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Sunday, June 28, 2009

rainbow anomaly... from karen

about a week ago, i posted about the rainbow karen and i saw on the day we got together to work on our book proposal. i suspected that what we were seeing was a very rare occurence, but one that was almost impossible to manifest given the conditions. the great thing about having a research assistant is that she checks out my hunches. here's what she sent me regarding our noon-time rainbow:


You were on the right track about conditions for and time of day for a
rainbow...See the link below. An excerpt is below.... Not only did we not
have local,morning rain to create refraction, we saw this rainbow between
12:15 and 1pm, when the sun is highest in the sky. No rainbows are seen
at noon when the sun is directly overhead. Quite the anomaly we witnessed! There is more info at the weather channel website, too.

Earth Science-Rainbow

A rainbow lasts only about a half-hour because the conditions that create it
rarely stay steady much longer than this. In many locations, spring is the
prime rainbow-viewing month. Rainfall is usually more localized in the
spring, and brief showers over limited areas are a regular feature of
atmospheric behavior. This change is a result of the higher springtime sun
warming the ground more effectively than it did throughout the previous
winter months. This process produces local convection. These brief,
irregular periods of pre cipitation followed by sunshine are ideal rainbow
conditions. Also, the Sun is low enough for much of the day to allow a
rainbow to appear above the horizon—the lower the sun, the higher the top of a rainbow.

The "purity" or brightness of the colors of the rainbow depends on the size
of the raindrops. Large drops or those with diameters of a few millimeters
create bright rainbows with well-defined colors; small droplets with
diameters of about 0.01 mm produce rainbows of overlapping colors that
appear nearly white.

For refraction to occur, the light must intersect the raindrops at an angle.
Therefore no rainbows are seen at noon when the sun is directly overhead.
Rainbows are more frequently seen in the afternoon because most showers
occur in midday rather than morning. Because the horizon blocks the other
half of a rainbow, a full 360° rainbow can only be viewed from an airplane.


Walk in the Woods said...


Kathy said...

Or maybe a hotflash, but that's very cool anyway!