On our drive up to begin the new week’s shift, we came across something unexpected part of the way up the Mount Washington Auto Road. Though the summit of Mount Washington was socked in the clouds, in the mountains just below there was a gorgeous rainbow that had just formed. It was by far the most well defined double rainbow I’ve ever seen in my life. Our timing could not have been more perfect, as the rainbow faded only a few minutes after we stopped to admire it.

 

 

Unfortunately, rainbows are a rare occurrence, especially on mountains. They require very specific conditions to be visible. Generally the sun must be behind the viewers, and low in the sky to get the proper angle. Any rain (or another form of water droplets such as sea-spray, mist, or a waterfall) must be in front of the viewer. When light strikes water droplets just the right way, they create the multicolored band we think of as rainbows.

 

 

As light from the sun enters a water drop, it is refracted by the water. The light is then reflected off the back wall of the circular water drop. As it leaves the water droplet it is refracted a second time as soon as it hits the air again. This second refraction allows the light to separate into the colors we expect to see from a rainbow. When the angle of reflection is 42 degrees, we get the classic arc rainbow.

 

 

If the light is reflected twice within the drop, a secondary rainbow can form above the primary rainbow. The rainbow yesterday was a classic example of a double rainbow. I’m looking forward to seeing more exciting weather like this in the weeks ahead.