Ducts
toptics
A
Highlights
A mock-mirage sunset sighted by Sandy Robertson at Alderney, the most northerly of the Channel Islands. Reddened rippled slits replace the sun continuing along the darkling sky as blue-black streaks. Ducting!
Image ©Sandy Robertson, shown with permission
Minutes earlier at Alderney.
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Novaya Zemlya Effect
Extreme ducting.
The sun appears to rise several minutes early or, in polar regions, days before predicted.
The duct images are multiple stacked slits separated by darkness.
More on this image and the NZE.
Air normally cools with increasing height. Sometimes a layer, or layers, of cooler air sandwich between warmer ones – a temperature inversion. Rays from a setting sun refract as they cross the layers to form a mock-mirage. The sun distorts and is often torn apart into three or more images.
Make the temperature differences severe and more happens. Sun rays trap in the colder layer. Then they might travel tens or even hundreds of miles around Earth’s curve before exciting into a fortunate eye. The rays bounce up and down in a ‘duct’.
Several 'bounce modes' may be possible - Each gives a separate slit image.
Rays passing from a denser substance (like cooler air) into a less dense one (warmer air) refract. They change direction and angle towards the interface. Dip a straw or pencil into a glass of water to see this. Make the air temperatures different enough and rays attempting to escape the cooler air are mirrored back. They reflect up and down, channeled along the duct.
