What causes the Sky to Turn Red

Dawn is the time of the day when the Sun is still below the horizon and the first signs of daylight begin to appear along the skyline. The sunlight during dawn is usually accompanied by a variety of low frequency tinges, including yellow, orange and red and this is most commonly due to the interaction between sunlight and the particles present in the atmosphere.  While the suspended air particles in the atmosphere scatter much of the blue light in the sky, longer wavelengths, such as red, orange and yellow light are able to get through the atmosphere and reach your eyes, therefore the sky in the morning appears red.

Electromagnetic spectrum

Visible light is a small part of the electromagnetic spectrum. The Sun emits energy in all frequencies of the electromagnetic spectrum, from radio waves to gamma rays and all other frequencies in between. When electromagnetic radiation reaches the atmosphere of the Earth, we’re only able to see the visible part of the spectrum, including red, orange, yellow, green, blue and violet light. The nitrogen and oxygen molecules and all other particles suspended in the atmosphere interact with the light from the Sun. Upon reaching the Earth’s atmosphere, sunlight can be scattered, reflected or absorbed by these particles.

Visible light

The visible spectrum consists of high and low frequency radiation. Red light, at one extreme of the visible spectrum travels through space at wavelengths that range from between 630-750 nanometers and frequencies in between 400-484 THz, while violet light at the other end of the visible spectrum travels at wavelengths of 380-450 nanometers and frequencies of 668-789 THz. The other colors in between these two extremes include orange, yellow, green and blue, with blue as the most energetic before violet. The interaction between visible light and the atmosphere is what causes a red dawn.

Why the Sun is red in the mornings?

At noon, when the Sun is high above in the sky, the Sun is brighter and appears white; however, during sunrise or sunset, sunlight reaches the atmosphere at a lower angle, in other words, the rays from the Sun have to travel through a thicker atmosphere, and as they do, the air molecules present in the atmosphere scatter much of the shorter wavelengths of visible light, allowing only the longer wavelengths of visible light, including red, orange and yellow reach the retina of the eye and producing a bright yellow-orange sky.

Rayleigh scattering

The scattering of light, according to Rayleigh scattering, is proportional to the fourth power of wavelength, thus, the shorter wavelengths of light, such as blue light will be scattered more than the longer wavelengths of light, including red, orange and yellow. While the longer wavelengths of light are able to pass through the atmosphere, the shorter wavelengths are scattered in all directions in the sky. Glancing towards the Sun during sunrise or sunset, the sun will acquire reddish to yellowish hue due to the longer wavelengths of light that were able to make it through the atmosphere.

If there are enough particles in the atmosphere whose diameters are longer than yellow light, even yellow light wavelengths will be scattered away and the sky will appear orange-red. The tinge of the Sun and the sky may increase further if the atmosphere becomes loaded with suspended particles, such as during a morning storm. If this is the case, then only the longest wavelengths are capable of penetrating the atmosphere, and the Sun is perceived as red. The more suspended particles present in the atmosphere. The more longer wavelengths of light are scattered, the redder the Sun is perceived.

Other causes of a red sunrise

Sulfur erupted from volcanic eruptions may produce a red sunrise or sunset. During powerful eruptions, sulfur dioxide ejected into the atmosphere. Under high moisture content, these particles serve as nuclei over which moisture will condense, forming highly reflective clouds of sulfuric droplets that produce red sunrises or sunsets for months after the eruption. Salt particles and water molecules are responsible for bright red suns which can be witnessed from the beach. According to sewanee.edu, a sunrise or sunset in the city differs from a sunrise or sunset in the country. This is most commonly due to the amount of suspended particles in the atmosphere.