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   Primary Rainbow 


Bright primary rainbow over Schlägl, Austria.

The sky is brighter inside the bow because rainbows are disks of light rather than sets of coloured rings.

Outside the primary there is a fainter secondary bow.

Imaged by Karl Kaiser (site) on August 28, 2002.

©2002 Karl Kaiser, shown with permission.

       To see a rainbow we need sunshine and falling rain. Rainbows are rarer than might be thought. In any one place in rainy England there are fewer than ten bright ones in a year. Halos occur much more frequently.

Early morning and late afternoon are the best times to see them because the sun must not be too high. Rainbows are always opposite the sun and their centres are below the horizon at the the antisolar point. The lower the sun the higher is the bow.

Red is always outermost in the primary bow with orange, yellow, green and blue within. Occasionally, when the raindrops are small, fainter supernumerary arcs of electric greens, pinks and purples lie just inside the main bow.

A rainbow is not just a set of coloured rings. The sky inside is bright because raindrops direct light there too. The primary bow is a shining disk brightening very strongly towards its rim.