..Adults, ..Children, ..Teens, Environment, Myths & Facts 
2 Comments 2,099 views Why are clouds white in colour?
Clouds may appear to us in various shapes, but most of the time, the clouds we see are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light.
Clouds will appear dark or gray when they are in the shadow of another cloud or when the top of a cloud casts a shadow upon its own base.
The darkness of a cloud also depends on the sky in the background. A cloud will look darker when it is surrounded by a bright sky and lighter when it is in front of darker sky. A dark cloud does not always mean that it will rain.
More often, the reason why we experience dark rainy days is because clouds are blocking the sunlight. Some of the brightest and purest light can be observed when dark clouds “break apart” and sunlight filters through.
Source:http://sci.odu.edu/sci
For more information on clouds, you can refer to these books!
All Rights Reserved, Tarrytown, N.Y. : Marshall Cavendish Benchmark, c2010
Cloudy Days by Trudi Strain Trueit
Call Number: JP English 551.57 TRU
Publisher: Tarrytown, N.Y. : Marshall Cavendish Benchmark
View item availability here
All Rights Reserved, Minneapolis, Minn. : Lerner Publications Co., c2010
It’s Cloudy Today by Kristin Sterling
Call Number: JP English 551.57 STE
Publisher: Minneapolis, Minn. : Lerner Publications Co
View item availability here
All Rights Reserved, Mankato, MN : Creative Education, 2011
Clouds by Bill McAuliffe
Call Number: Y English 551.576 MAC
Publisher: Mankato, MN : Creative Education
View item availability here
Posted By:
Ms Chen Wanying
Children’s Librarian
Children’s Services
The reason why clouds appear white in the daytime is the same reason why the same clouds appear reddish during sunrise & sunset. The size of the water molecules (H2O) that constitute Earth’s clouds are comparable to the wavelengths of the incident (ie. incoming) light, hence Mie Scattering comes into play. As Mie Scattering operates somewhat independently of visible wavelengths, all wavelengths of the incident light are *equally* scattered by the cloud’s water molecules.
If the incident light appears white (ie. consisting of all 7 wavelengths of the visible spectrum), the clouds will scatter all these 7 wavelengths & thus appear white to our eyes. If the clouds are large & dense, much of the incident light is scattered back to the direction where it originated, hence storm clouds & the the bases of towering clouds appear dark.
During sunrise/sunset, the incident light has to pass through a thicker layer of upper atmosphere (due to Earth’s curvature) before reaching the clouds. Since the atmosphere’s air molecules (eg. nitrogen, oxygen) are dimensionally comparable to the visible wavelengths of light, another physical process called Rayleigh Scattering takes place.
In contrast to Mie Scattering, Rayleigh Scattering is much more dependent on wavelength — the intensity of scattering is inversely proportionate to wavelength to the power of 4. Therefore in a thicker atmosphere, higher amounts of the shorter wavelengths (eg. blue, violet …) are *selectively* scattered away out of our view. What we end up seeing is the remaining longer wavelengths (red, orange) of the light. (This is why the sunrise/sunset sky looks red.) When this same reddish incoming light then passes through the clouds, the clouds continue to scatter all wavelengths equally (remember Mie Scattering) — except that all of these wavelengths are by now mainly red. Hence sunrise/sunset clouds look red.
As such, assuming you have a giant floodlight that shines purely green light, your water molecule (H2O) clouds will appear green as viewed from the ground during the day. Or if your planet has predominantly nitrogen dioxide (NO2) clouds & you shine white light on it, these aerosol clouds will appear reddish-brown in the daytime. And if you were to shine green light on the same NO2 clouds, they will appear blackish. When there is no light source (from sky or ground), all types of clouds possess the same absence of colour (ie. black), & thus appear invisible to the human eye in an equally black nightsky.
For the “kids at heart”, the physics of Rayleigh Scattering & Mie Scattering is summarized at: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
For younger kids: http://io9.com/5583751/why-are-clouds-white
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/opt/mch/sct.rxml
Hi Pat,
Thanks for enlightening our readers with your explanation! Our readers and I have learnt a lot about Rayleigh Scattering & Mie Scattering from the illustrations given above!