Sky: Why It’s Blue and Why It’s Red

blue skies
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The blue color of the sky is caused by a phenomenon called Rayleigh scattering. This happens when sunlight passes through the Earth’s atmosphere.

  • Sunlight consists of different colors, each with different wavelengths, ranging from red (longer wavelengths) to violet (shorter wavelengths).
  • When sunlight interacts with the atmosphere, it encounters gas molecules and small particles. Rayleigh scattering is more effective at shorter wavelengths like blue and violet than at longer wavelengths like red and yellow.
  • Although violet light scatters even more than blue light, our eyes are more sensitive to blue light and less sensitive to violet light. Moreover, some of the violet light is absorbed by the upper atmosphere.

These factors combined make the sky appear predominantly blue to us.

In summary, the sky looks blue because the shorter blue wavelengths of sunlight are scattered in all directions by the molecules and particles in the atmosphere, and our eyes are more attuned to seeing blue.

The reddish color of the sky during sunrise and sunset is caused by a combination of factors, including the Earth’s atmospheric composition and the scattering of sunlight. This phenomenon is primarily explained by Rayleigh scattering, but other factors, such as the angle of the sun and the thickness of the atmosphere, also play important roles.

  • During sunrise and sunset, the sun is near the horizon. As a result, sunlight must travel through a larger portion of the Earth’s atmosphere compared to when the sun is overhead at noon. The longer path increases the chances of scattering because the light interacts with more air molecules and small particles.
  • Rayleigh scattering causes shorter wavelengths of light (blue and violet) to scatter more than longer wavelengths (red and orange). When the sun is low in the sky, the blue and violet light is scattered out of the direct line of sight, leaving the longer wavelengths of red, orange, and yellow to dominate.
  • In addition to air molecules, larger particles such as dust, pollution, and water droplets (a phenomenon called Mie scattering) also play a role. These particles scatter light in different directions without a strong dependence on wavelength, contributing to the overall scattering effect. The combined effect of Rayleigh scattering (which is wavelength-dependent) and Mie scattering (which is not strongly wavelength-dependent) results in the reduction of blue light and the prominence of red and orange hues.
  • The angle at which sunlight enters the atmosphere during sunrise and sunset is much more oblique than during midday. This oblique angle causes the sunlight to pass through a thicker layer of the atmosphere, leading to more extensive scattering.
  • Certain atmospheric gases and particles can absorb specific wavelengths of light. While this effect is less significant than scattering, it can slightly influence the observed color spectrum.

In summary, sunlight travels a longer distance through the atmosphere at sunrise and sunset. Shorter wavelengths (blue and violet) are scattered out of the direct line of sight more than longer wavelengths (red and orange). The remaining light that reaches the observer directly is richer in red and orange wavelengths, giving the sky its reddish hues during these times.

Rayleigh scattering

Rayleigh scattering is the phenomenon of light or other electromagnetic radiation being scattered by particles much smaller than the radiation’s wavelength. It is named after the British scientist Lord Rayleigh, who first described it in the 19th century.

Important factors:

  • Particle Size: Rayleigh scattering happens when particles causing the scattering are much smaller than the wavelength of the light, typically molecules or very small aerosols.
  • Wavelength Dependence: The intensity of Rayleigh scattering is inversely proportional to the fourth power of the wavelength. This means shorter wavelengths, such as blue and violet light, scatter much more than longer wavelengths, like red light. Mathematically, the intensity I of the scattered light is proportional to  , where l is the wavelength.
  • Mathematical Description: The Rayleigh scattering cross-section, which quantifies the amount of scattering, is given by:

where:

  • σ is the scattering cross-section,
  • n is the refractive index of the scattering medium,
  • λ is the wavelength of the light,
  • d is the diameter of the particle,
  • V is the volume of the particle.

Rayleigh scattering is utilized in various scientific and technical domains. It plays a crucial role in comprehending atmospheric phenomena, remote sensing, and optical communications, as well as in the examination of particles and molecules in diverse environments.

Mie scattering

Mie scattering is a type of scattering of electromagnetic waves (including visible light) by particles that are comparable in size to the wavelength of the light. This phenomenon is named after the German physicist Gustav Mie, who first described it in 1908.

Important factors:

  • Particle Size: Mie scattering occurs when the diameter of the scattering particles is approximately equal to or larger than the wavelength of the incident light.
  • Wavelength Dependence: Unlike Rayleigh scattering, which is strongly dependent on wavelength, Mie scattering does not have a strong wavelength dependence. This results in a more uniform scattering of light across different wavelengths.
  • Appearance: Mie scattering tends to produce white or greyish colors because it scatters all wavelengths of light more or less equally. This is why clouds, which consist of water droplets much larger than the wavelengths of visible light, appear white or grey.
  • Angular Distribution: Mie scattering is highly directional, often exhibiting a strong forward scattering peak. This means that a significant portion of the scattered light continues to travel in the same general direction as the incoming light. This is why objects like the moon can appear to have a bright halo.
  • Mathematical Description: The mathematics of Mie scattering is more complex than Rayleigh scattering and involves solving Maxwell’s equations for electromagnetic waves interacting with spherical particles. The solutions are expressed in terms of infinite series and depend on the relative refractive index of the particles and the surrounding medium, the size parameter (which is the ratio of the particle circumference to the wavelength of the light), and the angle of scattering.

Mie scattering helps explain phenomena such as the appearance of clouds and the scattering of light by atmospheric aerosols, which can affect weather and climate models. Understanding Mie scattering is crucial in designing optical instruments and sensors, especially those operating in environments with particulate matter like smoke or fog. Techniques like optical coherence tomography (OCT) rely on scattering principles to image tissues and detect abnormalities.

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