Tuesday, April 22, 2014

Why are there double rainbows?

There seems to have been an unusual number of spectacular double rainbows around the Northwest during the past week, and several have you sent me some stunning pictures.  Let me show you a few samples.

 Picture Courtesy of Tom Kreyche from Capitol Hill, Seattle

Pictures courtesy of Seelye Martin in Laurelhurst, Seattle

Why TWO rainbows?  Most of the time only one is clearly apparent.

As many of you know, rainbows result when light from the sun enters a raindrop and experiences a reflection at the back of the drop and heads back to the observer (see schematics).  Sunlight has all wavelengths of visible light, from short wavelengths (purple and blue) to longer wavelengths (red and orange), with yellow and green in between.  As sunlight reaches the raindrop it is refracted (bent) upon entering the drop, reflected off the back of the drop, and then refracted again when leaving.  The interesting thing is that the amount of bending during refraction depends on wavelength, with longer wavelengths (e.g., red) bent less than shorter wavelengths (e.g., blue).  This is called dispersion, which results in the sunlight being broken up into a spectrum of colors:  the rainbow.




But sometimes, there are TWO reflections in the back of the raindrop, which produces another rainbow, with the colors reversed (see figure below).  This is the secondary bow, which is generally fainter than the primary bow.  Look at the pictures  above; can you see the reversal of colors?  Red is at the top of the primary bow, but at the bottom of the secondary bow.


Quiz time....is this picture of a double rainbow below real or fake?  The answer: FAKE.  No color reversal between the bows. Someone got carried away with Photoshop.


Spring is rainbow season here in the Northwest.  Why?  Lots of instability, so there are plenty of cumulus clouds and their transient showers. The showers are followed by sun, which helps produce the rainbows. Classic spring showers and sunbreaks!  A winter storm with a continuous cloud deck without breaks is no good for rainbows.

Why is it very unstable this time of the year?  Because  instability is associated with a big change in temperature with height and spring has this in spades.  The sun is strong and warms the surface, while the atmosphere aloft has not warmed appreciably yet.

There is one question about double rainbows I can't answer:  Are there two pots at both sides?  I will leave this important question to others more qualified to deal with it.


Come to think of it, maybe I can answer the question.  This is my kind of gold.  And they make it in Ellensburg!


Climate 101 Luncheon on Monday in Seattle

If any of you are interested, I will be participating in a Climate 101 luncheon sponsored by the Seattle Chamber of Commerce and mc'ed by Jeff Renner of KING TV.   There is some space left.  For more information go here.

9 comments:

richard583 said...

.. to make us smile. (not forgetting.)

A. Mekvold said...

There was a interesting rainbow today in Vancouver WA. It was really low to the ground, and really wide as a result. I've never seen one so close to the ground before. (sorry if this is a double post!)

JewelyaZ said...

This is my absolute favorite rainbow and other-bows page... the kids and I use this as a reference all the time.

Atmospheric Optics rainbows and other pages covering things like moonbows and rarer forms.

takadave said...

I'm feeling really stupid this morning. I don't see how the Wadsworth illustration explains the color reversal in the second rainbow.

Tim Burris said...

@takadave: I'm in the same boat. I understand conceptually that there are two reflections but afaict that picture explains the effect by showing two suns.

I've also been wondering about the cause of double rainbows lately. I never saw one as a kid but in the past few years any time I see a rainbow it's double. I was hoping for a confirmation or debunking of this anecdotal evidence of a change in atmospheric conditions. Is is because Seattle is further north than Portland where I grew up?

DEORTMAN said...

The double rainbow from Ballard was spectacular. In particular, the bottom violet was very visible. In fact below the main rainbow band there appeared a space and another band of violet (Running R-O-Y-G-B-I-V-space- V).

Any idea what might be causing a second band of violet to appear at the bottom?

DRYSIDECOUG said...

Fascinating explanation of the double rainbow. The most spectacular rainbow I ever saw was one evening over Hayden Lake in Northern Idaho at sunset. It was immense and intense.

Thanks Cliff.

Joe said...

A few weeks ago I was driving along the edge of a heavy rain storm and the bright sun and saw three rainbows. The third was very faint. The passenger in my car could also see it. It was pretty cool.

Chuck D said...

Thanks, JewelyaZ, for the Atmospheric Optics reference. These 2 interactive links show the double-rainbow cause very well.

http://www.atoptics.co.uk/rainbows/ord2form.htm#

http://www.atoptics.co.uk/rainbows/primrays.htm#

Once again, something new and interesting learned through Cliff Mass's Blog!