Why Are Some Clouds Pink?

The color of clouds depends on the angle of the sun’s light, in relation to the Earth’s atmosphere.

Though it is deemed as ‘white light’, the sun releases light in all colors. Once the sun rays reach the Earth’s atmosphere, all color wavelengths in the sunlight scatter in all directions. The shorter wavelengths – blue and violet – scatter first. That said, blue won’t scatter away as quickly as violet, so blue is dominant enough that people can see it. It’s also why our sky looks blue most of the time than any other color.

When the sun is at its peak, the sunlight travels through less amount of atmosphere and has a high angle. That means blue will dominate. During sunset, the sun angle is low and the light has to travel through more amount of atmosphere. This leads to the complete scattering of the blue, violet, and yellow wavelengths, leaving only red and orange wavelengths to be seen. Red color has the longest wavelength, and the last to scatter.

Pink is a shade of red and can typically appear around sunrise and sunset.

Speaking of the pink clouds, the clouds do not become pink; in fact, clouds lack an intrinsic color, they are just reflecting pink/red waves effectively.

Environmental factors such as dust, pollution, haze, and cloud formations may also contribute to different shades of light as the sun rises or sets.

What Causes the Sky to be Pink?

The sky is pink during sunset and sunrise for the same reason that the sky is blue.

Light is white, but when passed through a prism, it gives different colored light waves in the spectrum: red, orange, yellow, green, blue, indigo, and violet (VIBGYOR). Light travels in waves, sometimes as short ones and other times as long lines. Light will continue to travel in a straight line unless there’s interference from atmospheric molecules or a prism.

Atmospheric gases, clouds, and other particles scatter light, and since blue light travels on shorter waves, it causes charged particles to move faster, scattering more light. Thus, we see more blue in the sky. Human eyes are also a bit more sensitive to blue rays, which indeed helps.

As a general rule, the longer the light travels through the sky, the more color it loses. Some colors like red and orange do make it through, however. During sunrise, light has to travel a longer distance before it reaches our eyes. Pink being a shade of red, also reaches us. Thus, the morning sky appears with a blaze of pink or red.

What is a Rainbow Cloud?

If you’ve ever spotted multi-colored clouds high up in the sky, with the sun nearby, then you’ve most likely seen the fairly rare rainbow clouds. Also referred to as “fire rainbows” or an “iridescent cloud”, it isn’t a cloud at all; it’s the mere occurrence of colors in clouds.

Iridescence tends to form in the sky near clouds, like altocumulus, cirrocumulus, lenticular, or cirrus clouds on hot and humid days after a thunderstorm. When optimally aligned, the water droplets or ice crystals in the cloud act as a prism and the resulting refraction is reminiscent of a rainbow. The colors in a rainbow cloud are usually pastel, so you’ll see pink, mint, and lavender hues rather than red, green, and indigo.

The ideal conditions required for iridescence and formation of a “fire rainbow” are very precise.

• The sun has to be at an angle of 58° or more.
• The type of cloud is the second important factor. For example, high altitude, optically thin cirrus clouds. Because the temperature is low where cirrus clouds exist, they are mostly comprised of thin, plate-shaped ice crystals. Once the sun is higher than 58°, the light gets refracted through the ice crystals, producing beautiful rainbow-like colors.
• The angle at which sunlight enters the ice crystal – ideally about 10 degrees from the sun.

These conditions are also the reason why “rainbow clouds” are such a rare phenomenon.

What are Other Colors of Clouds and Their Meanings?

Some clouds appear white, puffy, while others are gray, black, pink, or even yellow. Why is that? What is the meaning of different cloud colors? Let’s find out.

Before talking about the different cloud colors, we need to understand what’s inside a cloud. Clouds are the condensed water droplets that were evaporated from the ground and sea below. Clouds also contain tiny ice crystals. When a cloud gets too dense or heavy, it precipitates and goes back down to the earth in the form of rain or snow, depending on the temperature.

When light is incident upon clouds, it is scattered equally at the same wavelength (Mie scattering), thus remaining white. However, clouds can look gray, black, or other colors at different points during the day.

1. Light Gray Clouds

Though clouds take white shade most of the time on an average day, you might as well see light gray shades in between, especially with puffier clouds. These are gray because of the different amounts of sunlight passing through them.

When the sunlight bounces upward or sideways, the parts of the cloud that receive more light remain white, while those that receive less light remains gray. At times smaller clouds may look gray because they might be hidden in the shadow of a nearby cloud that is bigger and opaque.

2. Blue-Gray/ Black Clouds

Clouds are denser and take blue-gray, dark gray, or sometimes black color when it is about to rain. This is because the clouds the billions of water droplets in the cloud combine together. Because they are denser and bigger, the droplets start absorbing more light but don’t reflect, instead just scatter different colors at once.

The taller and thicker the cloud, the lesser the light reaching its bottom. With zero to less light passing through it, the bottom of clouds appears dark gray or black and typically signals that a storm is brewing.

3. Yellow Clouds

The yellow-ish tint in the clouds is usually due to a combination of the angle of sunlight, Rayleigh scattering, and Mie scattering.

Mie scattering is caused by excess dust in the air, water droplets, pollen, or smoke in the lower atmospheric layers. Mie scattering occurs when the particles that cause the scattering are larger in relation to the wavelength of radiation.

The angle of the sun and Rayleigh scattering also influence yellow cloud formation. Rayleigh scattering is related to the chemical composition of the atmosphere and occurs the particles that cause the scattering of light are very small in relation to the wavelength of the light.

Often during forest fires, yellow smoke emits, which is then directly reflected within the clouds up above, causing a yellowish tint in the clouds.

Excess atmospheric nitrogen dioxide also causes yellow clouds; yellow clouds are more common in urban cities with high air pollution levels.

Unlike the pink, red, or gray clouds, orange/yellow clouds don’t have to be seen the sunrise or sunset; they’re typically the indication of the levels of pollution in the sky.

4. Greenish Clouds

Once suspected to be an optical illusion, green clouds have long been paired with a hail storm or a tornado. It turns out, a greenish cloud is not necessarily a sign of either, despite some interesting folklore. However, the exact cause of green clouds is still subject to debate today.

Many scientists believe that the green tint comes down to the electromagnetic spectrum and the way particles absorb and reflect sunlight. According to one theory, the water droplets in the thunderstorm clouds absorb red light and make the scattered light appear blue. When blue particles or objects are illuminated with red, the net effect is a green shade.

Another theory states that the thunderstorm clouds remove all other wavelengths by acting as a filter, for the green waves to shine. Computer modeling shows that this is absolutely possible with the right combination of water droplet size and cloud density/thickness.

Do Pink Clouds Appear More Often?

Pink clouds are quite common in some parts of the world, while rare or totally absent in others. They appear mostly sunset and sunrise. In certain areas of the world, especially highly polluted metro cities, the clouds will take a brighter pink to red shade – all thanks to the high concentration of aerosols in the atmosphere.

Aerosols are emitted from automotive vehicles, excess dust, sandstorm, or forest fires. So, deserts, and urban areas with higher traffic are likely to witness pink clouds than any other places.

References:

https://www.nationalgeographic.com/news/2013/10/131027-sunset-sky-change-color-red-clouds-science/

https://earthobservatory.nasa.gov/features/Pollution

https://www.sciencedaily.com/releases/2007/11/071108135522.htm