Dictionary tag: W

Perhaps the most common of atmospheric effects, the blueness of the sky, is caused by the way sunlight is scattered by tiny particles of gas and dust as it travels through the atmosphere.

The sky is blue because more photons corresponding with blue reach an observer than any other colour.

In outer space, the Sun forms a blinding disk of white light set against a completely black sky. The only other light is produced by stars and planets (etc.) that appear as precise white dots against a black background. The sharpness of each of these distant objects results from the fact that photons travel through the vacuum of space in straight lines from their source to an observer’s eyes. In the absence of gas and dust, there is nothing to scatter or diffuse light into different colours and no surfaces for it to mirror or reflect off.

All of this changes when sunlight enters the atmosphere. Here, the majority of photons do not travel in straight lines because the air is formed of gases, vapours and dust and each and every particle represents a tiny obstacle that refracts and reflects light. Each time a photon encounters an obstacle both its speed and direction of travel change resulting in dispersion and scattering. The outcome is that, from horizon to horizon, the sky is full of light travelling in every possible direction and it reaches an observer from every corner.

The following factors help to account for why blue photons reach an observer in the greatest numbers:

• The sky around the Sun is intensely white in colour because vast numbers of photons of all wavelengths make the journey from Sun to an observer in an almost straight line.
• In every other area of the sky, light has to bend towards an observer if they are to see colour. It is this scattering of light that fills the sky with diffuse light throughout the day.
• Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water in the atmosphere so scatter the least so few are redirected towards an observer.
• Shorter wavelengths (blue and violet) are just the right size to interact with obstacles in the atmosphere. These collisions scatter light in every possible direction including towards an observer.
• Because blue is relative intense compared with violet in normal conditions and in the absence of the longer wavelengths the sky appears blue.
• However, there is a whole band of wavelengths corresponding with what we simply call blue. As a result, different atmospheric conditions fill the sky with an enormous variety of distinctly different blues during the course of the day.

If we understand why the sky is usually blue it’s easier to understand why it can be filled with reds and pinks at sunrise and sunset.

Let’s review why the sky is blue

• In most weather conditions, the Sun and the area around it appear intensely white to an observer because vast numbers of photons of every wavelength make the journey from Sun to their eyes in an almost straight line.
• The Sun, and the area around it, appears white because it contains a mixture of all wavelengths of light (white light).
• In every other area of the sky, sunlight is striking billions of particles that make up the atmosphere and scattering in every possible direction.
• If it were not for this scattering (deflection of light in all directions), the sky would be as black as night. In reality, an observer is bathed in light arriving from every direction and the sky, as a result, appears to be full of diffuse light.
• Not all wavelengths of light behave in the same way when scattered by the small particles that make up the atmosphere.
• Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water so scatter the least.
• Shorter wavelengths (blue and violet) are just the right size and are affected by reflection, refraction and scattering as they strike successions of particles. It is these collisions that direct light in every possible direction including towards an observer.
• Because human eyes are more sensitive to blue than violet, in most atmospheric conditions, and in the absence of the longer wavelengths, the sky appears blue.
• A wide band of wavelengths corresponds with what we often describe as blue. As a result, the sky is filled with an enormous variety of distinctly different blues during the course of every day.

Why the sky is sometimes red

• A red sky suggests an atmosphere loaded with dust or moisture and that the Sun is near the horizon.
• In the morning and evening, photons must travel much further through the atmosphere than at mid-day.
• Assuming the air above our heads is around 20 km, the total distance light travels increases fivefold to around 500 km when the Sun is on the horizon.
• Remember that:
  • Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water so scatter the least.
  • Shorter wavelengths (blue and violet) are just the right size and are affected by reflection, refraction and scattering as they strike successions of particles.
• In the right weather conditions, light travelling horizontally through the atmosphere undergoes so much scattering that no yellow, green, blue or violet wavelengths remain.
• In these conditions, the light that reaches us, illuminating the sky and clouds and reflecting off every surface around us, is composed of wavelengths that bath the world in red and orange.