ph . Astronomy . Galleries . Atmospheric phenomena . Rainbow

A beautiful image of a rainbow, showing primary and secondary bows, by Tessa Joughin. The rainbow arcs over Mount Gilboa in the Witteberge (Ficksburg district), on the farm Franshoek approaching Thaba Thabo. Tessa used a Nikon D70s (f/9.5, 1/350th at ISO 100) to capture the image. Local time was 17:32.

A rainbow is an arc of light of many colours that is sometimes seen when the Sun shines on rain. In the image above, two bows can be seen, parallel to one another. The inner one is the primary bow, and the outer one is the secondary bow. Secondary bows are always broader and much less bright, and are not always visible.

The portion of sky enclosed by the primary bow is often noticeably brighter than that between the primary and secondary bows. The sky between the primary and secondary is, in fact, often much darker than the rest of the sky, and is named Alexander's dark band (learn more here.)

The outside edge of the primary bow is red, and the inner edge is blue or violet. The inner edge is also much more clearly defined. Colours in the secondary bow are the same as in the primary but they are less bright and the order is reversed, and Tessa's image shows.

The angular size of a rainbow is always 42°; the extent of the arc that is visible depends on the height of the Sun above the horizon. The lower the Sun, the more of the arc can be seen; when the Sun is on the horizon, a semi-circular arc is visible. To help estimate the Sun's altitude, keep in mind that when it is 45° above the horizon, your shadow is equal in length to your height.

Rainbow hunting tips: (1) check that the Sun is shining as it is raining; (2) the rain must be directly illuminated by sunlight; and (3) stand directly between the Sun and rain with your back to the Sun and look up at the sky in the direction of your shadow. For even better odds of seeing a rainbow, (4) try to be within a few hundred metres of the rain; (5) hope the rain extends over a portion of the sky large enough to accommodate the rainbow arc (angular size is always 42°); and (6) relocate the Sun so it is near the horizon.

Keywords: Astronomy, astrophotography, atmospheric phenomena, sunset, sunrise