Green Clouds

Is there a connection between the appearance of green clouds and hail or torndaoes? There is widely reported phenomena of green clouds which are often associated with severe weather. In South East Queensland, Australia, green clouds are strongly associated with heavy hail in large thunderstorms which are common in the summer months. In the Great Plains region of the U.S. green clouds are associated with storms likely to produce twisters or tornadoes. Green clouds have become strongly linked with severe weather.

The most likely colour of clouds is white. This is due to the random scattering of white light from the sun by small water droplets or ice crystals. Cloud colour can be changed by the cloud being illuminated by light with a colour cast, such as orange to red clouds at sunset. But a less obvious case is probably the cause of many green clouds and that is clouds that are illuminated by light reflected off green vegetation. It is not hard to imagine clouds shaded from direct sunlight by other clouds, but are illuminated by light reflected off large fields of bright green vegetation, such as large corn fields or heavily wooded forest. It is easy to imagine this occurring in the Mid West U.S. with substantial corn fields and relatively common occurrence of tornado producing storms. In this case however the colouration is going to be somewhat even and spread over a large area of cloud, maybe brighter towards the ground. Also the connection between the green clouds and tornadoes is coincidental; the conditions happen to be right for both tornado producing storms and clouds that appear green due to light reflected from green vegetation.

White Cloud
Typical white clouds

However there are many reports of green clouds in large thunder storm systems where the green colouration is well defined, coming from specific areas of cloud, and in manner that it is unlikely caused by ground reflection.

Greenish Cloud
Cloud with a greenish shade along the edge. Note the redish backlight. Photo by Dan Wharton, all rights reserved.

A study of thunderstorm clouds in 1995 and 1996 using a device to accurately measure the colour of the clouds did indeed confirm that many thunderstorms produce clouds with a distinct green hue. However it also found that the perceived colour varied dramatically with the measured colour, and that the actual colour varied from greenish to blue and yellowish colours. This is easy to believe as the human perception of colour is greatly influenced by surrounding colours and the intensity of the light.

This study also proposed an explanation and used a simple model to compare the theory with measurements. A good agreement was found supporting the explanation. The idea is that water is blue because is absorbed red light. If a thunderstorm contains enough water and is illuminated by sunlight which is reddish because the blue component has been scattered, such as at sunset, then the absorption of red light by the water will result in a green colour.

Blue Cloud
Storm cloud with a blue colour

Using this explanation green clouds are likely when thunderstorm with heavy rain are strongly illuminated from behind by reddish sunlight, such as at sunset. The high water content of the cloud absorbs red light, resulting in a green colouration. The studies model also predicts that the colour will be blue-green for larger rain drops, and yellow-green for smaller.

This explanation does not require hail in the cloud. However it does require a very high water content which may only occur in thunderstorms which are likely to also produce hail. Thus in SE Queensland it may be that storms that produce hail also appear to have green clouds, but the colour is coincidental and not due to the hail.

Colorimetric Observations of Green Thunderstorms and Their Implications for Current Hypothese - Frank W. Gallagher III - 22 October 1998

Blue Cloud
Storm cloud with a blue colour and blowing dust with a strong down draft

Responses to a posting on green clouds on an atmospheric optics email list

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