Page 7 - The World About Us

Steppes
1.1.1
1.1.1

What other factors influence biome distribution?

The general atmospheric circula on
is not the only factor which affects the
global pa ern of climates and,
therefore, determines the distribu on
of the planet's biomes.
One key factor is the presence of
high mountain ranges which act to
create rain shadows. As air carrying
evaporated water vapour is forced to
rise over mountain ranges, the
temperature of the air falls as it 'thins'
or expands. This is known as adiaba c
cooling.
Rising air, when saturated with
water vapour, will cool by around 5 C
for every 1000 metres it rises.
Adiaba c cooling causes the water fig.10 The Atlas Mountains, shown here with winter snow, January 2012.
vapour to condense into cloud droplets side of the mountains (the lee side) the woodlands in the centre of Asia and
and rain to fall on the windward side of opposite happens: the air warms as it North America, as well as the extreme
the hills. is compressed. Any cloud droplets now aridity of the centre of the Sahara
As the air descends on the opposite begin to evaporate back into vapour Desert.
and the lee side experiences less One final factor is the impact of
fig.11 The Himalayan mountain range. rainfall as the clouds dissipate. This is ocean currents. Along the coasts of
known as the rain shadow effect. some con nental areas an upwelling of
Significant rain shadows are found on very cold water from the Polar regions
the lee side of the Himalayas, Andes, cools the sea along these coasts. This
Rocky and Atlas Mountains (see fig.10).
very cold water chills warm, moist air
In addi on to rain shadows, the passing over its surface and results in
centre of con nents are so far from fogs forming. Most water, therefore,
the sea that most rain is precipitated precipitates out over the sea.
out of the atmosphere closer to the In the Atacama and Namib deserts,
oceans leaving li le for the centre of fog may be blown inland during the
con nents. This feature is known as
morning but this is quickly burnt off by
con nentality and helps explain the the Sun. Li le moisture gets further
dominance of grasslands rather than inland than a narrow coastal strip.

KEY TERMS
1. shown in a trophic pyramid? (1)
What is meant by the term 8. Explain how energy and nutrients pass
ecosystem? (1) through an ecosystem. (4) General Atmospheric
2. Outline the various components of an 9. What is meant by the word Circulation: the movement of
air throughout the troposphere
ecosystem. (3)
perturba on when referring to in six circulatory cells: the
3. What is meant by Net Primary ecosystems? (2) northern and southern Hadley,
Produc vity? (1) 10. Discuss why apex predators are o en Ferrel and Polar cells.
4. Explain why tropical rainforests are most at risk from perturba ons to the
Rain shadow: the drier region
more produc ve than hot deserts. (3) ecosystems in which they live. (3)
found on the lee side of
5. Explain why the different elements of 11. Outline why the centre of con nents mountain ranges caused by
an ecosystem are described as are o en drier than their coastlines. (2) rising air dropping its moisture
‘interdependent.’ (4) 12. Outline why the Atacama, Namib and as rain on the windward slopes.
6. Explain how global biomes are Western Sahara are deserts despite being The air warms as it descends
distributed in rela on to their la tude. (4) located along ocean coastlines. (3) the lee slopes, resulting in
clouds droplets evaporating and
7. What rela onship in an ecosystem is
less rainfall.
7

Ecosystems have distinct distributions and characteristics.