Why the Sky is Blue and Sunsets are Red

3 min de lectura

The sky appears blue during the day, while sunrises and sunsets often glow with reds and oranges. This phenomenon is primarily due to how sunlight interacts with Earth's atmosphere, a process known as Rayleigh scattering [1].

The Blue Sky

Sunlight, which appears white to our eyes, is actually composed of all the colors of the rainbow, each with a different wavelength [2]. The Earth's atmosphere contains tiny gas molecules, mainly nitrogen and oxygen [1]. When sunlight enters the atmosphere, these tiny molecules scatter the light in all directions [2].

Rayleigh scattering dictates that shorter wavelengths of light are scattered much more efficiently than longer wavelengths [1]. Blue and violet light have the shortest wavelengths in the visible spectrum, meaning they are scattered far more intensely by atmospheric molecules [3]. Because blue light is scattered across the sky, it makes the sky appear blue to our eyes www.nesdis.noaa.gov.

While violet light has an even shorter wavelength than blue and is scattered more intensely, our eyes are more sensitive to blue light. Additionally, some violet light is absorbed in the upper atmosphere, which is why we perceive the sky as blue rather than violet [4].

Red Sunsets

At sunrise and sunset, the sun is low on the horizon, meaning its light travels through a much greater thickness of Earth's atmosphere to reach our eyes weather.metoffice.gov.uk. Over this longer path, almost all of the blue and violet light is scattered away, effectively filtered out www.nesdis.noaa.gov.

What remains to travel directly through the atmosphere to our eyes are the longer wavelength colors: red, orange, and yellow. These colors are scattered much less efficiently and can penetrate the thicker atmospheric layer more directly, resulting in the dramatic reds and oranges seen during sunrises and sunsets weather.metoffice.gov.uk www.nesdis.noaa.gov.

Impact of Pollution and Dust

Air pollution and dust can significantly alter the colors of sunrises and sunsets [2, 7]. Particles like dust, smoke, and aerosols from pollution are generally larger than the gas molecules in the clean atmosphere. These larger particles also scatter light, and if present in moderate amounts, they can enhance the scattering of blue light, leading to even more vivid red and orange hues [2]. However, excessive amounts of large or very numerous particles can scatter all wavelengths of light indiscriminately, making the sky appear hazy or dull [2].

Skies on Other Worlds

Planets without a substantial atmosphere, like the Moon, have a black sky even during the day because there are no molecules to scatter sunlight [8]. On Mars, the sky is typically a butterscotch, yellowish-brown, or reddish-orange due to the pervasive fine dust in its thin atmosphere [8]. This Martian dust, rich in iron oxides, absorbs blue light and scatters red and yellow light more effectively. Interestingly, Martian sunrises and sunsets can appear bluish around the sun because the dust scatters blue light forward towards the observer when the sun is low on the horizon [8].