Clouds are collections of water droplets or ice crystals suspended in the atmosphere.
They are classified by a system based on Latin words describing their height and appearance. Cirrus (hair), cumulus (heap), stratus (layer) and nimbus (rain bearing).
Low clouds are up to 2.5 kms altitude, middle are 2.5 to 6 kms altitude and high clouds exceed 6 kms in altitude.
There are ten main types of cloud:
1. Cirrus - high cloud, with white wispy tufts made of ice crystals. No rain.
2. Cirrocumulus – high cloud, with small white, puffy ripples made of ice crystals. No rain.
3. Cirrostratus – high cloud resembles a thin transparent veil or sheet which covers large areas of sky. No rain
4. Altocumulus – middle cloud, with white ripples and dark shading. Possible light showers.
5. Altostratus – middle cloud, made of ice crystals and water droplets. Grey sheet- like appearance. Rain or snow.
6. Nimbostratus – low cloud, thick dark flat and uniform. Heavy rain or snow.
7. Stratocumulus – low cloud, appears as series of grey/white rolls with blue sky between. Drizzle.
8. Stratus – low cloud, appears horizontal, grey and constant cloud mass. Drizzle.
9. Cumulus – low cloud, with dark flat base and puffy white rolls or towers on top. Small cumulus clouds are associated with fair weather. Larger with rain.
10. Cumulonimbus – large low clouds, with flat, dark bases and huge towers which may reach heights of over 20,000 metres. May herald thunderstorms, heavy rain, hail and high winds.
Clouds usually appear white and opaque because of the effect of light. Sunlight appears as white light but it consists of the seven principle colours of the visible spectrum. Clouds contain an immense number of water droplets or ice crystals which reflect and scatter the different wavelengths of visible light uniformly, producing intact reflected white sunlight.
When clouds become high or thick enough to prevent sunlight from passing through, they creates a shadow, and this gives the cloud a grey or black appearance.
At sunset or sunrise when the sun is low in the sky, the sunlight must travel a greater distance through the atmosphere. The shorter wavelength colours are scattered out, leaving the longer wavelength colours of red, orange and yellow predominating when the sun is near the horizon. This light is reflected by nearby clouds producing the red, orange, yellow hued clouds that we associate with sunset and sunrise.
One of the most unusual cloud colours is the greenish tinge often associated with the deep, tall cumulonimbus thunderclouds that produce hail. The reason for this appearance is not fully understood. There are a number of theories, one is that the high water density of the clouds reflects blue/green light and another is that the reddish sunlight of a low sun shining on the bluish light below the cloud produces the green colouration.