The sky is usually blue. Gases scatter light in the sky in the same manner as one billiard ball collides with another, and sends it in a random direction. Since blue has a lower frequency, it makes charged particles move faster, which scatters more light. We see more blue than red because blue causes particles to get worked up more than do other colours. Our eyes are also more sensitive to blue light.
Sometimes, you have a purple sky
A purple sky might arise on account of the optical illusion of pink wavelengths illuminating the base of the cloud when the sun is at a low angle. Pink and dark blue combine to make purple. Low cloud, a setting sun and water droplets lead to purple skies after hurricanes. These saturate the air with moisture.
Or it might be because there’s a cannabis farm nearby
A purple sky could also mean that there’s a cannabis farm nearby. This happened in the small town of Snowflake in Arizona in the United States on January 10 2020. It looked like the Aurora Borealis – the Northern Lights – and you could see it from as far as 60 miles away. Resident Cara Smith posted a photograph she had taken on her way to work at 06:30 to social media, where it went viral.
What caused it
As the Weather Channel explained, this was the result of ultraviolet lights used on Copperstate Farms, which grows cannabis for medical purposes. Smith lives two miles away and works there. Ultraviolet light speeds up growth. This reflected off the snow on the ground. Water droplets in the atmosphere dispersed the light, and spread it across the sky. It was also cloudy. Some complained of light pollution while others found it pretty.
All about Copperstate Farms
Copperstate Farms opened in 2017, and grows 70 strains of cannabis for medical purposes. It boasts 40 acres of greenhouses. It’s the largest employer in Navajo County, with almost 300 people working in the greenhouse and a further 50 in other positions. It’s one of the largest cannabis farms in the United States.
It doesn’t even need to be a cannabis farm
The same phenomenon occurred in Charlottetown in Canada in December 2021. And it doesn’t have to be a cannabis farm. There was a purple sky over Trelleborg on the southern coast of Sweden in November 2020. This was because a tomato farm in Gisvlo, a small town 10 minutes east, started using a new energy-saving LED light system. This is good for the plants, and extends the growing season. After complaints from residents, operators turned the lights off between 05:00 and 23:00.