White rainbows exist! White rainbows are white in color. They are not classic rainbows: Colors in a classic rainbow are red, orange, yellow, green, blue, indigo, violet (ROYGBIV); the color white does not in a classic rainbow! Let’s look at white rainbows and how they occur.
Dr. MMM of AstroPicionary has taught astronomy to 10,000+ students over 15+ years at a USA Carnegie Level R1 University. Dr. MMM continues to teach astronomy and physics. She authors textbooks in astronomy that are in use worldwide. Here, Dr. MMM discusses an unusual siting seen from Earth – white rainbows.
Fogbow, Sea Dog, White Rainbow, Oh My!
White rainbows, more commonly known as fogbows, are white in color and appear in foggy conditions. To sailors of yore, fogbows are ghost rainbows due to their ghostly appearance in foggy conditions over water. To a British sailor, a fogbow is a sea dog.
The image below shows a fogbow over an atmospheric research tower at Summit Station in Greenland. This fog layer is about 60 feet thick. The primary fogbow is at top, with secondary and tertiary fogbows seen sequentially below.
As seen in this image, a fogbow has an arc or bow shape and appears in foggy conditions. As this bow is white and arches upward from the land, we can see why a fogbow is also known as a white rainbow.
Fogbows are not only above land, but are also above water. The fogbow shown below is just off the Florida coast, in the Gulf of Mexico.
We can see why sailors would call this white arch a ghost rainbow. No unique red, orange, yellow, green, blue, indigo, or violet colors are in this rainbow and conditions are ghost-like due to a wispy fog.
Where to See Sea Dogs
The Arctic and Antarctic are best locations to see white rainbows (a.k.a., Sea Dogs), as air temperatures are often lower than dew point temperatures. The image shown first in this post is a fogbow over Greenland. Greenland lies within the Arctic Ocean and lies east of the Canadian Arctic Archipelago. As seen, this image is a great example of a fogbow.
The second image shown in this post is a fogbow or white rainbow seen off the coast of Florida.
Although Florida is not in the Arctic or Antarctic, Florida weather conditions have air temperatures lower than dew point temperatures certain times of the year. As such, much moisture is in the air, generating fog. As such, Florida is a good location to observe white rainbows.
A third place to see white rainbows is in mountainous regions. Shown below is a fogbow over a prairie and a mountain in Red Rocks Lake National Wildlife Refuge. The road in the image threading through the fogbow is South Valley Road. As shown, ground fog is present.
Over water or over land, white rainbows, sea dogs, ghost rainbows, or fogbows can form. They can also form in cold locations like the Arctic or warm locations like Florida. Let’s look at how these ghost rainbows occur.
How White Rainbows Occur
For ghost rainbows to form, four conditions must be present: Fog, observer observing from anti-solar point, small size of for or cloud droplet, and low angle of sunlight. Let’s look at each of these.
Condition 1 – Fog
One condition for a ghost rainbow to form is fog. When temperatures drop to dew point temperatures, small droplets fall out of the air and form dew on a land surface. When small liquid droplets fall to a frozen surface, they freeze when settling on an icy or snowy surface.
For fog to generate, an ice surface or dewy land surface needs to heat up. For example, early morning sunlight can provide that heat: Sunlight evaporates a surface, which puts droplets into the air. After land or icy surface warms, the moist air above then warms. When air temperature becomes warmer than dew point temperature, fog develops. Humidity in the air has now reached 100%. As shown above in the Red Rocks Lake National Wildlife Refuge image, fog is present just above the ground.
Condition 2 – Observing Toward Anti-Solar Point
Another condition for generation of a ghost rainbow or fogbow is location of observer relative to sunlight: An observer must be facing away from the Sun and facing toward the fogbow. In addition, a ghost rainbow needs to be toward an anti-solar point. The video below from the YouTube channel AstroPictionary explains anti-solar point.
Shown below is a white rainbow over a hill in Twin Peaks park, which is in San Francisco, California, USA. This hill rises over 900 feet from sea level, which is the Pacific Ocean that is less than five miles from the park. Notice the observer’s shadow near the center of this 360 degree fogbow, with the observer’s head pointing in the direction of the anti-solar point. The observer and camera are pointing toward the fogbow and away from the Sun.
Condition 3 – Low Angle of Sunlight
A third condition to form a ghost rainbow is angle of sunlight, which must be lower than about 40 degrees to the horizon. The red arc in the sketch below marks a ~35o angle from horizon to a person’s head with the shadow’s head being at a corner of this triangle’s angle; this sketch is from AstroPictionary that describes anti-solar point, which is the red x in the sketch. In this sketch, the anti-solar point is below the horizon.
Ideal low sunlight angles for both fogbows and rainbows are during early morning or late evening. At these times, sunlight streams in nearer to the horizon. To see a potential fogbow or rainbow, an observer needs to have their back to the sunlight, which is Condition #2 above.
Also needed to see a fogbow or rainbow is Condition #1, which is fog; fog has moisture droplets in the air. As shown in the sketch below, incoming white light sunlight refracts upon hitting a water droplet surface. White light diffracts into colors of different wavelengths, each of which bends at different angle upon refraction. These colors then reflect inside a droplet (i.e., internal reflection) as shown on the far right side of this sketched droplet. These colors then refract out of the droplet to form, in this case, a rainbow.
Condition 4 – Small Droplet Size
A fourth condition to form a ghost rainbow is small droplet size. Small moisture droplets in fog result in formation of fogbows or ghost rainbows. Large water droplets in rain result in formation of rainbows. Both fogbows and rainbows need conditions 1-3 listed above. However, fogbows also need condition 4 to form, which is a small droplet size.
The image below shows fog droplets (left) and rain droplets (right). Fog droplets form on branches, as shown in the left image. Rain droplets form on a window, as shown in the right image. In a comparison, dew droplets in a fog are much smaller than droplets formed by rain.
The sketch under Condition #3 of reflection and refraction in a droplet that results in generation of a rainbow involves a raindrop. Refraction out of a water droplet keeps colors unique, resulting in formation of a rainbow. Smaller sized droplets result in individual colors blending to white within a droplet due to wave interference. Hence, white light streams out of droplets in a fog to generate a fogbow or a faint wispy white color of a ghost rainbow.
How To See Multiple Fogbows and Rainbows
The sketch shown below shows white light from the Sun streaming into a cloud that contains droplets of water. Light refracting into different colors, internally reflects, and then refracts out of these droplets. A net effect is arranging into concentric arcs of color, as shown, with red on the bottom and violet on top. The rainbow with the most intense and brightest colors is the primary rainbow. A less intense and dimmer color rainbow appearing below the primary is the secondary rainbow.
As the sketch shows, sunlight streams in from the left, and the observer has their back to this sunlight. Water droplets in a rain cloud is in front of the observer. The observer is facing toward the rain cloud and thus toward the rainbows.
Although fogbows are mostly white, some may have a red edge and violet edge like that of classic rainbow. Usually, the other colors blend to white. Thus this rarer fogbow has a violet upper edge and red lower edge, with wispy white in the middle. In these rarer fogbows, droplet sizes are larger than in white rainbows that only show white arcs. In a sense, the degree of whiteness indicates size of mist or fog droplet in a white rainbow.
The very first image in this post shows a primary white rainbow above a secondary and tertiary fogbows. The primary is all white and thus must contain small fog droplets. The secondary and tertiary fogbows show violet upper edge arches and lower red edge arches, with white in between. These secondary and tertiary fogbows have larger droplets, in comparison.
White Fogbows VS Colorful Rainbows
As their name implies, rainbows have rain associated with them and fogbows have fog associated with them. Rainbows appear in rainy conditions. Fogbows appear in foggy conditions. Fogbows do not usually appear in rainy conditions. Similarly, rainbows do not usually appear in fogbow conditions.
Besides their names being different, conditions to form fogbows and rainbows are different. Droplets in fogbows are smaller than droplets in rainbows. Similarly, fogbows form from much smaller clouds than those clouds forming rainbows.
A very noticeable difference between fogbows and rainbows is color. Rainbows have unique red, orange, yellow, green, blue, indigo, and violet colors where as fogbows appear mostly white. Hence, fog bows are also knows as white rainbows, sea dogs, or ghost rainbows.
Because of these differences, fogbows are not rainbows.
Conclusion
White rainbows look like ROYGBIV color rainbows, but are mostly white in color. White rainbows are commonly called fogbows. If white rainbows appear over water, they are sometimes called ghost rainbows. The British sometimes refer to these ghost rainbows as sea dogs.
Fogbows are different from colored rainbows. Besides color, fogbows have small fog or mist droplets whereas droplets forming rainbows are large.
Some white rainbows have an upper violet edge and a lower red edge, with white in between. These are rarer. They are sometimes present when multiple fogbows appear consecutively.
White rainbows have special conditions for formation such as foggy conditions, observing when Sun is low in the sky and observing with back to the Sun. These conditions are similar to those for observing rainbows. One difference is size of droplet, which needs to be smaller when trying to observe a fogbow.
When next outside a sunset or sunrise, with sunlight streaming toward fog, look to see if you can find a white rainbow or ghost rainbow. Enjoy!
