Dewbow over water, Finland Images by Marko Riikonen (site).
©Marko Riikonen.
This was the second rare phenomenon captured by atmospheric optics expert Marko from a boat on a Finland lake early on the morning of August 14 .  His first was a reflection glory. Soon after that he witnessed this dewbow formed over the water.

Dewbows on lawns are fairly common. Dewdrops formed during a humid night hang on leaves and blades of grass to reflect and refract sunlight like raindrops. But on water?

In this case the drops were supported on debris on the lake surface. “No algal or oily film was there. Just plant and insect litter. I saw different sized water droplets attached on the litter particles. Some had grown on single particles, others were on the projections of larger particles. The great majority of the particles had no droplets, droplets were rather scarce which is why I think the dewbow was visible only near the horizon.”

Larger dewdrops can also be flattened. But if the sun is low – as here - this does not hinder dewbow formation near to the lake horizon. The rainbow forming rays pass across a nearly horizontal and circular droplet section. Other parts of the dewbow could not form from such drops.

Marko held the camera only 5 cm above the lake to get the two upper rare images.

This was not the end of a day of atmospheric optics. That is the topic of the next OPOD.


Atmospheric
Optics

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Dewbows and water colours