Weatherwatch: When clouds turn green

http://www.theguardian.com/news/2013/sep/29/weatherwatch-green-clouds-tornadoes-hail

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Gathering clouds can sometimes appear to take on a sickly green colour, an effect frequently associated with hail storms. In parts of the US, green clouds can be a warning of an impending tornado; a particularly destructive 1980 storm in Michigan was dubbed the Green Storm because of its distinctive hue. But is the green colour real, and what causes it?

Green clouds were long suspected to be an optical illusion. Colour matching from memory is unreliable, light can change quickly, and people are more concerned with taking cover than taking pictures. In 1995, meteorologist Frank Gallagher decided to settle the matter by driving into the path of a storm in Oklahoma with a spectrophotometer to measure the exact colour of the light. Gallagher was awed as clouds above changed from slate grey to "a bright aqua-green." The spectrophotometer confirmed this observation, but explaining it was more difficult.

One theory says green light is reflected off fields below, but measurements show the colours do not match up. Another theory that clouds act as a backdrop to coloured afternoon light also failed to match observations.

The best theory is that storm clouds briefly act as a filter, removing other wavelengths, so only the green shines through. Computer modelling shows that this is possible with the right combination of water droplet diameter and cloud thickness. Oddly enough, although hail is often associated with green clouds, hail could not produce this effect. So green clouds retain some of their mysteries.

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