Danube Deluge  -   The river Danube surges upwards and over the pathway to flood low lying fields. Erik Németh (photos) captured this mirage near the village of Vajka in Slovakia.

Atmospheric
Optics
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14:10 on a sunny day, 4th April 2012. The Danube flowed cool while the sun heated the path. A slight breeze carried cool air left to right from the river up the shaded river bank and over the path.

The sky ‘reflections’ on the warm walkway signal an inferior mirage caused by warmer air beneath cooler. For the same reason inferior mirages are often seen in the afternoon on sun heated roads. Further away, the scene becomes blurred by turbulence from air convecting upwards from the path and mixing with the cooler Danube air. The blurring turbulent air is swept over to the lower level fields at the right.

The distant scene over the heated walkway bulges strangely upwards. The river surface itself appears raised compared to the undistorted horizon at left. Most of the nearby air is warm whereas the distant water near the horizon is cold. Rays are emerging through cold air into warmer. They are refracted by the ensuing thermal gradients making the horizon appear raised. The horizon bulges highest over the distant walkway, itself raised, where the air is heated most.

Warm air is forced down by the breeze into the low level land at right where it forms an extensive and turbulent inferior mirage. The light blue ‘water’ swirling downwards is actually an inferior mirage of the sky above. Patches of light blue miraged sky are also mixed with the darker blue of the Danube directly over the walkway.

We see most mirages looking across horizontal air layers at different temperatures. This Danube mirage is more complicated because there are temperature gradients from left to right and also along the line of sight. Compare it with another complex mirage, that of the long jetty at Alderney.

Thanks to Andrew Young for interpretation, errors are mine!