Triple Rainbow

One of the reasons I keep this blog going is the wonderful feedback from you, the readers, including the images some send me.  Yesterday, another one came in:  a very rare triple rainbow.

Here is the picture provided by Anna Jensen-Clem, taken around 3:15 PM Saturday in Kenmore just south of the intersection of 68th Ave NE and Bothell Way.

 You start by noticing two parallel bows than curve upwards to to the right.   These we see all the time.  The brighter one (to the right) is the primary bow and the second one (to the left) is the weaker secondary bow.  If you are very observant you will notice that the colors are reversed on them.

But there is another bow, considerably dimmer, that extends almost vertically from the primary bow, this is the unusual one.   And a nice example of something I have read about, but never seen personally, a reflection bow.  But what is it reflecting off?   Let's figure this out!

First, to get primary and secondary bows you need sun, rain, and the properly geometry.

Looking at the Camano Island radar image at 3:16 PM, we can see the shower in question over the northern portion of Lake Washington.  Got rain!

And the satellite image at 3:15 PM indicated it was mainly clear to the south.  Got sun.

 Now to get the normal primary and secondary rainbows you need the sun behind you and the rain in front of you.  As shown in the figure below, in the primary rainbow light is reflected once off the back of a raindrop and for the secondary raindrop there are two reflections.  That produces slightly different angle of the colors.  The colors are produced by the refraction of light entering and leaving the droplet, with different wavelengths of light (blue, red, yellow, etc.) being bent differently. The center of the rainbow, the center of the rainbow arc, is on the opposite side of the sky from the sun (the antisolar point)

So what about that rare extra bow photographed by Ms. Jensen-Clem?

The sun causing it appeared from the same direction as the primary bow but the bow is higher in the sky, which implies the sun would be coming from a  lesser angle.  This is a reflection bow, in which the sun's light is reflected off a water surface, thus coming from a lesser angle, and then producing a rainbow.  To get the geometry right, you need to have the sun relatively low (something true at 3:15 PM in late November).   Here is a diagram that shows what is going on.  The yellow represents the rays coming directly from the sun and the red the reflected rays. 

The difference in angle causes the rainbow to be shifted up. (see figure)

The water obviously needs to be behind the observer.  So where was the water for the Bothell bow?  Here is a map of the region.  The photo was taken a bit south of "A"--the intersection of 68th Ave. and Bothell Way.   Lake Washington provided the reflection surface..the geometry is just right.

It all fits...

 Finally, in a future blog I will tell you what many really want to know, particularly with holiday shopping season upon us:  how to find the gold at the end of the rainbow.  You also have to master dealing with Leprechauns, something we teach in our graduate classes in the department.