Should you be worried if you see green thunderstorm clouds?

If the sky turns a green colour when a thurnderstorm is approaching, does it mean that we are in for violent weather like tornadoes or giant hail stones? Lots of people associate a green stormy sky with imminent extreme weather conditions. However is this a physically proven reality or just an urban legend?

Sometimes when a storm is approaching the sky turns a green colour. According to popular belief, when the sky takes on a green tinge it is a clue that violent weather is about to erupt. Normally this includes tornadoes or giant hailstones. In regions where tornadoes are a common phenomenon – like in the centre of the United States – the sight of a green sky urges people to get themselves to safety either their basement or an adapted shelter. These associations have been spread as a sort of urban legend, so it is difficult to unravel the false from reality. What do researchers have to say about this phenomenon?

Green thunderstorms is the name given to the unusual storms which begin with a green hue in the clouds. One hypothesis suggests that the ground indirectly gives the clouds this greenish tint by reflecting the surrounding light when the land is covered in green vegetation (G.D Frier, 1992). However, measurements made by spectrophotometer prove this is not a necessary or sufficient condition for the phenomenon to occur. Between 1995 and 1996, American researchers chased these thunderstorms and recorded 15 having a hue considered green by a human observer. They were able to measure the storms’ light spectrum characteristics. Their results showed that the phenomenon is not significantly related to the nature of underlying soil *, but does however originate in the sky.

Researchers took measurements during storms with giant hail and discovered that both green thunderstorms and classic blue-grey thunderstorms produced this phenomenon. Same is true for tornadoes. A sky tinged with shades of green can not predict these two phenomena. So where does this colour come from?

Despite very little research on this subject, the most likely hypothesis concludes that this phenomenon is due to the rays of orange light when the sun is setting or rising. Filtering through the clouds, warm colours are absorbed by water droplets which appears green for an observer under the base of the clouds. Depending on the the size of the water droplets the colour of the sky can vary from blue-green to yellow-green when observed from below. This also explains why green storms are mostly observed in the late afternoon and early evening – the time when storms are most frequent and when the sun is setting.

There must be high cloud thickness and high cloud density for these rays of light to filter through and absorb warm colours in the clouds water partials. This is why this phenomenon is mainly associated with cumulonimbus clouds. The presence of thick clouds must be at the source of the link often reported between a green skies and a violent storm. Although cumulonimbus clouds are more likely to produce green skies when a storm is approaching, it does not mean the thunderstorm will produce extreme or violent weather.

Scientists believe these green thunderstorms may occur far more often than we think. Indeed, most of the time, sub-storm conditions are too dark for human eyes to detect these subtle colours. Contrary to popular belief, storms associated with a greenish sky do not produce tornadoes or giant hail any more so than ‘normal’ coloured thunderstorms without this hue. Green thunderstorms can of course still be violent particularly as they happen when there is thick cumulonimbus clouds! Precautions should always be taken when a thunderstorm approaches – regardless of its “colour” – and be aware of any potential weather warnings.

* For example, green thunderstorms can be found over Oklahoma red-brown lands.

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