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 Thermosphere  Mesosphere  Stratosphere  Troposphere – contains 75% of the mass of the atmosphere and almost all of the moisture and dust. 

 Temperature decreases by 6.4 o C every 1000m – lapse rate  Solar radiation heats the air by conduction  Contains most of atmospheric water, vapour, cloud, dust

 Steady increase in temp. caused by increasing concentration of ozone O 3 which absorbs ultraviolet radiation.  Winds light and increase with height  Pressure falls and air is dry

 Temperature falls rapidly as no water vapour, cloud dust or ozone to absorb incoming radiation  Lowest temperature –90 o C and strongest winds

 Temperatures rise rapidly with height – 500 o C due to increasing proportion of atomic oxygen which absorbs incoming UV radiation (like ozone)

 The atmosphere system involves inputs and outputs.  Incoming solar radiation is balanced by outgoing terrestial energy from the earth.  The balance between input and output is usually referred to as the Global Heat Budget.

Input begins with solar energy (insolation). Some insolation is:  Reflected by clouds and scattered by gas particles

 Absorbed by water vapour, dust and clouds.

Output is in the form of long wave radiation emitted from the earth – this balances the input of energy from the sun:  94% of this radiation is absorbed by water vapour and CO2 in the atmosphere.  6% is radiated back into space.

 As well as a vertical transfer of energy between earth and space there is also a horizontal transfer of energy between high and low latitudes.

 These energy variations are more extreme between the tropics and the poles. Such marked contrasts are referred to as the global temperature gradient and are the result of a number of factors:

 The curvature of the earth  Due to the curvature of the earth, the equator is closer to the sun than the poles and as a result, insolation at the equator is more concentrated.

Atmosphere penetration  The sun’s energy also passes through a greater depth of atmosphere at the poles causing a lot of energy to be diffused.

The Albedo effect  Ice and snow reflect more solar radiation back into space making them cooler whereas areas of dense vegetation absorb radiation making them warmer.

 Seasonal variations in amount of radiation received by the earth with latitude  Sun ‘appears’ to be at Tropic of Cancer mid-June, so northern hemisphere receives more insolation  Mid-December the sun ‘appears’ to be at the Tropic of Capricorn and so the southern hemisphere receives the maximum insolation

 Such an imbalance in energy receipt could theoretically result in the lower latitudes becoming warmer and the higher latitudes becoming even colder. In reality however, energy is transferred from areas of surplus to areas of deficit by atmospheric circulation and by ocean currents.

 The three cell model is a useful tool in describing atmospheric circulation and energy transfer.  Circulation 1 Circulation 1  Circulation 2 Circulation 2

 This extends from the equator to about 30 N and S of the equator. The intense heating at the equator causes the air to expand and become lighter, producing an area of low pressure.

 This warm, rising air contains large amounts of moisture which condenses to form cumulonimbus clouds and heavy rainfall. The rising air then spreads polewards and sinks to the sub-tropics. The sinking air produces high pressure resulting in clear skies

 And little precipitation (these areas correspond with the desert areas of the world)

 Air over cold surfaces will become cold, contract and become heavy. It will therefore sink and produce an area of high pressure. The sinking air moves towards lower latitudes where it will expand and rise back up creating a cell.

 The Ferrel Cell lies between the Hadley Cell and the Polar Cell. The HC and the PC are thermally direct cells (powered by temperature differences).

 The Ferrel Cell is a thermally indirect cell because it is powered by the other two. The FC transfers warm air from the Hadley cell to the high latitudes and transfers cold air form the PC to the low latitudes for warming.

 Atmospheric circulation is a lot more complicated than is suggested by the three cell model. Recent research questions the existence of the Ferrel Cell.

In place of the Ferrel Cell it is now argued that there are:  Alternating patterns of high and low pressure which travel at relatively low levels.  A series of high level, horizontal wavelike motions called Rossby waves.

 Rossby waves are very large, high velocity belts of wind operating in the upper atmosphere. They have a distinct wave like motion as they snake their way across the globe. At their core are long, narrow cylinders of very fast flowing air called jet streams.

 Teachers' Domain: The Effect of Jet Streams on Climate Teachers' Domain: The Effect of Jet Streams on Climate

 Whatever the actual workings of energy transfer and atmospheric circulation, broad global patterns of winds and pressure can be identified:

 The model of Global Wind Circulation is more complicated than it would appear due to several factors:

 The earth’s tilt and consequential seasonal contrasts.

 In June in the Northern Hemisphere, the earth’s axis is tilted towards the sun and the sun appears directly overhead in the Tropic of Cancer.

 In December, the earth’s axis tilts away from the sun and the sun appears to be overhead at the Tropic of Capricorn.

 The apparent movement of the overhead sun is important because it controls the belt of maximum heating which moves with the sun. This called the thermal equator.

The distribution of land and sea.  The wind pattern is more consistent in the Southern Hemisphere especially above latitude 30 degrees south as there are virtually no land masses to interrupt the winds and heating and cooling properties of the oceans means that a relatively consistent wind pattern results.

 Over the Northern Hemisphere, the large land masses result in an altered wind pattern due to the more extreme temperature differences experienced over the continents in summer and winter.

 Low pressure is dominant in the summer due to the intense heating of continental interiors (winds therefore spiral inwards anticlockwise towards the centre of the low pressure).

 High pressure dominates in winter and winds blow outwards in a clockwise direction.

 Sea water has a high thermal capacity, so the oceans are an effective store of thermal energy. In contrast with the land, the seas warm to a greater depth and also move and so redistribute this energy.

 Upper ocean currents are generated by prevailing winds blowing across the surface of the ocean. These are influenced by the rotation of the earth and the distribution of the land masses. The currents largely flow in loops called gyres.  ess05.sci.ess.watcyc.gulfstream/ ess05.sci.ess.watcyc.gulfstream/

 In addition to the surface ocean currents of the world, there is also and oceanic conveyor belt, or deep ocean circulation, that corresponds to the atmosphere’s climate.

 Antarctica is important in this pattern of movement, here vast amounts of water freeze into ice, this loss of fresh water causes the remaining sea water to become more saline and therefore more dense. This denser water consequently sinks and makes its way northwards towards the equator where it is warmed and returns southwards.

 Cold ocean currents flow from the poles.  Warm ocean currents flow from the equatorial regions.  urce/ess05.sci.ess.watcyc.convey2/ urce/ess05.sci.ess.watcyc.convey2/

 Below latitude 30 degrees, the west coast of continents have contact with cold ocean currents e.g. Peru, and the east coast of continents have contact with warm ocean currents e.g. Brazil current.

 Above 45 degrees, the position is reversed – west coast in contact with warm currents e.g. NAD, and east coast in contact with cold current e.g. Labrador current.

 In the Pacific and Atlantic oceans large loops (gyres) appear which are associated with cells of sub- tropical high pressure.

 RainfallVegetation

 Critical to our understanding of the varying rainfall totals and their seasonal distribution in tropical Africa is the seasonal movement of the ITCZ.

 The ITCZ is a belt of low pressure produced by the combination of equatorial heating and the convergence of trade winds, and migrates in response to the changing location of the thermal equator. (see hand out)

 As the airflows converge at the ITCZ, they rise and create a zone of clouds and rainfall. Once the air ascends it diverges and flows polewards, descending over a wide area centred around 30 degrees N and S. As it descends it is warmed and results in dry, cloudless conditions.

 The descending air at the subtropics will be affected by the air mass at the Earth’s surface. The most important air masses which affect Africa are Tropical Continental and Tropical Maritime.

 The Tropical Continental air mass (Harmattan) is a hot and dry air mass.  The Tropical Maritime is a hot and wet air mass.

The AFRICAN ITCZ REGION

 In July, the ITCZ has reached its most northerly extent and it pulls in hot, moist tropical maritime air bringing the Wet Season to West Africa.

 By January, in response to the changing position of the thermal equator, the ITCZ has migrated to the Tropic of Capricorn. Most of Africa north of the equator will experience its dry season at this time.

 West Africa is also influenced by Tropical continental air at this time bringing dry, dusty conditions.

 edu/carbone/module s/mods4car/africa- itcz/index.html edu/carbone/module s/mods4car/africa- itcz/index.html

Continental tropical air Maritime tropical air

79 Wet warm mT air HEAVY RAINS ATMOSPHERE S N Gulf of GuineaCoastal areas- equatorial climate Inland areas- savanna climate type Sahara- Desert climate type Moves this way Hot dry cT air ‘Harmattan’ wind IN JANUARY Copy diagram

Maritime tropical air Continental tropical air

Maritime tropical air

82 ATMOSPHERE S N Gulf of GuineaCoastal areas- equatorial climate Inland areas- savanna climate type Sahara- Desert climate type Wet warm mT air Hot dry cT air ‘Harmattan’ wind IN JULY Moves this way HEAVY RAINSLIGHT RAINS Copy diagram Compare the January and July diagrams.

htm Continental tropical air Maritime tropical air

 Causes of climatic change:  Natural causes  Man-made causes

 Variations in solar energy – sun spot activity occurring in cycles.  Milankovitch’s Cycle (wobble, roll and stretch theory)

 Composition of the Earth’s atmosphere – volcanic activity can add dust particles into atmosphere increasing the absorption and scattering of incoming solar radiation.

 Increased CO2 levels  Deforestation – increases CO2 levels  Flatulent cows - increased population pressure has led to increased food production – cows produce a lot of methane gas.

 Deforestation, soil erosion etc have increased the albedo effect.  Increased use of CFCs in aerosols etc.

 Predicted temp rise of 1.5 to 4.5 degrees C – this would threaten wildlife, affect agricultural areas, tropical diseases would spread

 Sea levels would rise – many low lying areas would be flooded.  Increase in extreme weather conditions.

 Reduce CO2 emissions  Reduce use of nitrogen fertilisers  Less intensive livestock production  Ban CFCs