Why is the sky sometime blue, sometime gray?

5 Answers

  • 1 decade ago
    Best Answer

    The Earth's sky is blue because the air molecules (largely nitrogen

    and oxygen) are much smaller than the wavelength of light. When light

    encounters particles much smaller than its wavelength, the scattered

    intensity is inversely proportional to the 4'th power of the

    wavelength. This is called "Rayleigh scattering," and it means that

    half the wavelength is scattered with 2**4 = 16 times more intensity.

    That's why the sky appears blue: the blue light is scattered some 16

    times more strongly than the red light. Rayleigh scattering is also

    the reason why the setting Sun appears red: the blue light has been

    scattered away from the direct sunlight.

    Thus, if the atmosphere of another planet is composed of a transparent

    gas or gases whose molecules are much smaller than the wavelength of

    light, we would, in general, also expect the sky on that planet to

    have a blue color.

    If you want another color of the sky, you need bigger particles in the

    air. You need something bigger than molecules in the air---dust.

    Dust particles can be many times larger than air molecules but still

    small enough to not fall out to the ground. If the dust particles are

    much larger than the wavelength of light, the scattered light will be

    neutral in color (i.e., white or gray)---this also happens in clouds

    here on Earth, which consist of water droplets. If the dust particles

    are of approximately the same size as the wavelength of light, the

    situation gets complex, and all sorts of interesting scattering

    phenomena may happen. This happens here on Earth from time to time,

    particularly in desert areas, where the sky may appear white, brown,

    or some other color. Dust is also responsible for the pinkish sky on

    Mars, as seen in the photographs returned from the Viking landers.

    If the atmosphere contains lots of dust, the direct light from the Sun

    or Moon may occasionally get some quite unusual color. Sometimes,

    green and blue moons have been reported. These phenomena are quite

    rare though---they happen only "once in a blue moon...." :) The dust

    responsible for these unusual color phenomena is most often volcanic

    in origin. When El Chicon erupted in 1982, this caused unusually

    strongly colored sunsets in equatorial areas for more than one year.

    The much bigger volcanic explosion at Krakatoa, some 110 years ago,

    caused green and blue moons worldwide for a few years.

    One possible exception to the above discussion is if the clouds on the

    planet are composed of a strongly colored chemical. This might occur

    on Jupiter, where the clouds are thought to contain sulfur, phosphorus,

    and/or various organic chemicals.

    It's also worth pointing out that the light of the planet's primary is

    quite insignificant. Our eyes are highly adaptable to the dominating

    illumination and perceive it as "white," within a quite wide range of

    possible colors. During daytime, we perceive the light from the Sun

    (6000 K) as white, and at night we perceive the light from our

    incandescent lamps (2800 K, like a late, cool M star) as white. Only

    if we put these two lights side-by-side, at comparable intensities,

    will we perceive a clear color difference.

    If the Sun was a hot star (say of spectral type B), it's likely we

    still would perceive its light as "white" and the sky's color as blue

  • 1 decade ago

    The sky is truly blue. It never becomes gray, it just looks like it is. When there are alot of clouds in the sky, or one big cloud, it seems like the sky is gray, when you're really seeing the bottom of the clouds.

  • 1 decade ago

    When it's grey, there's clouds. When it's blue, there's no clouds.

    Oh, and the sky is blue because of the way light hits the atmosphere, not because of the oceans.

  • 1 decade ago


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  • 1 decade ago

    The sky itself (believe it or not) has no colour, it reflects the colours of the earth.

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