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The
sun sets behind a distant hill. Tony
Cook (see his Tenerife image)
took this sequence looking up the Wharfe valley from Bramhope,
Leeds in Yorkshire, England. The hill edge is about 12
miles away and is a frequent source of flashes. The wind
was from the direction of the hillside. When air moves
across a hill the vertical temperature profile is compressed;
temperature gradients are sharpened and the isotherms
curved concave towards the earth. This favours miraging
and could be responsible for the inferior mirage like
flash taking place here above the astronomical horizon.
A related phenomenon is that stars and planets setting
behind a hill sometimes appear to creep along its rim.
The compressed isotherms enhance atmospheric refraction,
raise the planet and thus delay setting.
Exposures: 6th August 2005, Canon 10D, 1/4000s f8.0 at ISO 100,
Sigma 500mm APO telephoto with x2 teleconverter.
Images ©2005 Tony Cook, reproduced with permission.
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