Why People Are Seeing A Purple Sky In Cleveland

This article is dedicated to my wife because I know she likes Prince. When I saw the photograph below tweeted this week by Cleveland news anchor Neeha Curtis, the iconic song Purple Rain came to mind. However, the picture also evokes thoughts of Purple Haze by Jimi Hindrix. Don’t worry there is an explanation for why people in Cleveland might be seeing purple and it has nothing to do with Rock & Roll music even though the Hall of Fame is there.

Urban farming can explain the phenomenon. The Green City Growers greenhouse uses high-efficiency LED lighting in its operations. Douglas Guth wrote on the ProduceGrower.com website, “On cloudy days, growers at Green City Growers in Cleveland kick on LEDs to give their lettuce and basil an extra boost, speeding up the growing process and improving the plants’ look and taste.” Apparently the LED lighting are more energy efficient and more readily taken up by the plants for the process of photosynthesis. According to the Green City Growers website, they produce “tasty, nutrient-rich, pesticide-free produce year-round” including specialty greens, herbes, micro-greens, and lettuce.”

There is some serious science behind this urban gardening technique. A 2019 paper published in the journal Physiology and Molecular Biology found that light spectrum can be an important stimulus in the photosynthetic process. For some reason, I am having flashbacks to elementary school science projects that grew plants under different color lighting or filters. It turns out those projects were on to something. The aforementioned study found that shoot and growth rates for green basil were optimized under purple varieties of light.

The folks at Green City Growers turn on their LED lights early in the morning, particularly on cloudy mornings. Neeha Curtis and other Clevelanders are seeing the interaction of the LED lights with the cloud cover. By the way, there are other instances in which the sky can turn purple. During certain sunsets when the sun is low on the horizon, the process of scattering can cause the vi0let-blue part of the visible spectrum to be the dominant color seen by our eyes.