askST: Why do rainclouds appear dark?

Reader Tan Pin Ho wrote to askST with a meteorological puzzle: "Why are dark clouds dark in colour? All of us associate dark clouds with rain.

"But is it dark because it is too thick and, therefore, blocks the sun?

"Or is it because of some unknown rain-inducing particles?"

Environment reporter Audrey Tan answers.

The answer to this could also unlock the mystery behind the perennial question: "Why is the sky blue?"

Although many people will describe the light from the sun as golden, it is actually a mix of different colours from the visible spectrum.

An object appears to be a certain colour because light of a certain wavelength - which corresponds to that colour - is reflected more than light of other wavelengths.

So for instance, the sky appears blue because the air in Earth's atmosphere reflects blue light in all directions, explains the National Aeronautics and Space Administration (Nasa).

Similarly, small clouds usually appear white as they are very effective in scattering the visible spectrum from sunlight into white light in all directions - even within the cloud, explained Assistant Professor Winston Chow, from the National University of Singapore's (NUS) geography department.

But rainclouds appear dark because they contain more water droplets that absorb, instead of reflect, light. When this happens, less sunlight penetrates the clouds.

"The combination of more light being absorbed by water droplets and less light passing through being scattered thus makes the base of these clouds appear darker," added Prof Chow, who does weather research.

He said there are generally four major cloud types, which are grouped according to the height in which they occur. They are:

- High (cirrus, cirrostratus and cirrocumulus clouds, which occur between 6km and 18km above ground),

- Middle (altostratus and altocumulus, which occur between 2km and 8km above ground)

- Low (stratus, stratocumulus and nimbostratus, which occur less than 2km above ground)

- Low to High. These are also clouds that can extend from 2km all the way until 18km (cumulus and the cumulonimbus clouds)

Cumulus clouds are associated with fair weather, while cumulonimbus clouds are associated with afternoon thunderstorms experienced commonly in Singapore, Prof Chow said.

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