The primary rainbow must be the most well-known atmospheric optical phenomenon to people. However, it is actually relatively uncommon to see in relation to natural weather. It is caused by light being refracted and internally reflected by spherical raindrops over an angle of 138 deg. Due to the refraction the coloured arc is produced, having a radius of 42 degrees. Usually, the bow is brightest near the horizon, because falling water droplets take on the form of ellipsoids with their long axes horizontal. Because of that, a horizontal cross section of the droplets is more like a circle than a vertical cross section, and the quality of the resulting rainbow is, in the end, dependent on the roundness of the cross section.

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