Children are naturally curious about their world. A question often raised is: Why is the sky blue?
Light is scattered as it travels through the atmosphere by air molecules; blue wavelengths tend to be more strongly scattered.
The Sun’s Light
The Sun emits light over an array of wavelengths (or colors). Our eyes, however, perceive its light most vividly as blue due to scattering; sunlight traveling through Earth’s atmosphere is dispersed into all directions by gas molecules in such a way as to produce diffuse scattering rays whose longer wavelengths pass unimpeded through; violet and blue wavelengths however are strongly scattered and this scattering process explains why our skies appear blue on clear days.
As for why oceans and seas appear blue, that’s due to water’s ability to absorb long wavelengths of sunlight. Once absorbed, these wavelengths scatter at the surface so the light rays remaining appear bluer.
Though numerous factors contribute to our world appearing blue, one of the key contributors is sunlight from the Sun. While its light can cover an array of wavelengths, our eyes are most responsive to blue and violet lights from it; when hitting our atmosphere these wavelengths disperse more rapidly than any others giving our eyes a predominantly blue view of our surroundings.
Open water appears blue because of this process; therefore it is hard to see the bottom of a lake or sea when its waters are bustling with life.
But the color of the sky doesn’t come from reflecting oceans and lakes – rather, its hue is created by how sunlight interacts with Earth’s atmosphere.
The Earth has a thin atmosphere composed of nitrogen and oxygen gases that scatter visible light wavelengths like blue and violet wavelengths in all directions, giving its sky its characteristic hue. This phenomenon also accounts for why other planets lacking an atmosphere – like Mars which only has an extremely thin atmosphere – appear blue when their skies don’t actually exist – such as when their skies appear blue even during daylight. On the contrary, however, Moon lacks any atmosphere at all, resulting in its surface appearing black even during bright days.
John Tyndall discovered in 1859 that passing white light through clear fluid (such as water) scatters blue wavelengths more strongly than red ones, with this effect amplified when particles like dust or droplets of water vapor were present. By 1911 however, the full understanding of sunlight’s interaction with atmospheric molecules had come into focus, eventually becoming known as Rayleigh Scattering.
Rayleigh Scattering is the phenomenon in which light is scattered all around when it interacts with molecules in our atmosphere, as Lord Rayleigh established in his mathematical model of it. Rayleigh discovered that light scatters more strongly when particle sizes are smaller than wavelengths – this explains why we see blue skies, as oxygen and nitrogen molecules make up much of what composes our atmosphere and these smaller molecules refract light differently from visible wavelengths of visible light.
Red and orange wavelengths of light are less likely to be scattered by molecules in our atmosphere and therefore travel directly towards your eyes, while shorter blue-violet wavelengths take a longer path through it before arriving there.
Cloudless skies appear more intensely blue due to how sunlight travels further through our atmosphere after it goes below the horizon, meaning more blue wavelengths than red ones get scattered, taking longer paths through it all, thus intensifying their color further.
At dusk and dawn, when the sun is lower in the sky and traveling through more of our atmosphere, the blue wavelengths become more scattered and absorb a larger portion of light that reaches your eyes than red wavelengths – giving them more intensity overall than their counterparts. Red wavelengths, on the other hand, tend to be absorbed more readily by our eyes so they appear much less intense.
On a clear day when sunlight illuminates the atmosphere, its rays encountering gas molecules in the air can be seen to scatter in all directions – with blue-tinted light scattering more readily due to shorter wavelengths being more easily absorbed by these gas molecules than longer red wavelengths.
At sunrise or sunset, when the Sun is low near the horizon it has to travel a much longer journey through the atmosphere, which scatters shorter blue wavelengths more widely while simultaneously allowing longer red and orange wavelengths through. This gives rise to vividly hued skies at these times of day.
If you were on the Moon without atmosphere, however, the Sun would appear very dull and almost black due to lack of gas molecules to scatter light – hence why its surface appears flat and featureless when compared with planets in our Solar System.
Tyndall and Rayleigh postulated that dust particles in the atmosphere and droplets of water vapour in clouds were responsible for giving sky its distinctive blue hue, however they miscalculated a key factor: for maximum scattering power they must be significantly smaller than light wavelengths – something our atmospheric molecules of oxygen and nitrogen provide in abundance!
Strongly scattered blue wavelengths of sunlight stimulate our eyes by activating blue cones more strongly, while also stimulating green and yellow cones less strongly, leading us to perceive the sky as blue with a slight greenish tinge. Red wavelengths do not influence this perception and therefore do not impact our perception of its color.
High Elevation Scattering
As sunlight travels from low elevation to higher altitude, its light is scattered more readily, with blue wavelengths being scattered more vigorously than others – one reason why higher altitude skies appear bluer.
At sea level, the color of the sky varies significantly due to weather, local terrain (dust), human activity (pollution), and composition of our air. Some mountainous regions feature famous blue hazes caused by aerosols made up of terpenes from vegetation recombining with ozone in the atmosphere to form particles approximately 200nm in size; such particles can cause Rayleigh scattering while larger than wavelengths tend to scatter all colors equally (known as Mie scattering).
Cloud droplets are much smaller than the molecules that compose our air, and therefore scatter all wavelengths of light equally to produce white clouds. Smog in urban environments often contains various sizes of pollution; its predominant constituent particles (those around 10nm in size) scatter blue and red light equally while less frequent dispersions allow other hues to shine through more clearly.
As we ascend higher into the atmosphere and outer space, there is no longer any air to scatter light and make the sky appear black – as William Shatner experienced during his Blue Origin suborbital flight.
Answering why the sky is blue requires understanding three simple factors. First is direct sunlight which has passed through a lot of our Earth’s atmosphere containing molecules which preferentially scatter blue-colored light; our eyes then respond to that blue-colored light sensitivity. Thirdly is that sunshine contains light from many wavelengths which our atmosphere scatters differently; finally there is our eye’s sensitivity towards such blue-colored light sensitivity.