Jennifer’s Tips for Chasing Moonbows

I still photograph the Yosemite moonbows quite a bit, but often don't decide whether to go until the last minute with work, weather, and the late drive home. Saturday, June 2 is the next predicted date, but I have not decided whether I'll go that night or not. Water levels may be dropping off some by then, but with the full moon being so early in June this year, hopefully it will still work. I anticipate June likely will be the last opportunity for 2012, though, as I cannot imagine the water levels will make a July attempt worth the trouble unless the park gets a significant amount of late June rain.


I should have added that I fear the days of having the moonbow nearly to myself are behind me. Last weekend, there were easily over 100 people in the viewing area for the lower falls, and probably at least that many spread out along the bike path near the chapel for the upper falls. At the lower falls, we ended up sitting on the ground in front of people standing, which was okay since the ground wasn't wet and since Robert and I can still manage to get down to and up from the ground at our ages. However, that is not a good solution for many club members, and having dry ground at the lower falls at moonbow time is quite unusual. If there is a group of any size trying to stay together, they either need to show up extra early, or try the upper falls as the probable better option.



Just what is a moonbow, anyway?

A moonbow is a lunar rainbow created by the refraction of bright moonlight off spherical water droplets at an angle of nearly 42 degrees.

Where can I find a moonbow?

Yosemite National Park’s Yosemite Falls is the best-known local location for moonbows. Other popular locations for moonbows are Cumberland Falls in Kentucky and Victoria Falls in southern Africa. For those who cannot travel, apparently it is possible to create a moonbow with spray from a garden hose, although I have not attempted that.

When does the Yosemite Falls moonbow occur?

The moonbow can occur any time the conditions are favorable (darkness, clear sky near the moon, bright moonlight, ample water/mist, and proper geometric angle). A team of astronomers and physicists at Texas State University in San Marcos, Texas created a program to predict the occurrence of the Yosemite Falls Moonbow, assuming favorable conditions. For their predictions, see It appears that the predictions are slightly conservative. Others have photographed the moonbow a few days before/after the dates predicted.

*** Unfortunately, the low water levels this year have been to the moonbow’s detriment. I had very little refraction in my photos taken on the night of May 31 (although on the positive side of things, the camera and I both stayed dry, even from the viewing area footbridge where it normally is like standing in a shower this time of year.) There are predictions given for June 28 through July 1, 2007, but conditions are not looking favorable for those dates unless the top of the falls receives a lot of rain near that time.

I went to the Lower Yosemite Falls viewing area at an ideal time, but I didn’t see anything that looked like a moonbow. What happened?

Possibly a couple different things went wrong. First, were you looking in the right place? The moonbow forms in different locations in relation to the falls at different times and on different dates. When there is a high arch across the falls, it is easier to see than when it has dropped down into the rocks at the base of the falls or when it forms to the right of the falls instead of arching across the water from the viewer’s left to right.

That is another reason to check the Texas State predictions. They not only give the times, but they advise whether the moonbow should form to the left, right, or in the falls at a particular time. Also, the moonbow is yet another prime example of the camera and the eye seeing differently. The long exposure on an SLR camera will capture the colors, but the human eye has less color sensitivity in dim light, so most people see the moonbow as a white, gray or silver arch rather than full color.

How can I get a good moonbow photo?

Unless you can hand-hold a camera steady for 30 seconds or longer (good luck!), use a tripod. If you have a remote or cable release for the shutter, use it. Any camera movement likely will blur the image, and redoing 2+ minute exposures is time-consuming. You also need a camera that will allow you to make exposures of 30 seconds or more. Set both the camera and the lens to manual. (Take a deep breath, the manual settings are not that scary!) Turn the lens focus to the infinity setting (a sideways 8 on most marked lenses, usually reached by turning the focus ring most or all the way to your left). Set the camera’s manual setting to bulb. Find and enable the camera’s mirror lockup setting. If shooting digital, you may want to enable long exposure noise reduction as well..

A small flashlight can be useful to check the camera and lens settings (and light the pathway on the walk to the falls), but compose the image in the viewfinder first. You won’t see much of anything in the viewfinder right away if you’ve been looking at the light from the flashlight!

Chose the ISO setting (lower is less noisy, but requires longer exposures) and bracket exposures. I generally check focus and composition at ISO 800 or 1600 where I can make a 20-30 second exposure, then reduce the ISO when I am ready to take the actual photo (if it isn’t too wet to make exposures of over 30 seconds in length).

At ISO 100 and f/4, I start with an exposure of about 2 minutes and bracket up/down from that. Go early or at the beginning of the predicted times. The moonbow at the lower falls drops down and gets harder to photograph as the hour gets later. If it is too wet, try moving out to Cook’s Meadow and photographing Upper Yosemite Falls instead, as moonbows also can form there. Good luck!

Jennifer Doerrie