Atmospheric
Optics
Road Blink & Mirage ~ A scene in Idaho, USA pictured by Alan Clark. The land is a flat lava field and a major potato growing area. Here, all is snow covered except for the dry roads. Their seeming wetness is a complex inferior mirage. The roads form dark patches, 'road blink', in low cloud and mist. The three patches indicate the twinned highway and a service road. Arctic dwellers exploited similar 'blinks' to navigate through the darker 'leads' of water surrounding ice floes.
All images ©Alan Clark, shown with permission
Complex Inferior Mirage - The dark road surface has previously seen sunlight and is warmed. In turn it warms a layer of air above it. Above that is colder air. Rays from the cars and sky refract across the temperature and air density gradients to form the mirage. We see beneath each car an inverted image. The 'water' is a mirage image of the cloudy sky.
Many inferior mirages stop there. The cars have traces of another mirage image - this time erect - beneath the inverted one. Inferior mirages can form a series of alternately inverted and erect images. In this case the undulating road helps.
Blink - Cloud and mist can be so dense that it is 'optically thick'. No direct light passes through. Rays entering the cloud are scattered in all directions by collisions with many small water droplets. The cloud surface appears diffuse and the same colour as the incident light.
Rays from the snow covered fields light the cloud surface in places and some light scatters back downwards. The cloud is less lit above the dark road.
Snow is not needed. Coloured fields similarly light and tint low clouds.
