Download Suggested answers

ATMOSPHERE
Suggested answers
ANSWER 1.
A). DESCRIBE THE LATITUDINAL VARIATION OF EARTHS ENERGY
There is a net gain in the tropics because insolation has to pass through a relatively shallow depth of atmosphere, as
it strikes the atmosphere at right angles, reducing the likelihood of reflection or absorption by dust, clouds or
gases. Rich vegetation in rainforests for example then absorbs solar energy. In addition, tropical areas receive
concentrated sunlight throughout the whole year. Thus in the tropics more solar energy is received than is
radiated. As one moves towards the poles, the angle of incidence becomes more oblique, which means that the
sun’s energy is dissipated over larger areas, many of which are snow covered with very sparse vegetation and
so are highly reflective. These areas also experience greater seasonal variations in insolation, and there may be
up to four months where little or no insolation is received.
B). EXPLAIN THE THREE CELL MODEL
The three cells in the model, together with the resulting surface winds are directly caused by thermal energy
differences. At the Tropics, the Hadley Cell is caused by intense heating, which results in air rising, causing
low pressure. This warm air rises and begins to move polewards, carrying heat energy with it and eventually
falling at about 30º of latitude. This downward movement of air causes the sub-tropical high pressure and
energy is transferred to air in the adjoining Ferrel Cell. Cooler air then moves as the Trade Winds back to the
Equatorial low pressure area to begin this process again. At the poles, intensely cold air falls and blows
towards the equator as polar easterlies, becoming warmer as it does so. At about 60º the air is in contact with
warmer land and sea surfaces: it becomes unstable and begins to rise. The sinking air of the sub-tropical high
and the rising air of the temperate low pressure areas cause the third cell – the Ferrel Cell – to move through
friction. It receives heat energy from the Hadley Cell, carries it polewards in the mid-latitude westerly winds,
and then transfers it to the adjoining Polar Cell. In this way, heat energy is transferred from the equator to the
poles, with colder air being moved to the equator. The resulting surface winds do not blow directly northsouth, but are deflected to the right in the N Hemisphere and to the left in the S Hemisphere by Coriolis Force.
ANSWER 2.
P16 OF NOTES
Identify air masses and describe source and origin
a). The tropical continental airmass originates in the sub-tropical high pressure zone over the Sahara Desert, which
means that the sinking air is stable, hot and low in humidity. As it blows towards the ITCZ it brings hot dry
weather to West Africa, where it is known as the Harmattan. Prolonged periods of activity can bring drought.
The tropical maritime airmass, by contrast, originates over the Gulf of Guinea, which is cooler than the
Sahara. This airmass is therefore warm with high humidity. On reaching the hotter landmass, the air becomes
unstable and brings wetter, cooler weather than the Harmattan.
Describe and explain the varying rainfall patterns (ITCZ)
b). The varying amounts of rainfall in this area are caused by the seasonal migration of the zone of maximum
insolation, which is due to the tilt of the Earth as it orbits the Sun, which in turn affects which airmass will
cover the area. Areas closer to the Equator, and along the coast are under the influence of the moist, unstable
mT airmass for at least 9 months of the year and so have a high rainfall total (Libreville has 2510mm during
11-12 months of rain and Enugu, with 1784 has 9-10 months). As one moves north, the rainy season and total
rainfall decrease rapidly to Menaka, where there is a very short rainy season of only 1-3 months, with 203mm.
The zone of maximum insolation is marked by the Inter-Tropical Convergence Zone (ITCZ), where rising
warm air draws unstable warm moist tropical maritime air northwards from the Gulf of Guinea and hot dry
stable tropical continental air southwards from the Sahara. In January the ITCZ lies along the coast, so only
coastal fringes experience the mT airmass and the rain it brings. As the zone of maximum insolation moves
north, so does the ITCZ, pushing back the dry cT airmass and drawing the unstable mT air further north. It
follows then that areas further north will only be affected by the mT air for a short period, whereas those
further south will have longer rainy periods.
ANSWER 3
QUESTION 1 (b), 1991
“Describe the role of either atmospheric circulation or oceanic circulation in the redistribution of energy
over the globe”.
5 marks
The world’s pattern of ocean currents is a mechanism whereby surplus heat energy is redistributed from
tropical areas, where there is a surplus, to higher latitudes where there is a deficit. The pattern of currents is
affected largely by the world’s wind patterns, however the ocean currents do not follow the winds exactly, as
they are interrupted by the earth’s landmasses. Currents move away from tropical seas, carrying heat energy
towards the poles, and this water is then replaced by cooler water moving towards the equator. This can be seen
in the Atlantic, for example where the North Equatorial Current flows west, but is then guided north, then
north-east by the coasts of first South, then Central and finally North America. This current, now called the
Gulf Stream, and assisted by South Westerly winds, carries warm water north-eastwards across the North
Atlantic, where it is known as the North Atlantic Drift, and into the Arctic Ocean. Cold water flows equatorwards at a lower level in the ocean, for example in the Greenland, Labrador and Canaries Currents, to be heated
and so to continue the cycle. Without this oceanic circulation, there would be far greater variations in
temperature between areas of energy surplus and areas of deficit.
Similar patterns may be seen the other northern oceans, where landmasses have warm currents on their western
coasts and cold currents to the east. In the southern oceans this pattern is reversed, although here the currents
are more similar to the wind pattern, as there is less land to interrupt them.
ANSWER 6.
QUESTION 1, 2001
“Suggest physical and human factors which might have contributed to the variations in temperature shown in
the diagram.” 5 marks
The general tend in global temperatures between 1850 and 1990 was upwards throughout the period, from an
approximately 0.2ºC below the mean to around 0.4ºC above it. This may have been due to long-term astronomical
issues such as those highlighted by Milankovic, but there have also been changes on a shorter scale, which may
have had human or physical causes.
Between 1850 and 1900 the steady rise in temperatures may have been caused by the increased discharge of
greenhouse gases from burning fossil fuels during the Industrial Revolution in Europe and North America, although
the sudden drop in the early 1880s, from 0.2ºC below average to 0.4ºC below average could be attributed to dust
and ash from the 1883 eruption of Krakatoa preventing some solar radiation from reaching the earth.
The peak and subsequent trough between 1910 and 1920 may have been due to the increased industrial output
caused by World War 1. From this point until 1945 there was a relatively rapid rise in temperatures, from 0.4ºC
below the mean to approximately the mean. This was possibly due to the increased use of motor vehicles, although
there was a levelling off during the 1930s, which coincides with the Great Depression. Closure of industries at this
time could have reduced the output of greenhouse gases.
From 1945 until 1970 there was a slight fall, which may be attributable to the amount of dust forced into the
atmosphere by the testing of atomic bombs: this could have reflected insolation on a global scale.
The rapid rise in temperatures from 1970 until 1990 may have been caused by greater affluence leading to wider
car ownership, destruction and burning of the rainforests, which not only destroyed the plants that absorbed carbon
dioxide, but it also created a great deal of the gas through the burning of the trees. Another cause was the rapid
industrialisation of some developing countries, (NICs), who frequently used outdated and polluting industrial plant.
Both of these factors would place large quantities of oxides of carbon, sulphur and nitrogen into the air, trapping in
long-wave radiation from the earth.
The slight dip in the early 1960s and 1980s might be attributable to the eruption of Icelandic volcanoes and Mt St
Helens respectively, which put millions of tons of dust into the atmosphere, reflecting some of the sun’s rays.