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Highway Mirage
Thu Sep 08, 2005
Listen in RealAudio 
Ahead the highway pavement appears covered with water pools. But you can never
reach these pools, for they are only mirages: highway mirages.
Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
Before highways spider-webbed our national landscape, such mirages were generally
considered as creatures of the hot desert. Highway mirages are actually very common
visions on pavement on any sunny day, even during the winter.
Your imagination is not playing tricks; mirages are real images seen by our eyes, but
incorrectly interpreted by our mind. What we see as "water pools" are actually light rays
emanating from clouds and sky above and ahead of us. These rays are bent, or
refracted, upward in a broad, bow-shaped path after passing through a layer of less
dense air near the surface. Our brain, however, interprets the rays as having originated
from the ground rather than from the sky.
Technically, both the desert and highway mirages are inferior mirages, so called in
reference to the position of the perceived image being seen as below, or "inferior" to,
its actual position. Inferior mirages usually form when light rays pass through a
relatively hot layer of air formed near the ground by strong solar heating — the hotter
the surface air relative to that above, the greater the bending effect. The temperature
difference between the air layers is more important for mirage formation than the
absolute temperatures.
Above a paved road, the full sun can generate surface temperatures 20-30 F degrees
hotter than the air only inches above, even in winter. Thus, while highway mirages are
commonly seen over dry pavement during the summer months, they can appear on
any day when the atmosphere is just right.
The Weather Notebook receives support from The National Science Foundation, and
Subaru of America.
Today's Links
Introduction to Mirages
http://mintaka.sdsu.edu/GF/mirages/mirintro.html
More on Atmospheric Optical Mirages
http://www.geocities.com/TheTropics/Beach/7002/mirage.htm
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