Environmental Engineering Reference
In-Depth Information
sunspots and solar flares. The solar wind interacts with the magnetosphere, the magnetic
field that surrounds Earth, and this interaction is visible as the aurora .
Let us look in more detail here at the diurnal and seasonal effects of radiation received
at the top of the atmosphere.
Figure 2.10 Energy distribution on an intercepting surface
depends upon the angle of the incoming energy rays. Energy
distribution is more concentrated on a perpendicular surface
( A) than on a surface at a lower angle ( B).
DIURNAL VARIATION
As Earth rotates on its axis a different portion of the top of the atmosphere will be
exposed to the incoming solar radiation (often abbreviated to insolation ). At dawn the
sun will be low in the sky and the amount of radiation passing through a unit area normal
to the line from the sun will be spread over a large area (Figure 2.10). As the sun rises in
the sky the surface area decreases, and so intensity increases. If our surface is eventually
at right-angles to the solar beam it will receive the maximum intensity of radiation - the
surface area is at its smallest. As well as the angle between the sun's rays and the top of
the atmosphere and Earth's surface, the length of daylight will also affect the amount of
radiation received. At the equator the day remains approximately twelve hours long
throughout the year. At the poles it varies between zero and twenty-four hours, depending
upon the time of year.
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