Environmental Engineering Reference
In-Depth Information
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FIGURE 12.4
Seasonal variations in day length as a function of latitude (degrees).
FIGURE 12.5
Eighteen-year annual mean daily total shortwave radiation for the United States (MJ m
-2
day
-1
). (Courtesy of the National Center for Atmospheric Research, DAYMET program.)
The relationship between light penetration and water quality has been known for a long time. In
1865, Angelo Secchi, an Italian astrophysicist commissioned to measure water transparency in the
Mediterranean Sea, developed a method to measure light penetration, or the transparency of water.
He invented a device that consists of a disk with alternating black and white quadrants (Figure 12.9).
The disk is mounted on a pole or a line and is then lowered through the water column until the pat-
tern on the disk is no longer visible. The depth at which this occurs is known as the Secchi depth.
This device is still in common use today.
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