Primary rainbow

(Section 3.2.3.5.1)

This is a coloured bow or arc that appears on a “screen” of water drops when light from a bright light source (the Sun or Moon) falls upon them. A thick “screen” of raindrops will produce more vivid colours than a thin curtain of fewer drops. If the screen of drops is only present for some of the arc, only a fragment of a bow is visible.

The size of the drops or droplets determines which colours are present and the width of the band occupied by each of them. It is unusual to distinguish all of the so-called “colours of the rainbow” (red, orange, yellow, green, blue, indigo and violet). When the Sun is near the horizon, dust may cause a red rainbow. This happens because the aerosols preferentially scatter the shorter blue and green wavelengths of light.

A rainbow is seen in a part of the sky opposite the light source (Sun or Moon), and the arc is always centred on a point relative to the observer directly opposite the light source (that is, an extension of a line from the light source through the observer, and typically into the ground. This point is known as the antisolar point when the Sun is the light source, and the antilunar point when the Moon is the light source. In all cases, violet is on the inside (radius of the arc is 40°) and red is on the outside (radius is 42°). The sky is darker outside the bow than inside, a phenomenon known as Alexander's dark band.

The amount of arc seen by the observer also depends on the elevation of the light source. Over flat terrain near sunrise or sunset (moonrise or moonset) the primary bow can be almost a semicircle. As the Sun or Moon rise in elevation, the centre of the arc, and so of the primary rainbow, lowers below the horizon. When the light source has an elevation approaching 40°, the arc of the primary bow (with a radius of 42°) will almost sit on the horizon with only the top of the bow visible in the sky. If the Sun or Moon rise higher than 42°, the primary bow will be below the horizon.

From an elevated vantage point such as a hill, an observer may see rainbow arcs greater than a semicircle. From a very high vantage point such as an aircraft, a rainbow forming a large part of a circle, or even a complete ring, may be seen.

In exceptionally rare circumstances (when raindrops of different size coexist) the primary bow appears to split into two equivalent bows, giving a “twinned bow rainbow”. The inner bow does not at all resemble a supernumerary bow.