Atmospheric
Optics
Pollen Corona
A reminder of summer imaged by Karin Parker in Finland.
"It was taken June 21st, 22.20 EET in Oulu. An hour or so before, there had been sudden rain and there was mist on the ground. The sun was breaking through the clouds when I saw this and pulled the car over in shock to get the photo. The pine pollen season had been delayed a little here in Finland because of the weather, but a week previously the green dust started appearing in large quantities so I guess the conditions were just right.
There's been no post processing at all- this is just off the camera card."
Corona image ©Karin Parker, shown with permission
The diagram shows just two scattering points.
Pine pollen imaged by Jari Luomanen. Each grain has two air sacs.
More than a corona. At the optically thin cloud edges, small water droplets are scattering (diffracting) light to produce a 'silver lining'.
The shadow? The cloud is casting its shadow through lower mist. The viewing angle and position makes it appear above the cloud.
Bright. Colourful. Several oval rings. Bright patches. Sneezing. Very likely a pollen corona rather than the more commonplace one from small water droplets.
Pollen grains from the same plants are of identical size. That produces the more saturated colours and multiple sharper rings.
Some, but not all, pollen coronae are oval with brighter patches. The grains producing them have air sacs to aid wind dispersal and the sacs orient the grains. the resulting diffraction pattern no longer has circular symmetry.
Pollen grains are opaque, or at most translucent. That does not affect their ability to form coronae.
Light waves scatter from their surface. Each scattering point gives a train of outgoing spherical waves. The waves overlap. In directions where the wave crests coincide they reinforce and there is light. At other angles the waves cancel one another and there is darkness. The resulting diffraction pattern is the corona.
