When I was knee high to a grass hopper I loved to watch clouds. Maybe all children love to play the game of spotting animals in the shapes of the clouds. Bright white clouds on a clear blue sky look wonderful, and even as an adult it is easy to get lost in them.

White Clouds

But why are clouds white? The simple answer is that they reflect very well the light that falls on them. Clouds are illuminated by light from the sun, and light from the sun is seen as white by our eyes, a mix of all the colours of the rainbow - which produces white. Clouds are made up of many small water drops and ice crystals. Light can reflect and scatter so many ways from and in a cloud that when illuminated directly it ends up looking an fairly even and bright white.

White clouds.

Red Clouds

Not all clouds are white. As mentioned above, the colour of a cloud depends on the colour of the light that illuminates it. At sunset or sunrise the colour of sunlight can be yellow to deep red due to the scattering of the blue component of sunlight as the light travels a longer path through the atmosphere.

Red cloud at sunset.


A red cloud at sunset.

Blue Clouds

There are also more subtle ways the colour of a cloud can change such as the colour of the light changing after reflecting off a cloud due to scattering on it's way to the observer, or due to scattering of light by air between the cloud and the observer. In the first case the blue light will scatter resulting in a white cloud taking on a yellow to red tint. In the second case a white cloud will take on a blue tint due to the blue light scattered by the air between the cloud and the observer. This is the affect that makes mountains in the distance look blue.

Grey Clouds

Clouds can also look dark or gray. This can be caused a number of different ways. But it is also due to perception by our eyes. A light gray cloud on a bright white background will look much darker than the same cloud on a dark or black background, in which case it might look white and bright.

A cloud can look dark or gray because it is partially transparent and the blue sky behind it can be seen through the cloud. This will happen in light wispy clouds with little water content, and more often in ice crystal clouds. Ice crystal clouds can spread out more as the ice crystals last longer once they move out

of saturated air (air with 100% relative humidity) due to it taking longer for ice crystals to sublimate (change directly from ice to water vapor), than water drops to evaporate.

The second photo is taken with a polarizing filter which darks the background sky. In both the photos the clouds are about the same colour and brightness, but with a light background they look dark, and with the darker background they look white.

Dark clouds are also caused by shadows by other clouds, or the cloud itself. A large cumulus cloud lit from behind will shade the surface seen by the observer, and most likely shade other clouds. And again, how dark or gray the cloud appears due to the shading will strongly depend on the brightness of the background.

Green and Blue Clouds

Finally there are combination of effects. A cloud can look blue if it is heavily shaded and is illuminated by the scattered blue light of the blue sky.

Probably the most unusual colour for a cloud is green. This is thought to be due to a cloud containing a large amount of water being back lit by reddish light, such as at sunset. Water is naturally blue because it absorbs red. The light falling on the cloud appears red because the blue component has been scattered, and then the red component is absorbed by the water in the cloud, leaving green.

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.

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.

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.

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