Sky science explained: How do dawn and dusk get their colours

References to the raging sky, the vivid crimson sunrise, and the twilight hue are predominant throughout Shakespearean texts. The eye-catching colours of the sunrise and sunset can be seen everywhere if the weather permits. But have you ever stopped and wondered why the colours change? Here’s why.  

Colour waves 
The riot of colours we see in the sky is determined by various wavelengths of sunlight. The electromagnetic radiation, generally called the visible light spectrum, contains various coloured wavelengths that the human eye can process. The common phenomenon of white light hitting a glass prism to disperse into the seven colours of the rainbow is relevant here too. When the light in the sky interacts with air molecules, it undergoes the process of scattering, absorbing, and reflecting, creating hues whose colour waves do not reach us. 

The hue of sunrise 
When you look up at the sky during the day or at sunrise, the scattered blue and violet light reaches your eye. The human eye is more perceptive of blue frequencies; hence, we see the blue sky. The small air particles are responsible for the light to change directions. The most common meteorological process, the ‘Rayleigh scattering’ happens in the sky, where the air particle is smaller than the light’s wavelength causing no change, thus, emitting blue colour. It is the mere positioning of the sun in the sky that determines the colour of sunrise or sunset. 

Sunset and its colour 
The sun appears to emit white light from a distance because it is high up in the sky and all the wavelengths of visible light reach the observer’s eye with equal intensity. The same sun, which, on sinking close to the horizon, emits sunlight from a much lower angle. This enters the atmosphere reaching the eyes of the observer. This time, the air molecules ‘scatter’ away from the shorter wavelengths of light including violet and blue. Through this process, the only light left to enter the atmosphere are yellow, orange, and red with much longer wavelengths. 

The lower the sun is positioned along the horizon; the more molecules will get the chance to pass through the light. This, in turn, means more scattering, owing to the creation of bright colours such as red, orange, and hues of purple gathering in the sky. Another important point of focus here is that short colour waves scatter more than long ones, thus, producing the scenery of those fiery red waves along the sky. 

Dust and pollution 
The size and concentration of air particles determine the type of sunrise or sunsets. Most wavelengths of light reach the observer's eyes with nearly equal intensity when sunlight encounters very few particles in the atmosphere. The reduced scattering causes white or yellow sunsets, which are common in the rocky mountains. In these areas, the atmosphere contains fewer dust and other particles. 

Shorter and intermediate wavelengths of light (violet, blue, and yellow) are scattered away when there is a high concentration of particles in the atmosphere that are slightly larger than air molecules (like smoke, dust, and pollutants). As a result, the sun appears orange-red because only the longer wavelengths (orange and red) reach the observer's eyes. Due to the high concentration of salt particles suspended in the air over the oceans, red sunsets are frequently observed from a beach. Shorter wavelengths of light are effectively scattered by these particles, resulting in red sunsets. Volcanic eruptions can also cause red sunsets by injecting dust and ash particles into the atmosphere.