Page 131 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 4 atmosphere and Surface Energy Balances 95
For atmospheric particles larger than the wave- lengths of light (such as many pollutants), the Rayleigh scattering principle does not apply. Mie scattering is the process that works on these particles. In a sky filled with smog and haze, the larger particles scatter all wave- lengths of visible light evenly, making the sky appear al- most white.
The altitude of the Sun determines the thickness of the atmosphere through which its rays must pass to reach an observer. Direct rays (from overhead) pass through less atmosphere and experience less scattering than do low, oblique-angle rays, which must travel farther through the atmosphere. When the Sun is low on the horizon at sun- rise or sunset, shorter wavelengths (blue and violet) are scattered out, leaving only the residual oranges and reds to reach our eyes.
Refraction As insolation enters the atmosphere, it passes from one medium to another, from virtually empty space into atmospheric gases. A change of medium also occurs when insolation passes from air into water. Such transi- tions subject the insolation to a change of speed, which also shifts its direction—this is the bending action of refraction. In the same way, a crystal or prism refracts light passing through it, bending different wavelengths to different angles, separating the light into its component colours to display the spectrum. A rainbow is created when visible light passes through myriad raindrops and is refracted and reflected toward the observer at a precise angle (Figure 4.4).
Another example of refraction is a mirage, an image that appears near the horizon when light waves are re- fracted by layers of air at different temperatures (and consequently of different densities). The atmospheric dis- tortion of the setting Sun in Figure 4.5 is also a product of refraction. When the Sun is low in the sky, light must penetrate more air than when the Sun is high; thus light is refracted through air layers of different densities on its way to the observer.
Refraction adds approximately 8 minutes of day- light that we would lack if Earth had no atmosphere. We see the Sun’s image about 4 minutes before the Sun actually peeks over the horizon. Similarly, as the Sun
▲Figure 4.4 A rainbow. Raindrops—and in this photo, moisture droplets from the niagara River—refract and reflect light to produce a primary rainbow. note that in the primary rainbow the colours with the shortest wavelengths are on the inside and those with the longest wavelengths are on the outside. in the secondary bow, note that the colour sequence is reversed because of an extra angle of reflection within each moisture droplet. [Bobbé Christopherson.]
▲Figure 4.5 Sun refraction. The distorted appearance of
the Sun as it sets over the ocean is produced by refraction of the Sun’s image in the atmosphere. Have you ever noticed this effect? [Robert Christopherson.]
Sun’s image
Sun’s
actual position
What is happening in the photograph?
Refraction by the atmosphere
Observer Earth
Georeport 4.1 Did Light Refraction Sink the Titanic?
an unusual optical phenomenon called “super refraction” may explain why the Titanic struck an iceberg in 1912, and why the California did not come to her aid during that fateful april night. Recently, a British historian combined weather records,
survivors’ testimony, and ships’ logs to determine that atmospheric conditions were conducive to a bending of light that causes ob- jects to be obscured in a mirage in front of a “false” horizon. Under these conditions, the Titanic’s lookouts could not see the iceberg until too late to turn, and the nearby California could not identify the sinking ship. Read the full story at www.smithsonianmag .com/science-nature/Did-the-Titanic-Sink-Because-of-an-Optical-Illusion.html?.