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Summary Atmosphere- International A-Level Geography notes including AS-Level
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A- level Atmosphere,Geophysical hazards
Definitions of weather and climate; the structure and composition of the atmosphere and the role
different gases play in climate
Weather refers to short term atmospheric conditions
Climate is the weather of a specific region averaged over a long period of time, usually 30 years or
more → expected weather
Climate change refers to long-term changes
Structure of the atmosphere:
Divided into 5 layers:
● Troposphere
● Stratosphere
● Mesosphere
● Thermosphere
● Exosphere
,Troposphere:
- Troposphere is the lowest layer. Contains about 90% of the earth's air and is where weather
occurs. Starting at Earth's surface, it extends 7-20km above sea level, being the widest at the
equator and narrowest at the poles. Air pressure and density decrease as you travel higher
through the troposphere.
- A temperature gradient in the troposphere, from about 15 degrees to –57 Celsius, warmest air
near ground level and coolest in the tropopause (boundary between troposphere and
stratosphere)
- The height of the tropopause depends on latitude, season and whether it is day or night. Near
the equator, the tropopause is about 20 km above sea level. In winter near the poles the
tropopause is much lower.
Winds and the troposphere:
- The troposphere is heated from below. Solar radiation warms the ground and oceans, which in
turn radiate heat back into the air above. The warm air rises. Convection, updrafts and
downdrafts mix the air.
- Due to the uneven heating of the various regions of the troposphere by the sun (warmest at the
equator, coolest at the poles) convection currents are produced, leading to large-scale wind
patterns that distribute heat and moisture around the Earth. The Earth’s rotation causes the air
to be deflected as it moves from the poles to the equator, causing belts of surface winds to
move from east to west (easterly winds) in tropical and polar regions, and from west to east
(westerly winds) in the mid-latitudes.
Jet streams:
- Jet streams (‘rivers of air’) with wind speeds of around 400 km per hour, form just below the
tropopause.
Stratosphere:
- The second layer of the atmosphere extends from the troposphere to an altitude of about 50
km. At the equator the bottom is around 16 km; it is around 10 km in the mid-latitudes and
around 8 km at the poles. It is slightly lower in winter at mid and high latitudes, and slightly
higher in summer. The boundary between the stratosphere and the mesosphere is the
stratopause.
- The stratosphere is very dry; the air contains little water vapour. As a result there are very few
clouds in this layer.
Mesosphere:
- The mesosphere sits above the stratosphere and extends from 50 km above Earth’s surface to
an altitude of 85 km, ending at the mesopause (the boundary between this layer and the
thermosphere above.
- In the mesosphere the temperature gets colder as altitude increases. The top of the mesosphere
is the coldest part of the Earth’s atmosphere and can be as low as -120 degrees Celsius.
Temperature in this layer is very low and the air is very thin.
Thermosphere:
- The thermosphere lies directly above the mesosphere, extending from about 90 km to between
500 and 1000 km in altitude. Temperature in the upper thermosphere can range from 500
degrees Celsius to 2000 degrees Celsius; however the air would still ‘feel’ cold because the
hot gas particles (oxygen, hydrogen and helium) are so far apart.
Exosphere:
- The exosphere is the top layer of the atmosphere, extending to an altitude of 10000 km above
Earth. It contains very atmospheric particles, because atoms molecules escape into space.
, Hydrogen and helium are the main components, present in extremely low densities. This is
the area where many satellites orbit Earth.
Atmospheric circulation
The general circulation of the atmosphere, and ocean circulation, redistributes heat energy across the
planet and influences the locations of high and low pressure areas. (1)
Nitrogen N2 78.08%
Oxygen O2 20.95%
Water H20 20.95%
Argon Ar 0.93%
Carbon dioxide CO2 0.04%
Neon Ne 0.00%
Helium He 0.0005
Methane CH4 0.00017
Hydrogen H2 0.00005%
Nitrous oxide N20 0.00003%
Ozone O3 0.000004%
What is global atmospheric circulation
● In the atmosphere above the earth, air masses circulate around the globe
● This circulation occurs in cells
● The different cells help to determine the climate and winds at different latitudes
● Atmospheric circulation is the basis for our everyday weather and it is driven by the sun
● The circulation of the atmosphere and ocean redistributes heat energy across the planet
There are hot temperatures at the equator:
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