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Chapter 4

BIO120H1 Chapter Notes - Chapter 4: Horse Latitudes, Westerlies, Great Lakes

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James Thomson

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Patterns of climate from global to habitat scales
Tropic of Cancer: the latitudinal lines around the earth at 23.5 degrees north
Tropic of Capricorn: the latitudinal lines around the earth 23.5 degrees south
Tropical region: the belt bound by the tropic of cancer and the tropic of capricorn
Solar equator: The line of latitude closest to the sun (always within the tropical region)
Latitudinal patterns: how the sun heats the earth:
Temperature is a very important environmental variable to organisms.
oTemperature variation occurs from the latitudinal gradient in temperature: hot
near the equator, grading toward cold at the poles.
This pattern arises from the uneven distribution of radiant electromagnetic
energy that is continuously blasted at the earth from the sun.
The spherical shape of the earth makes it so that the sun's rays strike the
earth that we can consider the incoming photon flux as a uniform stream of
parallel rays.
Hence the reason why photon density is highest are the equatorial
regions, where the surface of the earth is essentially perpendicular to the
vector of the incoming photons.
Photon density per unit are declines as we move toward the poles
because the angle of incidence declines from 90 degrees to 0 degrees (at
the poles).
Seasonal variation in climate arises because the earth's axis is tilted at about 23.5
degrees off the vertical.
oAs it makes its annual revolution, different parts of the earth experience the sun
as being directly overhead at noon.
Taking into account the atmosphere, it seems that the incoming solar light energy
transfer is weak- this is not true:
oA lot of energy transfer happens when light hits surfaces other than air (solid
surfaces, and water).
When photons hit those surfaces, they are absorbed and reradiated at
longer, IR wavelengths.
These IR light waves are absorbed by the atmosphere (especially
the bottom of the atmosphere). This IR heating is closest at the equator.
Atmospheric circulation:
Solar (IR) heating from the bottom of the atmosphere sets up atmospheric circulation
Large-scale patterns of atmospheric circulation will be discussed later.
Examples of small-scale patterns of atmospheric circulation are thunderstorms,
tornadoes, hurricanes and typhoons.
Atmospheric pressure: when the weight of a column of air compresses the air at the bottom
Adiabatic lapse rate: the relationship of temperature drop to altitude gain (explained by ideal gas
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