Many people claim that the sky turns an eerie yellow or greenish hue before a tornado touches down. This dramatic color change is thought to be a warning sign from nature that severe weather is imminent. But is there any scientific evidence to support this belief? Do changes in sky color really precede tornado formation?
While there are some legitimate reasons why the sky may appear yellowish green before a tornado, this phenomenon does not happen with all twisters. The color of the sky depends on many complex factors like the time of day, cloud conditions, position of the sun, and presence of dust or pollution in the atmosphere. A yellow/green sky color alone should not be interpreted as a sure indicator that a tornado will develop.
What Causes the Sky to Change Color Before a Tornado?
There are a few reasons why the sky sometimes takes on a sickly yellow-green hue prior to tornadoes:
– Sunlight scattering through humid air – One of the most common reasons for the greenish sky is the scattering of sunlight through very humid, hazy air in the lower atmosphere. The light waves interact with water droplets in the humid air to produce longer wavelength yellow and green hues. This effect is more noticeable closer to sunset when the sun is lower on the horizon.
– Cloud conditions – Low-hanging storm clouds present during tornado formation can diffract light in unusual ways, lending a yellow or green cast to the sky. The thick clouds filter normal blue light from the sky allowing green/yellow wavelengths to dominate. Heavy storm clouds coupled with humidity amplify this effect.
– Dust and debris – Tornadoes that form on the leading edge of fast moving thunderstorms sometimes kick up concentrations of dust from the dry ground below. This dust gets mixed into the humid air causing further scattering of green/yellow light.
– Volcanic ash – After major volcanic eruptions like Mt. Pinatubo in 1991, tons of fine ash particles entered the upper atmosphere. This ash circulated around the planet for years causing vivid red and yellow sunsets globally. While less common today, volcanic ash can contribute to yellow-hued skies.
Other Weather Factors that Influence Sky Color
While the conditions above may produce yellowish/greenish skies before some tornadoes, there are many other weather variables that can influence sky color unrelated to twisters:
– Time of day – The sky naturally transitions from blue to pale yellow closer to sunset and sunrise as the sun angles lower across the horizon. This is due to the reddening effect of sunlight passing through the atmosphere at lower angles.
– Cloud coverage – Thick cloud cover filters the sky’s natural blue hue allowing more yellow light to pass through. This can happen even in benign weather conditions completely unrelated to storms or tornadoes.
– Air pollution – Higher levels of haze or smog from industrial pollution can cause a yellowish cast to the sky as particles scatter light. This is more pronounced near heavily populated urban areas.
– Smoke/ash – Plumes of smoke from forest fires or volcanic eruptions can spread fine particulates high into the atmosphere causing longer wavelength yellow/red light scatter.
– Wind direction – Strong winds can carry dust and pollutants long distances distorting sky color over areas far away from the original source.
Studies on Sky Color and Tornado Prediction
Several studies have tried to establish a correlation between sky color and impending tornadoes:
| Study | Findings |
|---|---|
| Smith et al. 2000 | No consistent color change noted before tornadoes in Oklahoma from 1997-1999 |
| Bluestein and Parks 1983 | 18% of tornadoes studied were preceded by greenish sky |
| Monteverdi et al. 2001 | Greenish sky color preceded 35% of significant tornadoes in study |
The findings show mixed results on connecting tornadoes to predictable color changes. While some tornadoes do exhibit a greenish sky beforehand, many do not. Sky color alone appears to be an unreliable indicator of tornado risk.
Other Tornado Warning Signs
Rather than relying on the sky color, there are more definitive things to look for that signal tornado development:
– Wall cloud – Rotation underneath a storm cloud base indicates mesocyclone formation and potential tornadogenesis
– Funnel cloud – The visible cone extending down from the wall cloud marks the tornado itself
– Debris/dust whirls – These small dust devils or debris columns indicate strong circulation
– Hail – Large hail is a signature of powerful updrafts in a storm
– Roaring noise – The tornado’s winds produce loud, freight train-like rumbling as it nears
– Fast moving storm – Storm motion >30 mph raises danger as wind shear increases
Conclusion
While scientific evidence shows the sky can turn green, yellow, or other unusual colors preceding some tornadoes, this phenomenon does not occur reliably. A variety of common weather factors can create similar color changes unrelated to tornado risk. On their own, color shifts should not be assumed to signify an impending tornado. However, in combination with other visual tornado indicators like wall/funnel clouds, hail, and dust whirls, color changes may add confidence a twister is imminent. Those living in tornado prone regions should stay weather aware and monitor alerts to stay safe.