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RAINBOW TIDBITS

METEOROLOGIST JEFF HABY

a. A rainbow will not be seen by a surface observer if the sun is greater than 42 above the horizon. Rainbows are most common in the last 4 hours of sunlight due to this time having a good combination of rain being more likely and the sun angle being lower than 42 above the horizon.

b. Red will be the outermost color and violet will be the innermost color of the primary rainbow. This color scheme is reversed for the secondary rainbow.

c. The secondary rainbow encompasses an 8 arc above the primary rainbow. Thus, the secondary rainbow is bigger in diameter.

d. The secondary rainbow is always dimmer than the primary rainbow. The number of reflections within a raindrop determine if the light that eventually escapes from a drop will be seen as that coming from the primary, secondary rainbow or higher order rainbow. Since reflections reduce the amount of light energy (because some energy escapes through the drop upon each reflection), the primary rainbow will always be the brightest.

e. The triple rainbow (tertiary rainbow) will be on the opposite side of the sky as the primary and secondary rainbow. The triple rainbow is rarely seen since it is even dimmer than the secondary rainbow and is on the same side of the sky as the sun. Most people do not know where to look for the tertiary rainbow. It is erroneously thought that the third rainbow is above the secondary rainbow since the secondary rainbow is above the primary rainbow.

f. Segments of a rainbow will often be brighter than other segments. The part of the rainbow that is near the horizon tends to be brighter than the top of the rainbow since there are often more and larger raindrops for the sun's rays to travel through near the horizon. Cloud shadowing will also result in some parts of the rainbow appearing brighter than other parts will. The rainbow will be absent in regions sunlight or raindrops are absent.