Keith Holmes, of Dunedin, asks :-

What causes the colour in clouds, from white to grey to blue-black? Steam in the kitchen is transparent but would it become coloured if in great depth, similar to water? Bath water is colourless but the ocean is green or blue. In both cases are other ingredients involved?

Esther Haines, a physicist at the University of Otago, responded.

If we are looking at clouds from the sunlight side, say from a plane, then they appear white. This is because the light reflected by the cloud has been scattered many times by the small droplets of water that make up the cloud.

If we are looking at clouds from the other side they appear white, grey or black depending on how thick the cloud is and how many droplets there are in each cubic metre of the cloud. A thick cloud with many droplets appears black because relatively little light makes it all the way through the cloud. A thin cloud with few droplets can appear bright white because most of the light makes it through the cloud.

A thick cloud of steam appears white since the water droplets in the steam scatter light back to our eyes just as clouds do when viewed from a plane.

Water is blue. A sheet of white paper viewed through 3m of water appears pale blue. This is because water molecules weakly absorb red light. A glass of water, or even a bathtub full of water, does not absorb enough red light for the blue colouring to be perceptible.

When we look at the ocean the colour we see changes depending on whether the sky is clear or cloudy, whether there are any small particles such as sediment or algae in the water, and the angle from which we are viewing it. If we are looking nearly straight down at the sea, for example, from a plane or a high cliff, then we see the colour of the water, which is blue. If we are looking from a beach then we see light from the sky just above the horizon reflected by the sea. This could be blue, white or grey depending on the weather. Small algae in the water can make the ocean appear green.