Environmental Engineering Reference
In-Depth Information
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The tilt of the Earth on its axis points some regions toward the sun and others away from the sun.
Areas angled toward the sun receive more direct energy than areas angled away from the sun. The vari-
ous seasons on Earth are caused by the tilt of the planet's axis and the rotation of Earth around the sun
over the course of a year.
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Earth's surface is moving faster at the equator than the poles.
This is the reason for the phenomenon
known as the Coriolis effect (the apparent deflection of a moving object in a rotating reference frame).
As solar energy heats the Earth's surface, some heat is transferred to the atmosphere by radiational heating
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This energy warms the gases, the gases expand, they become less dense and then rise, whereupon they cool and
fall back to the Earth's surface to be reheated again, thus creating a continuous cycle. This constant heating and
cooling creates vertical currents called convection currents. On a global scale, these convection currents are
called the Hadley, Ferrel, and Polar cells:
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Hadley air circulation cells
occur close to the equator. The surface air in this region is warmed from the
strong solar radiation, causing the air to rise and expand. This process releases moisture and provides
high amounts of rain, a major contributing factor for the tropical rainforests in the equatorial region. The
air, now holding less water, heads north and south, ultimately cooling and sinking back towards the sur-
face. Now containing very little water, the arid air helps to produce deserts. The Hadley cell is the
strongest of the three air circulation cells.
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The Ferrel air circulation cells
generally occur at mid-latitudes between the Polar and Hadley cells. Be-
cause of the sinking air coming from the Hadley cells and the rising air brought in by the Polar cells,
westerly surface winds are produced in these regions.
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The Polar air circulation cells
are the northernmost of the three types of cells and contain dense, cold
air moving towards the poles. As the air reaches the poles, it sinks and then moves south back towards
the mid-latitudes. In this process, air starts to warm and rise again, creating low pressure areas.
Another component of atmospheric circulation is the
Coriolis effect.
This occurrence creates the deflection of
objects from otherwise moving in a straight line. This is due to the rotation of Earth. For example, if you spin a
disk and try to draw a line from the center to the edge, the result will be a curved line. This is caused by the
Coriolis effect. This effect causes winds in the Northern Hemisphere to deflect to the right and in the Southern
Hemisphere to deflect to the left. The Coriolis effect contributes to the creation of the three air circulation cells
and has effects on weather patterns and ocean circulation.
