Ask WECT

September 6, 2010

Why do we see rainbows half moon shape?

Filed under: Uncategorized — WECT @ 6:11 pm

Angie asks…

Question? I have wanting the answer to this question for a long time….Why do we see rainbows half moon shape? The rain is coming down straight, sideways, wind blown, left, and right. So whay is it not coming down zigzag or up and down shape? Now a lot a people I have asked this question to did not know the answer at all. So maybe you can solve this mystery. Thanks.

We asked evening meteorologist Robb Ellis to weigh in on this one.

A rainbow is technically always a full circle, however, usually about half of the circle remains below the horizon. For a rainbow to occur, the sun must be to your back (and below a certain angle on the horizon (42 degrees)), and the rain must be in front of you, so the sunlight can hit the individual droplets just right. This is why rainbows are seen in the late afternoon or early morning, never at high noon. The rainbow itself is merely an optical illusion, so it doesn’t matter how quickly or in what direction the droplets are moving. The light refracts (bends) inside the droplet making the rainbow.

Fuller rainbows can be seen from above (in an airplane) or you can try it with a garden hose, spraying fine water droplets below you.

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