BIO120H1 Chapter Notes - Chapter Section 4: Axial Tilt, Sonoran Desert, Northern Hemisphere

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27 Nov 2018
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BIO120: SimUText Biogeography (Section 4 Global Patterns in
Physical Conditions)
In association with Lecture 13 & 14 and Reading Quiz 6
Climate and Plant Communities
- Alexander von Humboldt spent five years traveling through Americas visited variety of places
including steamy tropical rainforests along Orinoco river, high altitude llanos grassland of
South America, hot and dry Sonoran desert, and temperate forests of eastern United States
- Humboldt noticed plant communities in similar climates all looked similar, even though they
contain completely different species
- Climate predicts not only number of species likely to occur in a place, but also physical
appearance of those species
- Global patterns in temperature and precipitation influence distribution of plant communities
worldwide
Differential Energy Input From the Sun
- Climate patterns across Earth are shaped by uneven input of energy from Sun
o E.g. Earth tends to be warmer near Equator than near poles because intensity of solar
energy in equatorial regions is greater
- At equator, rays of sunlight hit planet at very direct angle; at higher latitudes near poles, incoming
rays are at an indirect, oblique angle angle of entry affects amount of energy reaching surface
in two ways:
1. Incoming light intercepts a larger surface area near poles, so same amount of
incoming radiation spread over a larger area of land or water
2. Light reaching poles hits at angle rather than directly overhead, so rays must travel
through more atmosphere, which dissipates more of their energy, leaving less to
reach surface
- These two factors cause equator to be more strongly heated by sun, both land and water
The Tilt and Orbit of the Earth Create Seasons
- Amount of sunlight hitting Equator fairly constant throughout year, but at higher latitudes,
combination of Earth’s axial tilt and orbit around Sun cause seasonal changes in sunlight
o At peak of summer, planet positioned such that Northern Hemisphere is tilted toward sun
important factor is not actual proximity to Sun, but more direct angle at which light
rays penetrate atmosphere and hit surface of Northern Hemisphere
- Tilt in Earth’s axis explains why summer days in high latitudes are long while winter days are
short during Northern Hemisphere’s summer, North Pole is entirely in light, while during
winter, NP is entirely in shadow; as Earth moves between solstices, days in NH get shorter while
days in SH get longer
- When Earth is midway between two positions, called September equinox, every place on glove
experiences a day and night exactly 12h long
Atmospheric Circulation
- Different parts of Earth’s surface receive different amounts of solar energy differential heating
effects temperature and drives global patterns of air and water circulation
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- Primary circulation pattern is Hadley Cell large convection cell that circulates air and
determines large scale patterns of precipitation and moisture as well:
1. Hadley Cell Formation Intense solar heating at equator warms air near surface,
allowing absorption of substantial amounts of moisture
2. Warm Air Rises As air warms, it expands, causing it to rise
3. Expanding Air Cools As air expands, it rises and cools, allowing clouds to form
4. Moisture Condenses Cold air can hold less moisture, so moisture condenses and
falls as rain over tropical regions
5. Dry Air Pushed Poleward As more air rises, it pushes the now dry air towards the
poles
6. Dry Air Falls As dry air continues to cool, it grows denser, eventually sinking near
30˚N and 30˚S; dry air creates bands of deserts at these latitudes
7. Return to Equator Dry, dense air flows back towards Equator, where it is once
again warmed
8. Warm Wet Air Rises Warm air picks up moisture and begins to rise, completing
the cell
9. Polar and Ferrel Cells Solar heating near 60˚N & S drives Polar cells which are
similar to Hadley cells, but less intense; Ferrel Cells of mid latitudes are driven by
convective orces of Hadley and Polar circulation cells
- Hadley cell is largest and steadiest of atmospheric circulation cells; Earth operates with a 6 cell
system, with three circulation cells in each hemisphere
- Atmospheric cells disperse heat from solar energy, drive weather systems, and determine patterns
of surface winds across the globe
Bands of Wet and Dry
- Atmospheric circulation cells are coupled with uneven distribution of solar energy and combine
to create patterns of rainfall across globe influences vegetation on large scales
- There are dark brown bands of desert at ~30˚N & S latitude between these strips of desert,
there’s lush green vegetation of inter – tropical convergence zone; it lies where two Hadley cells
converge
Coriolis Effect and Wind Patterns
- Earth’s topography affects how air flows across its surface atmospheric circulation cells create
north and south flowing air currents at surface; winds may be deflected by tall mountains and
continental coasts
- Rotation of Earth also affects circulation patterns, causing winds to be deflected to right in
Northern Hemisphere, and to left in Southern Hemisphere, through Coriolis effect
- Coriolis Effect
o All points on spinning sphere, Earth, complete single rotation in same amount of time,
but they move at different speeds E.g. in one day, point on Equator and Anchorage,
Alaska will each rotate around Earth’s axis once, but equatorial point will travel
~40,074km, while Anchorage will travel only ~10,372km
o Difference in velocity means that trajectory of moving air and water towards south or
north over Earth’s surface appears to be deflected objects moving away from equator
appear deflected eastward, objects moving toward equator appear to be deflected
westward
o Coriolis effect is what drives rotation of large storms like hurricanes
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