Cloud Forest Bow, Costa Rica
Imaged at Monteverde by Alex Knopman.

This rainbow is apparently seen EVERYDAY in Monteverde, which is famous for its cloud forests. You can see that the rainbow is just within the cloud that perpetually hangs over the area. The second pic is a close up of the same rainbow with its supernumeraries.
  ©Alex knopman

Here we have a not too often seen ‘missing link’ between a true rainbow and the diffuse and almost colourless fogbow. It is broader than a normal rainbow and its colours are less saturated. There is even a band of (almost) white along its centre. The other difference is that its supernumerary is farther spaced than usual from the main primary bow.

Water drops less than 0.25 mm diameter made it compared with raindrops (1mm dia and more) and fog droplets (less than about 0.1 mm). Diffraction has broadened the bow but to a lesser extent than occurs with the smaller droplets of fogs and mists.

These ‘intermediate’ bows are unusual and not often seen except sometimes at waterfalls and in the artificial sprays of lawn sprinklers.


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Rainbow to fogbow:

Numbers are the diameter of the water drops in micron (1/1000mm). As the drop size decreases diffraction effects are increasingly dominant. The bow broadens and loses its colours. The supernumeraries retain more colour and move inwards from the primary bow.

At right an Airy function simulation for a lognormal drop size distribution with a standard deviation in diameter of 15% of the mean. Uniform sized drops would give more supernumeraries than are customarily seen in nature.