Pollen Corona A pollen corona crowns the castle of Giulietta at Montecchio Maggiore, Italy. An image by Francesco Della Stella Dalla Stella. Pollen grains from a particular tree are all the same size and shape. That�s perfect for making a corona. Each grain diffracts the sun�s rays in the same way to give us a sharp diffraction pattern in the sky with undiluted colours. Ordinary water droplets in clouds are less good. They have a range of sizes. Even worse, their sizes can change dramatically across the corona�s angular extent. That gives us broken coronae and iridescence. Still spectacular but in a different way.

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