Pictures of pink skies above New York City have started to appear on social media and users are speculating about why they are happening.

However, this is actually a relatively common phenomenon, particularly over busy cities.

In October 2017, Storm Ophelia turned an afternoon orange in London, prompting a slew of jokes online about the advent of the apocalypse.

Earlier that year, the Chinese city of Shanghai was smog cloaked the city in a magenta cloud.

The reason we see blue light in the sky on a clear day is because blue and violet wavelengths of light are more readily scattered and dispersed when they encounter the molecules and small particles present in our air.

Of the two, we see the sky as blue rather than violet because our eyes are more attuned to that colour.

The extent of the wavelength and the size of the particle it encounters determines the colours we see in the sky.

In the case of the London incident last autumn, Storm Ophelia had picked up dust from the Sahara desert and smoke particles from summer wildfires on the Iberian Peninsula. These were blanketed it over the London skyline, giving the sky a strange colour that day.

This is also why we are more likely to see unusual streaks of colour in the sky at sunrise and sunset.

​“Because the sun is low on the horizon, sunlight passes through more air at sunset and sunrise than during the day, when the sun is higher in the sky,” Professor Steven Ackerman of UW-Madison told the Science Daily website.

“More atmosphere means more molecules to scatter the violet and blue light away from your eyes. If the path is long enough, all of the blue and violet light scatters out of your line of sight.

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“The other colours continue on their way to your eyes. This is why sunsets are often yellow, orange and red.”

In the case of New York, high atmospheric pressure could be containing dust and small particles in the air, increasing the density and effectively serving as a filter.