Rainbow over Nanticoke, August 7 2008

Last Thursday I stopped at my house across town to mow the lawn. I was in a race against time the whole way there: the radar had shown a storm bearing down on Nanticoke, and it was unclear if I would be able to get the whole lawn done before it hit. I did, but just barely; thunder had been rolling around the sky for the last twenty minutes or so of my task, and the first drops of rain began to fall as I wheeled the mower back to its position of shelter on my back porch.

(The thunder didn't deter the two guys putting on a roof a few houses away. Neither did the rain, or the lightning.)

I changed back out of my lawnmowing gear and got ready to leave. I noticed that while the setting sun was peeking out of the clouds and through the windows on the back of the house, it was still pouring on the street in front.

Oh, boy. Here it comes. Wait for it.

I sat on my rocking bench on my front porch and waited. I didn't have to wait long.

Rainbow over Nanticoke, August 7 2008, 7:34 PM

The houses in my neighborhood are packed closely together, and most of the houses have old-fashioned front porches. You can look to your left and your right and see who is out on the porch at any given time. When I first saw the intense right leg of the rainbow in the North South,* exactly where I expected it, there was just one neighbor out, and I had to yell to him a few times before he understood what I was wildly gesticulating about. But before long there were neighbors popping onto their porches up and down the street, many of them armed with cameras. I may have been the only one using a camera phone.

One of the earliest pictures I took on my camera phone was an almost-identical shot on July 13, 2007. Although this older picture was taken five minutes earlier in the day, the sun was actually much closer to setting in the more recent photo. So the rainbow in the picture from last week is bigger, and its legs meet the horizon more nearly vertically.**

Rainbow over Nanticoke, July 13 2007, 7:39 PM

I drove across town to my mom's house and grabbed my Nikon Coolpix L4. Nine minutes after my first photo the rainbow was still there, though its intensity was starting to diminish, and the secondary bow had vanished.

Same rainbow (in a manner of speaking) as in the first image, August 7 2008, 7:43 PM

Not that this was really the same rainbow. No two observers see the same rainbow. A rainbow is made up of refracted and reflected light that has gotten bent as it made one bounce and two bends on its trip through a spherical water droplet. You are seeing the result of these reflections and refractions, or at least of those that took place in a locus of points where the reflected rays happened to be aimed back at your eyeballs - that is, a circle centered on your head. Each observer will see a rainbow formed from reflections in different droplets. (I don't want to risk saying something completely off-base here, so I'll just direct you to here for further reading.)

The left leg of the rainbow

But having said that, it's now easier to understand why you only see rainbows when the sun is behind you and the water in the atmosphere is in front of you. And why sometimes you can see the rainbow in front of nearby houses and other objects if the rain has just passed through your locality but is still falling over there. It also explains why the legs of the rainbow were now visible...

The right leg of the rainbow

...but the upper reaches were not: the rainbow had grown so large, and the storm so distant, that there were no raindrops to reflect the parts of the rainbow that weren't showing up. The rainbow, from my point of view, was above the rain! (This is more obvious in the photo of the left leg of the rainbow, where falling rain miles to the East could be seen as dark vertical bands that cut off abruptly at the same level that the rainbow ends.)

(Note also that the right leg of the rainbow touches the horizon behind the mountains in the distance, rather than in front of them like in a photo I took a few months ago.)

Colors in close-up

Finally, I took one last shot of the colors of the rainbow in close-up.

If you didn't happen to be in Nanticoke last Thursday, or if you were and somehow missed this - well, now you know when and where to look. Next time it rains and the sun comes out afterwards, go outside and look for the shadow of your head!



*My left-right confusion also applies to North-South. North is where Santa lives, South is where the penguins and Old Ones live. Geez, what could be simpler to remember?

**A rainbow forms a circle of constant size centered on an imaginary point corresponding to the center of the head of the observer's shadow. The higher the sun in the sky, the lower that center point will be, and less of the rainbow will be visible - imagine that the rainbow is a complete circle, but you can only see the parts sticking up above the horizon. Closer to sunset the center point approaches the horizon, so more of the rainbow can be thought of as being above the horizon - and you can see a bigger rainbow. At sunset, the rainbow may be at most semicircular, but...well, go back to the post and read on...