Few of the worlds natural phenomena have inspired as much poetry and song as rainbows. Unlike most unusual events of the sky, rainbows have been perceived more often with wonder and delight than fear. Perhaps this is because their appearance immediately following rain storms makes them so easily associated with optimism, cheerfulness, and good fortune.
Rainbows are the result of the sun’s rays refracted and reflected by falling raindrops. Sunlight passing through a raindrop is sorted by wavelength, with each color sent out from the drop on a slightly different path. Thus drops high in the sky send red wavelengths to an observer, while those below send yellow, and those farther below send blue.
A rainbow bows because it is the visible portion of a perfect circle. The center of the circle is below the horizon at a point equal to the height of the sun above the opposite horizon. At sunrise or sunset, the bow will be high, a nearly complete half-circle, with its center resting on or only slightly below the horizon. When the sun is higher in the sky, the center of the circle is lower and we only see a section of the top of the circle, causing the arc of the rainbow to appear much smaller.
The best opportunity for viewing rainbows is immediately after a rain shower passes, when the sky behind the storm begins to break up and the sun to shine through. Typically, in most parts of the world, a rainbow will occur in the late afternoon, when the prevailing winds are sweeping the storm toward the east, and the sun in the west is shining on the dark backside of the clouds.