A* Summary Notes: Unit 3 - Atmosphere and Weather Systems
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Course
Unit 3 - Atmosphere and Weather Systems
Institution
PEARSON (PEARSON)
Detailed notes on the structure of the atmosphere and atmospheric circulation; weather systems, including the jet stream, ITCZ, tropical cyclones, mid-latitude depressions, anticyclones, fronts and air masses. These notes are based on the IAL Edexcel Geography specification, but the content is also...
3.3.1 -
Global
Enquiry question: What are weather and climate and how are they influenced by the
atmospher
ic global climate system?
circulation
Definitions of weather and climate; the structure and composition of the atmosphere
and the role different gases play in climate.
Weather: the short-term day-to-day variation in atmospheric conditions of a place (hourly basis).
Climate: the long-term trends (30 years or more) in climatic events, such as rainfall, temperature, over a
region; the weather of a specific region averaged over a long period of time, usually 30 years or more.
The Structure and Composition of the Atmosphere
Troposphere
Troposphere: lowest layer in atmosphere that
extends about 20km above sea level at the
equator and 7km at the poles.
Contains about 75% of all of the Earth’s air and
99% of all water vapour.
Is widest at the equator and narrowest at the
poles.
Is where weather occurs.
Air pressure and density decrease going up the
troposphere.
Temperature gradient from about 15°C to -57°C,
with the warmest air at ground level and coolest
at the tropopause.
Winds in the Troposphere
Weather Troposphere heated from below by solar
takes radiation, which warms the ground and oceans,
place which radiate heat back into the air above.
within The warm air rises, and convection, updrafts and
the downdrafts mix the air.
context Uneven heating of the troposphere by the Sun
of the (warmest at equator, coolest at poles) results in
general convection, leading to large-scale wind patterns
circulatio that distribute heat and moisture around Earth.
n of the Earth’s rotation causes air to be deflected as it
atmosphe moves from the poles to the equator, causing
re 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.
Tropopause
Tropopause: boundary between troposphere and stratosphere.
Height depends on latitude, season and time of day (day or night).
Near the equator, it is about 20km above sea level.
In winter, near the poles, the tropopause is much lower.
Jet streams are winds with speeds around 400km/hr that form just below the tropopause.
Temperature inversion – temperature stops falling with increasing altitude.
Stratosphere
Stratosphere: second layer of the atmosphere that extends from the troposphere to an altitude of
about 50km.
At the equator the bottom is around 16km, mid-latitudes: 10km, poles: around 8km.
Slightly lower in winter at mid and high latitudes, and slightly higher in summer.
Very dry – air contains little water vapour, so there are very few clouds.
Temperature increases from -51°C to -15°C at the top of the stratosphere.
Ozone layer (a concentration of O3) is found between 20-30km altitude in the stratosphere.
Stratopause: boundary between stratosphere and mesosphere.
Mesosphere
Mesosphere: third layer of the atmosphere that extends from the stratosphere to an altitude of 85km.
Temperature gets colder as altitude increases.
o Top of mesosphere is the coldest part of the Earth’s atmosphere – as low as -120°C.
, Temperature very low and air very thin.
Mesopause: boundary between mesosphere and thermosphere.
Thermosphere
Thermosphere: fourth layer of the atmosphere that extends from 90km to between 500-1000km
altitude.
Temperature in upper thermosphere ranges from 500-2000°C, but the air still feels cold because the
hot gas particles (O2, H2, He) are so far apart.
Thermopause: boundary between thermosphere and exosphere.
Exosphere
Exosphere: top layer of the atmosphere, extending to an altitude of 10,000 km above the Earth.
Contains very small atmospheric particles because the molecules escape into space.
H2 and He are the main components, and are present in extremely low densities.
Area where many satellites orbit Earth.
The Role of Gases in Climate
Name Formu Percentage % What it does
la
Nitrogen N2 78.1 Required for plant growth
Oxygen O2 20.9 Animal/human respiration
Ozone O3 Trace Filters out some harmful UV rays
Argon Ar 0.9 Trace gas
Carbon Greenhouse gas; reflects outgoing radiation back to Earth.
CO2 0.04
dioxide Plant growth and photosynthesis.
Neon Ne 0.002 Trace gas
Helium He 0.0005 Trace gas
Methane CH4 0.0002 GHG; reflects outgoing radiation back to Earth.
Water Cloud formation; scatters some incoming radiation back into
H2O 0.001-5
vapour space.
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)
Global Atmospheric Circulation
Atmospheric circulation is the large-scale movement of air masses, and the method by which heat
energy is distributed around the planet.
Air masses circulate in cells in the atmosphere. These cells help determine the climate and winds at
different latitudes.
Atmospheric circulation is the basis for weather and is driven by the Sun.
Atmospheric circulation and ocean circulation redistribute heat energy across the planet,
and influence the locations of high and low pressure areas.
Atmospheric and ocean circulation are driven by the imbalance in insolation received.
o The equator receives more heat than the poles, so warm equatorial air rises and expands until
it reaches the tropopause. At the tropopause, the air spreads and moves towards the polar
regions. At the same time, cold dense air at the poles sinks and moves towards the equator,
replacing the warm air that has
risen.
Differential Heating
Different parts of the Earth heat up as
different speeds due the different amounts
of insolation received – responsible for the
tri-cellular model.
Factors affecting the amount of insolation
received:
o The curvature of the Earth:
insolation at the Poles is stretched
and so weaker because of larger
surface area.
o The thickness of the atmosphere:
the further from the equator, the
, greater the amount of atmosphere the radiation has to penetrate, so more is lost through
scattering, absorption, reflection.
o Albedo: poles have a higher albedo than the equator due to ice caps being light/white in colour.
o Tilt of the Earth: as Earth tilts on its axis (throughout seasons), the angle of the Sun’s rays
changes throughout the year.
Curvature of the Earth is responsible for:
o Different concentrations of insolation received.
o Different distance insolation needs to travel through the atmosphere.
At the Equator
Insolation (incoming solar radiation) is most concentrated at the equator due to the curvature of the
Earth, which means that the Sun’s rays most directly hit the equator (at roughly 90° angle), and are
more concentrated over a smaller area.
This means that the equator heats up more than other latitudes further N or S.
The Sun is directly over the equator at the equinoxes, so the Sun’s rays strike most directly. On June
21st, the Sun is over the Tropic of Cancer, and on December 21st it is over the Tropic of Capricorn.
At the Poles
The Earth’s curvature means that the Sun’s rays strike at a low angle. This curvature also means that
the Sun’s rays are more widely dispersed, so insolation is less intense. (Insolation is spread over a
much larger surface area at higher latitudes.)
The curvature of the Earth means that the Sun’s rays need to travel through more atmosphere at the
poles than at the equator, so the rays are scattered and dispersed more/there is more chance of
reflection of the Sun’s rays.
Poles have higher albedo due to ice and snow (which has an albedo of 85%) reflecting the Sun’s rays,
so less heating occurs.
As the Earth tilts on its axis, there are times of the year where no insolation reaches either one of
poles.
Global Energy Imbalance
This uneven distribution of energy means that:
o At the equator and 40° N and S of it (at low latitudes), there is an energy surplus.
o 40° N and S onwards (at high latitudes), there is an energy deficit that is greatest at the poles.
This imbalance in energy causes the Earth to redistribute the heat energy through the tri-cellular
model through ocean currents and atmospheric circulation.
The Tri-Cellular Model
Air Pressure Climate Biome
Hadley Low pressure at equator – Equatorial climates are hot Thick and plentiful vegetation,
Cell warm, moist rising air above and wet – high temperatures such as tropical rainforests at
(equator the equator creates a band of and heavy precipitation. equator.
to 30° N low pressure.
and S of 30° N and S, climate is hot
it) High pressure 30° N and S of and dry – high temperatures Deserts form 30° N and S.
equator – descending dry air and low precipitation.
creates high pressure.
Ferrel Low pressure 60° N and S of Moderate temperatures and Temperate deciduous forest at
Cell equator – rising warm air. moderate precipitation. 60° N and S.
(betwee
n
30° and
60° N
and S of
, equator)
Polar High pressure – descending Cold and dry climate – cold Tundra at poles.
Cell (60° air. temperatures and low
to the precipitation.
poles)
Hadley Cell
Air close to the ground is heated by it through conduction due to the high temperatures at the equator
caused by the high concentration of insolation.
This warm, moist air rises above the equator to a height of about 18km, and spreads out underneath
the tropopause because it cannot rise any further, flowing northward and southward towards the poles.
The northward flow deflects to the right in the Northern Hemisphere due to the Coriolis effect, and the
southward flow deflects to the left in the Southern Hemisphere.
This rising air creates a band of low pressure at the equator, and this air contains moisture, which cools
and condenses, forming clouds and then rain, leading to thunderstorms at the equator, so equatorial
climates are hot and wet.
As the now dry air (because moisture is lost through precipitation at the equator) moves towards the
poles, it cools and some of it sinks at about 30° N and S of the equator. The descending dry air spreads
north and south as it nears the Earth’s surface, producing a band of high pressure, which brings dry
conditions, so conditions 30° N and S of the equator tend to be hot and dry, forming deserts at these
latitudes.
At the surface, some air flows towards the lower pressure area at the equator, becoming the trade
winds or tropical easterlies.
Ferrel Cell
Located between 30° and 60° latitude.
Warm air from the equator and cold air
from the poles meet. The warm rises
above the cold air, and they mix. This
leads to the formation of a low pressure
area, bringing rain and thunderstorms.
Rainfall occurs along fronts.
Low pressure at 60° latitude due to rising
warm air causes the air to rise and form
clouds. Some warm air returns to the 30°
latitude to complete the circulation cell.
Much further from the equator, so less
heating by the Sun, so temperatures are
cooler.
Flow in opposite direction to Hadley and
Polar cells and transport heat from the
equator to the poles, resulting in semi-
permanent areas of high and low
pressure.
Leads to formation of westerly winds at
the mid-latitudes – some of this sinking
air at 30° latitude continues travelling
towards the poles and the Coriolis force
deflects it to the right in the Northern
Hemisphere and left in the Southern
Hemisphere.
Not closed-loop convection cells because they do not have the heat source of the equator or the cold
heat sink of the poles to drive convection, meaning they can be affected by passing weather systems.
Polar Cell
Between 60° N and S and the poles.
The smallest and weakest cells.
Air is cooler and drier than at the equator, but is still sufficiently warm and moist to undergo
convection and drive the circulation cell.
Cold dense air sinks, forming an area of high pressure, bringing dry conditions.
At the air sinks, it spreads back towards the equator. The Coriolis force is strongest at the poles, so the
air moving towards the equator is deflected sharply to the right in the Northern Hemisphere and to the
left in the Southern Hemisphere.
Cold dense air descending in polar regions flows at low levels 60-70° N and S. As the air leaves the
polar regions, it starts to warm and rise, returning to the poles at high levels.
Conditions are dry and cold, leading to the formation of tundra.
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