Unusual colors of the sky

The unusual colors of the sky are all other than natural blue. Yes, it can be many other colors. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn what unusual colors the sky can be and why sometimes the it turns shades that look more associated like atmospheres of other planets.

Green sky

On the evening of July 6, 2022, the sky over the US city of Sioux Falls turned green. At least one friend probably sent these photos to you on social media.

According to meteorologists, a rare type of hurricane is to blame - a derecho. Hurricanes like this consist of a fast-moving group of thunderstorms up to 400 kilometers long, where wind speeds can reach up to about 200 kilometers per hour.

When there are lots of large water drops in rain clouds, they can scatter blue light. It makes them look bluish instead of dark gray. But when it comes to sunset or dawn, in addition to blue, clouds scatter yellow and red light. When all these shades are superimposed, our eyes see green.

This is exactly what happened in Sioux Falls. Scientists note that green skies are a rare, but natural phenomenon that may indicate the approach of a large hail.

Orange sky

At the end of September 2009, the sky over almost the entire East Coast of Australia turned orange and red.

During the day, the color of the air seemed yellow, but closer to nightfall it turned very red — then, the most spectacular photographs were taken.

The reason for this was the largest dust storm in 70 years. There is very little vegetation in those places, and the soil contains many minerals with iron that give in an orange shade.

This way, the storm lifted mostly orange and reddish particles into the air, the sun illuminated them and scattered its light on them, turning Eastern Australia into a kind of Mars for a while.

Pink sky

In July 2022, residents of Antarctica's polar stations noticed that the sky had turned bright pink.

At this time of the year, it's dark almost around the clock in Antarctica, and only in the middle of the day the sun's rays break through the horizon creating a short twilight — that was the moment the sky turned pink.

According to New Zealand scientists, the eruption of a giant underwater volcano near the Pacific archipelago of Tonga, which happened back in January, led to the glow.

Particles ejected by volcanoes during eruption can persist in the atmosphere for months or even years.Researchers say volcanic aerosols can scatter and refract sunlight during sunrise and sunset so that turn the sky a bright pink or purple shade.

What is the connection between these phenomena?

You may have noticed that in all three cases, the unusual shades of the sky are associated with dawn or sunset, when the sky turns red, and the scattering effect. What is this all about?

Everything around, including light, is made up of particles. Because light behaves like a wave, every particle of light has a wavelength. Our eyes perceive different wavelengths as different colors.

Every particle, both light and atom, 'oscillates'. Different colors of light have different frequencies of oscillation. If the frequency of oscillation of a light particle is close to the frequency of oscillation of the atom it collides with, a resonance occurs and the atom 'reradiates; this light around itself — that's what we call 'scattering' light.

Rayleigh scattering. Wave lengths (below) and the percentage of sunlight particles scattered in the atmosphere. Valerya Milovanova / Windy.app

Atoms of oxygen and nitrogen which make up most of our atmosphere resonate most with blue' and 'violet' light wavelengths, but over the course of evolution, our eyes have not adapted well enough to see monochrome purple. As a result, the sky looks blue during the day.

When the Sun is near the horizon, light has to interact with a much larger number of particles in the atmosphere before reaching our eyes than at noon, when the Sun is directly above us, or at least higher than at dawn and sunset.

Because of this, more light is scattered, and we finally notice how the air scatters particles with 'red' and 'yellow' wavelengths.That way, we see red sunsets and sunrises and unusual colors during rare weather events.

Text: Jason Bright, a journalist and a traveller

Cover photo: Kenny Eliason / Unsplash