Rainbow clouds, also known as cloud iridescence, are a real phenomenon that occurs when sunlight interacts with water droplets in the atmosphere. The water droplets act like prisms, refracting the white sunlight into the colors of the rainbow. This refraction separates the sunlight into various wavelengths, producing vibrant bands of color in the cloud.
What causes a rainbow cloud?
Rainbow clouds are caused by a combination of the right atmospheric conditions and the right viewing angle. Here are the key requirements for rainbow clouds to form:
- Water droplets – The cloud must contain water droplets or ice crystals. The droplets diffract and reflect the light.
- Sunny skies – Direct sunlight is required to produce the rainbow effect in the clouds. Rainbow clouds do not form on cloudy or overcast days.
- High altitude clouds – Cirrus, cirrostratus, and altostratus clouds at 20,000 ft altitude or higher are typically involved. The high altitude means the sun’s rays hit at just the right angle.
- Angle of the sun – The sun needs to be at an angle of 58° or greater above the horizon. This allows its rays to refract properly in the cloud droplets.
- Angle of view – The rainbow effect is only visible from a specific range of viewing angles on the ground, around 42° from the anti-solar point. If you move out of this range, the rainbow will disappear.
When all these conditions are just right, the sun’s white light will refract into spectacular rainbow colors in parts of the cloud.
What types of clouds can produce rainbows?
Some of the main cloud types that can generate rainbow colors include:
- Cirrus – Thin and wispy high-altitude clouds made of ice crystals.
- Cirrostratus – Translucent, whitish high-altitude veils of ice crystal clouds.
- Altostratus – Grayish or bluish mid-level clouds composed of water droplets and ice crystals.
- Altocumulus – Mid-level rippled clouds made of water droplets and ice crystals.
- Stratocumulus – Low puffy, rippled clouds made of water droplets.
Cirrus and cirrostratus clouds produce the most vivid and colorful rainbow clouds because of their high altitude. Altostratus, altocumulus, and stratocumulus clouds can also generate rainbow colors when conditions are right.
Where do rainbow clouds occur?
Rainbow clouds can be seen all over the world when the right conditions exist. However, here are some locations around the globe where rainbow clouds are more commonly witnessed:
- United States – Montana, Iowa, Idaho, North Dakota, South Dakota, Nebraska, Minnesota, Wisconsin
- Canada – Alberta, Saskatchewan, Manitoba, Ontario
- Europe – Switzerland, France, Germany, Austria, Northern Italy
- Russia – Moscow, St. Petersburg area
- China – Provinces like Inner Mongolia, Xinjiang, Heilongjiang
- Australia – High plains region like Canberra and southern New South Wales
- New Zealand – South Island high country
These regions tend to have the right mix of high-altitude cloud conditions, direct sunshine, and flat terrain needed for rainbow cloud viewing. But they can appear anywhere given the right circumstances.
When are rainbow clouds most likely to occur?
Timing is everything when it comes to catching a glimpse of a rainbow cloud. Here are the optimal times they tend to form:
- Season – Rainbow clouds are most common in winter and early spring.
- Time of day – They tend to appear in the afternoon when the sun angle is ideal.
- Weather – Rainbow clouds require sunny skies. They are almost always seen in cold, clear weather after frontal passage.
Winter and early spring have the right mix of high cirrus clouds, abundant sunshine, and cold temperatures that allow ice crystals to form. The sunny weather after a cold front moves through also produces good conditions.
The angle of sun in the afternoon maximizes the rainbow effect. Midday sun is too high in the sky and morning sun is too low to get that perfect refraction of light.
How long do rainbow clouds last?
The ephemeral nature of rainbow clouds is part of what makes them so magical, yet frustrating to photograph. Some key points about their lifespan:
- Rainbow clouds typically last anywhere from 5 to 15 minutes, although puede durar más tiempo bajo las condiciones adecuadas.
- The lifespan depends on how quickly the high cirrus clouds move and change shape at high altitudes.
- Fast upper level winds will make the rainbow colors dance and quickly distort.
- Slower moving cirrus clouds produce longer lasting rainbow colors.
It takes a delicate balance of cloud height, thickness, and wind speed for rainbow clouds to persist more than 15 minutes or so. Most fade quickly as the clouds morph and the angles change.
Why do rainbow clouds disappear so quickly?
There are a few key reasons why rainbow clouds seem to vanish as quickly as they materialize:
- The high cirrus clouds rapidly change shape and move across the sky.
- The angle of incoming sunlight shifts ever so slightly.
- The viewer’s perspective or angle changes over time.
- Cloud thickness and height fluctuate, altering the light refraction.
With such precise requirements for everything to align just right, it doesn’t take much change for the rainbow colors to disappear. The constantly morphing nature of the high altitude cirrus means the magical sight is often fleeting.
Are rainbow clouds rare?
Rainbow clouds are uncommon, but not extremely rare weather phenomena. Their rarity depends on location and weather factors:
- They are most common at higher latitudes like northern U.S., Canada, and Northern Europe.
- Rainbow clouds tend to be more frequent in winter and early spring months.
- Ideal weather conditions, like sunny skies after a cold front, must be present.
- Viewing angle and position of sun also play a key role.
In areas and seasons where the perfect mix of weather conditions exist more regularly, brilliant rainbow clouds happen a few times a month. Elsewhere they may only be seen a few times a year.
Frequency of rainbow cloud sightings
| Location | Annual sightings |
| Northern U.S./Canada | 10 – 15 days |
| Central U.S. | 5 – 10 days |
| Southern U.S./Mediterranean | 1 – 5 days |
| Tropics | 1 or fewer days |
As the table shows, higher latitude locations tend to see rainbow clouds more frequently each year. Tropical areas only experience them rarely while northern regions can witness 10 or more rainbow cloud events per year.
Do rainbow clouds predict weather?
Rainbow clouds do not directly predict upcoming weather, but they provide some clues about atmospheric conditions:
- Their presence indicates high thin cirrus clouds are present above 20,000 ft.
- The air is humid enough for these icy clouds to form at high altitudes.
- Rainbow clouds mean the upper atmosphere is stable and cold.
- Sunny skies are required for rainbow cloud formation.
So while not a direct weather forecast tool, rainbow clouds confirm the upper atmosphere is cold and clear. Their unique beauty results from a delicate balance of weather needed for these colorful clouds to form.
Are rainbow clouds related to iridescent clouds?
Iridescent clouds and rainbow clouds are very closely related phenomena. They both involve water droplets diffracting sunlight into spectacular colors. The main difference is:
- Rainbow clouds – Colors only visible in certain patches of clouds.
- Iridescent clouds – Entire cloud displays rainbow colors across its full extent.
However, both are caused by nearly identical processes and require the same precise conditions. So iridescent clouds can essentially be thought of as rainbow clouds that completely cover the entire sky!
Conclusion
In summary, rainbow clouds are a very real and scientifically explainable phenomenon. These rare and stunning clouds form when sunlight refracts through icy clouds high in the atmosphere. Their ephemeral nature arises from the precise alignment of conditions needed. But this fleeting rainbow sight serves as a reminder of the beauty that exists in the skies around us.