Stephen Bay's Photography Blog

How to Predict Rainbows and Plan Photographs

Rainbow over downtown San Diego

Here’s a recent photo I made of a rainbow above San Diego’s downtown. I’ve been trying to get this image for a while now, probably a year or more. But I always kept missing. I would go to a spot where I thought a rainbow would appear and wait but I had no success.

Rainbows are difficult to photograph in San Diego because it doesn’t rain much and when it does rain you need to be on the edge of the cloud system in order to catch the sun shining through. The clouds can move rapidly and hence you have to predict where they’re going to be and get in position well ahead of time. If you wait until you can see one before heading out with your camera, the clouds may be gone by the time you arrive at your shooting location.

I made a number of attempts but I wasn’t having any success. So I did some research into how best to photograph them and I’ll share here what I found.

Where does a rainbow appear?

A rainbow forms when light rays enters rain droplets which then refracts the light into different colors like a prism. The Wikipedia article on rainbows goes into more detail on the physics but from a photography perspective, the important thing is noting that

  • A primary rainbow appears at an angle 42 degrees away from the sun. If you draw a straight line from the sun to your position, the rainbow will appear in a 42 degree arc around that line.
  • A secondary rainbow appears at an angle 53.5 degrees around the line from the sun. This secondary rainbow is usually fainter and may be harder to see but it should always be present.

Rainbow over downtown San Diego Image from Cmglee/Wikipedia

The above wikipedia image shows the arc of the primary and secondary rainbows in relation to the viewer and the sun. The primary rainbow occurs from a single internal reflection in rain droplets and the secondary from a double reflection.

Because of this angular relationship, the location of the rainbow is deterministic (if it does appear). This means that we can predict the rainbow’s location for a specific camera and sun position. In turn, this helps us plan an appropriate photographic composition.

Planning a composition

To plan ahead, you can use a website like suncalc to find the location of the sun at any given time. It shows both the azimuth (compass direction) and altitude (angle in the sky). From this and the viewer’s location, the position of the rainbow is completely determined. However the exact position of the arc in the sky and where it touches the ground can be a bit complicated to calculate so here are some rules of thumb:

  • The rainbow appears centered directly opposite the sun.
  • The higher the sun, the lower and flatter the rainbow will appear.
  • If the sun is higher than 42 degrees you wont see a (primary) rainbow at all.
  • The rainbow’s arc will be the largest and highest when the sun is at the horizon (i.e. at sunrise/sunset). The top of the rainbow’s arch will be 42 degrees in the sky and will touch down 42 degrees on either side of the center.
  • When the sun is higher in the sky, the touch down points are closer to the center.

The following animated gif shows how the rainbow’s position changes over the course of an afternoon (assuming there is rain and direct sunlight for it to appear). It starts with most of the rainbow underneath the horizon (i.e. not visible) and then rises in the sky until it is nearly a half circle.

Rainbow over downtown San Diego Animated graphic showing the location of a rainbow in the sky depending on the time of day. This is based on the sun’s position as viewed in San Diego on 11/8/20. Graphic based on screenshots from PlanIt.

If a more precise position is needed, especially in relation to landmarks, I use the PlanIt app which shows where the rainbow will be on both an overhead map and on a simulated camera view. The camera view takes into consideration the focal length of your lens and also displays any buildings or other structures that you add. For example, the two screenshots below show my setup in PlanIt for the rainbow photo.

PlanIt’s overhead view of the rainbow position PlanIt’s simulated view of the rainbow as it would appear in the viewfinder

PlanIt provides two views of the rainbow position: (left) an overhead view on top of a map, and (right) a simulated view showing what the camera viewfinder would see with any landmarks that have been entered by the user.

Where is it raining?

For a rainbow to appear you need rain of course.

You can check the hourly forecast on any weather site and use that to plan where you should go. Based on this information you could head out during a period when the rainfall should be intermittent and then wait. However, I found this not particularly effective, the hourly forecasts are not very accurate and it could just leave you sitting around for hours with no action.

Instead of an hourly forecast, I like to look at the weather radar. You can find this on some websites (just google “weather radar”) or on phone apps like My Radar (free) or Radarscope (paid). Most of the time they animate the radar and show the previous hour with a projection into the next hour. So you get much more granular and real-time view. You can see how the rain is moving and then decide where to go based on where you think the rain will be in the future.

Doppler Radar view of rain clouds

Screenshot of the My Radar app showing location of rain (green and yellow regions). The slider controls the time and reveals how the rain is moving.

This gives you only a small amount of lead time, so you should be ready to go out the door immediately. You probably also should have a composition in mind so that you when you arrive you can just setup and shoot. For my shot, I had about 2 minutes after I arrived to get the rainbow before it started to fade.

Tips for shooting

When trying to get a rainbow photograph, you should have a few places in mind as shooting locations. At each spot, scout the location ahead of time and make note of your desired composition and shooting direction. It will be easier if you have some leeway in where the rainbow can appear.

On a day where there is forecasted rain, check to see where the rainbow will be at a few times during the day and compare it to your list of spots. If there’s a match and the weather conditions start looking promising (i.e. rain clouds breaking up with sunlight) you will know exactly where to go and can head out with minimal delay.

During shooting, you can make the rainbow pop and appear stronger by using a polarizer. You have to rotate the polarizer to maximize the effect of the rainbow. However, if you’re not careful you can also eliminate the rainbow if you turn the polarizer to the wrong angle. So you have to look through your viewfinder and stop turning the polarizer when it pops the most.

impact of polarizer A polarizer can make the rainbow appear stronger or eliminate it completely.

Also if it’s raining at your spot, you should periodically check the front of your lenses for water droplets and wipe them off as they accumulate.

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