Clouds turning pink is a fascinating atmospheric phenomenon caused by the scattering of sunlight. When certain conditions are just right, clouds can take on a pink or reddish hue that stands out brightly against the blue sky. While pink clouds may look unusual, they have a simple scientific explanation. Understanding what makes clouds turn pink also provides insights into how sunlight interacts with the atmosphere. This amazing sight is an example of nature’s beauty and the wonders of meteorology.
What Causes Pink Clouds?
For clouds to turn pink or red, a few specific things need to happen:
- The sun must be relatively low in the sky, close to the horizon.
- There must be high clouds present in the upper atmosphere.
- The high clouds must consist of ice crystals.
- The lower atmosphere must have high humidity.
When these conditions are met, sunlight entering Earth’s atmosphere is scattered in a particular way that preferentially removes green and blue wavelengths of light. This leaves more red light to shine through, causing the clouds to take on a pinkish hue.
Low Sun Angle
The sun’s position in the sky has a strong influence on the colors we see at sunrise and sunset. When the sun is high overhead, its light passes through less atmosphere and scattering is minimized. But when the sun is near the horizon, its light must travel through more air. This leads to dramatic scattering effects that cause long reddish and pink shadows during sunrises and sunsets.
The same principle applies to pink clouds. For the clouds to become pink, the sun must be low in the sky so that its light encounters enough atmosphere to scatter significantly. That’s why pink clouds are usually only seen in the hours around sunrise and sunset, especially at higher latitudes during winter when the sun stays lower in the sky all day.
High Clouds
For pink clouds to form, there must be clouds located high in the upper troposphere, typically 6,000 meters (20,000 ft) or more above the ground. Being so high puts them in just the right position to catch the reddened sunlight during sunrise or sunset. Any clouds lower in the atmosphere would be in shadow and not illuminated by the colored light.
High clouds may consist of ice crystals or supercooled water droplets depending on the temperature. Cirrus, cirrostratus, and high altitude cirrocumulus clouds are the types most likely to take on a pink coloration.
Ice Crystals
Ice crystals in the high clouds are especially important for the creation of pink clouds. As light enters the upper atmosphere and encounters ice crystals, it is refracted and scattered. Shorter wavelengths like blue and green are scattered most strongly, while longer wavelengths like orange and red pass through more easily. This process filters out greens and blues from the sunlight before it reaches the clouds, leaving them awash in red and pink hues.
Water droplets in the high clouds can also cause scattering, but they are less efficient than ice crystals. So clouds with a mix of supercooled water and ice crystals, like cirrocumulus, are especially prone to turning shades of pink and red.
High Humidity
An atmosphere with high humidity in the lower troposphere can enhance the pink cloud effect. The abundance of water vapor leads to additional scattering of sunlight as it passes into and out of the lower atmosphere. This reinforces the reddening and pinkness of the light reaching the upper clouds.
Therefore, the most vivid pink clouds tend to occur when high cirrus clouds are present above a humid lower atmosphere. The combination of ice crystal scattering high up and water vapor scattering down low maximizes the impact on the color of sunlight hitting the clouds.
Optical Scattering Effects
The overall process that causes clouds to turn pink is called scattering. As sunlight enters Earth’s atmosphere, it encounters gas molecules, water droplets, ice crystals, and other particulates. These particles scatter and diffuse the white sunlight, altering its color before our eyes perceive it.
Different colors of light are affected in different ways by scattering. Here are the key principles:
- Rayleigh scattering – Shorter wavelengths like blue and violet are scattered most.
- Mie scattering – Longer wavelengths like orange and red are scattered least.
- More scattering overall – Sunlight appears redder and pinker.
- Less scattering overall – Sunlight appears bluer and whiter.
In the case of pink clouds, the combined scattering effects of ice crystals, water vapor, and the low sun angle remove much of the green/blue light. What remains is the long wavelength red light that can illuminate the high clouds in dramatic shades of pink and red.
Rayleigh Scattering
Rayleigh scattering describes the way that light scatters off air molecules and smaller particulates that are much smaller than the wavelength of light. Blue and violet wavelengths are scattered much more strongly than long red wavelengths under Rayleigh scattering. This phenomenon explains why the sky normally appears blue – the blue components of sunlight are scattered in all directions as sunlight passes through the atmosphere.
In situations with an abundance of Rayleigh scattering, such as when the sun is low on the horizon, shorter blue wavelengths are scattered away even more. This gives sunlight a stronger red appearance by the time it reaches our eyes or illuminates clouds high in the sky.
Mie Scattering
Mie scattering occurs when light encounters larger particles similar in size to its wavelength, such as water droplets, dust, and smoke. Mie scattering shows less wavelength dependence than Rayleigh scattering. Long red wavelengths are not scattered quite as freely, so the net effect is less reddening of the remaining light compared to pure Rayleigh scattering.
But Mie scattering does reinforce the tendency for shorter blue wavelengths to be scattered away more easily. So it contributes to the overall reddening effect that creates vivid red and pink clouds at sunrise and sunset.
Where Pink Clouds Occur
Pink clouds can happen anywhere in the world when conditions are right, but they tend to be most common and vivid in certain locations:
- Higher latitudes – Longer sunsets and sunrises increase scattering effects.
- Tropics – Abundant high cirrus clouds are present.
- Polluted regions – More particulates to scatter light.
- Mountainous areas – Enhanced scattering at high altitude.
- Coastal regions – Humid marine air causes scattering.
Let’s look closer at why these types of locations are hotspots for observing pink clouds.
High Latitude Locations
Places farther away from the equator see the most striking and long-lasting pink cloud sunrises and sunsets. At higher latitudes, the sun remains low on the horizon for a longer period of time around sunrise and sunset each day. This sustained low angle maximizes scattering as sunlight passes through the atmosphere at a slant.
Locations like Alaska, Scandinavia, Canada, and Russia often produce vibrant pink clouds for an hour or more at the start and end of each day in the right weather conditions. The effect is especially dramatic in winter, when the sun barely rises above the horizon even at midday.
Tropical Locations
Tropical and subtropical regions are prime areas for pink cloud formation because the upper atmosphere is full of high clouds. Tropical air is very moist and unstable, promoting the formation of wispy cirrus clouds at high altitudes where temperatures are below freezing.
Places near the equator like Hawaii, Florida, and the Caribbean frequently experience cirrus and cirrocumulus clouds that catch the reddened sunlight during sunrises and sunsets. The abundance of high clouds make vivid pink colors almost a daily occurrence in the tropics when other conditions align.
Polluted Areas
Locations with higher levels of air pollution see more dramatic pink cloud events. Smoke, haze, and anthropogenic particulates increase the amount of Mie scattering that sunlight encounters on its way to illuminating clouds from below. This reinforces the color-altering effects of Rayleigh scattering for even richer red and pink hues.
Urban areas and industrialized regions tend to have more pollutants that intensify sunset colors. Locations like China, India, and Mexico sometimes experience remarkable bright pink clouds thanks to the scattering boost from pollution.
Mountains
Higher elevations also increase the vividness of pink clouds because the atmosphere is thinner. Sunlight has less air to pass through, resulting in sharper scattering effects. And mountains are often situated in ideal positions to catch the low angle light refracting through the atmosphere at sunrise and sunset.
That’s why mountainous regions like the Rockies, Alps, and Himalayas frequently see vivid pink clouds accenting their imposing snow-capped peaks at the start and end of the day.
Coastal Areas
Being situated on the coast also enhances an area’s potential for spectacular pink cloud events. Abundant moisture is evaporated off the ocean surface, resulting in more water vapor in the air. This additional humidity provides more opportunity for scattering as light passes into and out of the lower atmosphere.
Coastal locations from California to New Zealand to Western Europe often experience remarkable blazing pink clouds at sunrise and sunset when other factors like cirrus clouds are also in place.
Special Cloud Variations
While most pink clouds occur at high altitudes, some special cloud variations can take on red and pink hues even when they are lower in the sky:
- Mammatus clouds – Pouch-like protrusions on the underside of a cloud.
- Iridescent clouds – Thin clouds diffracting sunlight into spectra of color.
- Noctilucent clouds – Night-shining clouds forming at the edge of space.
Let’s examine how these unique cloud types can also turn pink at times.
Mammatus Clouds
Mammatus clouds are pouch-like lobes of cloud that hang below the base of a larger cloud. They often form on cumulonimbus storm clouds, giving them a knobby appearance. Mammatus clouds can take on pink, orange, and red hues when they catch the colors of the sunrise or sunset.
Unlike most pink clouds, mammatus clouds are typically relatively low in the troposphere, only about 2-3 km up. But their bulbous shape allows them to be illuminated from below by the reddened sunlight passing through gaps in the atmosphere near sunrise or sunset.
Iridescent Clouds
Iridescent clouds are thin, wispy clouds that diffract sunlight into the colors of the spectrum. As light waves pass through these clouds, the different wavelengths separate into distinct red, orange, yellow, green, blue, indigo, and violet bands of color.
The most common type of iridescent clouds are nacreous clouds that form in polar regions 15-25 km up. But thin patches of cirrus, cirrocumulus, and altocumulus clouds can also diffract light at just the right thickness, causing iridescent patterns including pink and reddish hues.
Noctilucent Clouds
Noctilucent or night shining clouds represent an extreme version of high-level clouds that can turn pink. These clouds form at the boundary of Earth’s atmosphere 50-85 km in altitude – at the edge of space.
Noctilucent clouds still get illuminated by the sun even after sunset since they are so high. Their icy composition scatters light into delicate bluish and pinkish colors that shine luminously long after night has fallen below.
Conclusion
Pink clouds never fail to impress as one of nature’s most beautiful wonders. These colorful cloud formations have a logical scientific origin. But they still retain a sense of marvel and mystery when they illuminate the sky with their dazzling display. Now that you understand the basic ingredients for making clouds turn pink, you can appreciate the next sunset or sunrise of dramatic colorful clouds even more.