|
|
| |
|
|
| |
|
|
|
|
| |
 |
| Visible light is
only a small portion of the full electromagnetic
spectrum. |
The
Color of Clouds
The color of a cloud depends primarily upon the color of
the light it receives. The Earth's natural source of
light is the sun which provides 'white' light. White
light combines all of the colors in the 'visible
spectrum', which is the range of colors we can see.
Each color in the visible spectrum represents
electromagnetic waves of differing lengths. The colors
change as the wavelength increases from violet to indigo
to blue, green, yellow, orange, red and deep red.
As a light wave's length increases, its energy
decreases. This means the light waves that make up
violets, indigo and blue have higher energy levels than
the yellow, orange and red.
One way to see the colors of sunlight is by the use of a
prism. The velocity of light decreases slightly as it
moves into the prism, causing it to bend slightly. This
is called refraction. The degree of refraction varies
with the energy level each wave.
The lowest energy light waves refract the least, while
the highest energy waves exhibit the greatest
refraction. The end result is a dispersion of light into
a rainbow of colors.
Rainbows are partly the result of sunlight refraction
through a rain drop, which acts like a prism. |
|
 |
| A prism will allow
you to see the individual colors that comprise
the source light. In this case, sunlight
entering the prism is divided into the colors of
a rainbow based upon the wavelength of each
component. |
So, if sunlight
is 'white', why is the sky blue?
The atoms and molecules comprising gasses in the
atmosphere are much smaller than the wavelengths of
light emitted by the sun.
As light waves enter the atmosphere, they begin to
scatter in all directions by collisions with atoms and
molecules. This is called Rayleigh scattering, named
after Lord Rayleigh.
The color of the sky is a result of scattering of ALL
wavelengths. Yet, this scattering is not in equal
portion but heavily weighted toward the shorter
wavelengths.
As sunlight enters the atmosphere much of the violet
light waves scatter first but very high in the
atmosphere and therefore not readily seen. Indigo color
light waves scatter next and can be seen from high
altitudes such as jet airplanes flying at normal
cruising altitudes.
Next, blue light waves scatter at a rate about four
times stronger than red light waves. The volume of
scattering by the shorter blue light waves (with
additional scattering by violet and indigo) dominate
scattering by the remaining color wavelengths.
Therefore, we perceive the blue color of the sky.
If the sky is blue, why are
clouds white?
Unlike Rayleigh scattering, where the light waves are
much smaller than the gases in the atmosphere, the
individual water droplets that make up a cloud are of
similar size to the wavelength of sunlight. When the
droplets and light waves are of similar size, then a
different scattering, called 'Mie' scattering, occurs.
Mie scattering does not differentiate individual wave
length colors and therefore scatters ALL wave length
colors the same. The result is equally scattered 'white'
light from the sun and therefore we see white clouds.
Yet, clouds do not always appear white because haze and
dust in the atmosphere can cause them to appear yellow,
orange or red. And as clouds thicken, sunlight passing
through the cloud will diminish or be blocked, giving
the cloud a grey color. If there is no direct sunlight
striking the cloud, it may reflect the color of the sky
and appear bluish. |
|
 |
| In this sunrise
image, the blue sky, yellow Cirrus clouds and
orange Altocumulus clouds result from both
Rayleigh and Mie scattering. Rayleigh scattered
produces blue sky and the color the clouds
receives. Mie scattering is responsible for the
color we see. Even with Rayleigh scattering
taking place in the atmosphere, over one-half of
the sun's 'white' light continues through the
atmosphere reaching the earth's surface. |
Rayleigh and Mie
Some of the most picturesque clouds occur close to
sunrise and sunset when they can appear in brilliant
yellows, oranges and reds. The colors result from a
combination of Rayleigh and Mie scattering.
As light passes through the atmosphere, most of the
shorter blue wavelengths are scattered leaving the
majority of longer waves to continue. Therefore, the
predominate color of sunlight changes to these longer
wavelengths.
Also, as light enters the atmosphere, it refracts with
the greatest bend in its path near the earth's surface
where the atmosphere is most dense. This causes the
light's path through the atmosphere to lengthen, further
allowing for more Rayleigh scattering.
As light continues to move though the atmosphere, yellow
wavelengths are scattered leaving orange wavelengths.
Further scattering of orange wavelengths leaves red as
the predominate color of sunlight.
Therefore, near sunrise and sunset, a cloud's color is
what sunlight color it receives after Rayleigh
scattering. We see that sunlight's color due to Mie
scattering which scatters all remaining wavelength
colors equally. |
|
 |
| A depiction of three
hypothetical waves of light passing through the
earth's atmosphere. A) Sunlight barely enters
the atmosphere with only violet and indigo
colors scattered. B) With violet and indigo
colors scattered first, sunlight penetrates
further into the atmosphere where the greatest
portion of blue scattering occurs. There is some
bending of light by the atmosphere due to
refraction adding some length to the light's
path. Just as the light path begins to leave the
atmosphere the color is predominantly yellow. C)
Greatest refraction and longest light path with
the most Rayleigh scattering. |
The Color of
Perception
Sometimes, under direct sunlight, clouds will appear
gray or dark gray against a blue sky or larger backdrop
of white clouds. There are usually two reasons for this
effect.
The clouds may be semi-transparent which allows the
background blue sky to be seen through the cloud.
Thereby giving it a darker appearance.
A more common reason is the contrast between the
background (blue sky or additional clouds) and
foreground cloud overwhelms our vision. In essence, our
eyes are tricked with our perception of foreground
clouds appearing dark relative to the overwhelming
brightness of the background.
This latter reason is why sunspots look dark. Brightness
of the sun is based upon temperature and a sunspot's
temperature is lower than the surrounding surface of the
sun.
Relative to the surface of the sun, sunspots appear
quite dark. However, if sunspots were isolated from the
surrounding brightness, they would still be too bright
to look at with the unprotected eye. The contrast in
brightness between the two is what causes sunspots
appear dark. |
|
|
 National
Weather Service: The Color of Clouds |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Search Fun Easy English |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
About
Contact
Copyright
Resources
Site Map |
