational attraction and
largely rotates with it.
4. Who studies the Atmosphere?
Learning about different states of the
atmosphere enables science to understand
and predict changes on a range of scales.
Meteorology is the subject that studies
the chemical and physical properties of
the atmosphere together with its fields of
motion, mass and moisture.
5. Formation of the Atmosphere
At the time of the Earth's formation around 4.5
billion years ago there was probably no
atmosphere. It is believed to have come into
existence as a result of the volcanic expulsion
of substances from its interior, mainly water
vapour, with some carbon dioxide, nitrogen
and sulphur. The atmosphere can only hold a
certain amount of water vapour, so the excess
condensed into liquid water to form the
oceans.
6. Formation of the Atmosphere
It is thought that the first stage in the
evolution of life on Earth required an oxygen-
free environment. Later primitive forms of
plant life developed in the oceans and began
to release small amounts of oxygen into the
atmosphere as a waste product from the cycle of
photosynthesis: H2O +CO2 + sunlight →sugar + O2
7. Formation of the Atmosphere
This build-up of atmospheric oxygen eventually led
to the formation of the ozone layer. This layer,
approximately 8 to 30 km above the surface, helps to
filter the ultraviolet portion of the incoming solar
radiation. Therefore, as levels of harmful ultraviolet
radiation decreased, so plants were able to move to
progressively higher levels in the oceans.
This helped to boost photosynthesis and thereby the
production of oxygen. Today, this element has
reached levels where life has been sustainable on the
surface of the planet through its presence, and it
should be remembered that oxygen is an element
which is not commonly found in the universe.
8. Composition of the atmosphere
Main Elements = 99%
78%
21%
Nitrogen (N2)
Oxygen (02)
Trace Elements = 1%
Argon (Ar)
Carbon Dioxide (CO2)
Neon (Ne)
Helium (He)
Methane (CH4)
Nitrous Oxide (N2O)
Xenon (Xe)
Ozone (O3)
Nitrogen Dioxide (NO2)
Iodine (I)
Carbon Monoxide (CO)
Ammonia (NH3)
Water Vapour (H20)
9. Vertical structure of the atmosphere
The atmosphere is divided
into four isothermal layers or
'spheres': troposphere,
stratosphere, mesosphere
and thermosphere.
Each layer is characterised by
a uniform change in
temperature with increasing
altitude.
In some layers there is an
increase in temperature with
altitude, whilst in others it
decreases with increasing
altitude.
The top or boundary of each
layer is denoted by a 'pause'
where the temperature profile
abruptly changes .
10. Troposphere
The troposphere contains about 80% of the atmosphere and is
the part of the atmosphere in which we live, and make
weather observations.
In this layer, average temperatures decrease with height
6.4oc/1000m, as there is less air in contact with the ground to
heat up. This is known as Environmental Lapse rate (adiabatic
cooling brought about by changes in temperature caused by a
decrease in pressure at height).
This sphere mixes vertically by convection, conduction and
turbulence more than any other sphere. These vertical motions
and the abundance of water vapour make it the home of all
important weather phenomena.
The troposphere is around 16 km high at the equator, with the
temperature at the tropopause around –80 °C. At the poles,
the troposphere reaches a height of around 8 km, with the
temperature of the tropopause around –40 °C in summer and –
60 °C in winter. Therefore, despite the higher surface
temperatures, the tropical tropopause is much cooler than at
the poles at the thickness is increased – more cooling occurs.
11. Stratosphere
Temperatures in the stratosphere rise with increasing
altitude (creating a temperature inversion) this is
caused by concentrations on O3 which absorbs
ultraviolet radiation. This is greatest around 50 km at
the edge of the stratopause. Temperatures range from –
30 °C over the winter pole to +20 °C over the summer
pole according to latitude and season.
As well as a noticeable change in temperature, the
move from the troposphere into the stratosphere is also
marked by an abrupt change in the concentrations of
the variable trace elements. Water vapour decreases
sharply, whilst ozone concentrations increase. These
strong contrasts in concentrations are a reflection of
little mixing between the moist, ozone-poor
troposphere and the dry, ozone-rich stratosphere.
The stratosphere extends up to around 48 km above the
surface, and together with the troposphere, they
account for 99.9% of the Earth's atmosphere.
12. Mesosphere
Temperatures in the mesosphere decrease rapidly as
there is no water vapour, cloud, dust or ozone to absorb
incoming radiation.
Temperatures at the mesopause go as low as – 120
°C with very strong winds – 3000km/hr.
As in the troposphere, the unstable profile means that
vertical motions are not inhibited. During the summer,
there is enough lifting to produce clouds in the upper
mesosphere at high latitudes
13. Thermosphere
The thermosphere extends upwards to altitudes of
several hundred kilometres, where temperatures
range from 250oc to as high as 1,700oc, getting warmer
with increasing height.
Temperature ranges depend on the degree of solar
activity and as there is more atomic oxygen there (like
ozone) to absorb the heat.
The temperature changes between day and night
(Diurnal) amount to hundreds of degrees.
Above 500 km temperatures are very difficult to
define. Molecules are so widely spaced that they move
independently, and there is no reason why their
temperatures should be the same.
14. Earth’s Annual Heat Budget –
Heating of the Atmosphere
At ‘A’ the sun’s energy is
more concentrated on a
small land area – intense
heating.
At ‘B’ the sun’s energy is
spread over a wider
surface area – leading to
less direct heating.
‘A’
‘B’ ‘B’
15. Heat Budget in OUR Winter
In the
North in
OUR Winter
there is a
deficit
In the
South in
OUR Winter
there is a
surplus
At the
equator
there is a
surplus
Of course the Earth’s heat
budget is not that simple!
You have to remember the
effects of seasonality – the
seasonal shift in the trace
of the sun on the Earth’s
surface during it’s orbit.
16. Heat Budget in OUR
Summer
In the South
in OUR
Summer
there is a
deficit
In the North
in OUR
Summer
there is a
surplus
At the
equator
there is a
surplus
17. Earth’s Annual Heat Budget
So over the pattern of a year the Earth’s heat budget
changes with the seasons, however there is always a
surplus at the equator.
The uneven distribution of heating
across the Earth is what drives the
air to move (WIND) in an effort to
redistribute the heat to areas of
deficit.
