Why Blue Sky?

why blue sky

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Air particles in our atmosphere are smaller than the wavelengths of light, meaning they scatter blue light more readily than any other color – giving the sky its signature blue hue during daylight hours.

1. The Sun

As the Sun rises or sets, its light travels through the atmosphere to our eyes. Depending on time of day and elevation of observer, that light may appear blue or reddish orange depending on which wavelengths pass through our atmosphere.

The Sun emits all colors of the visible spectrum, from violet through greens, blues, and reds. As white sunlight reaches Earth’s atmosphere it is redirected and scattered by molecules of air and gas (primarily nitrogen and oxygen) at just the right size to scatter blue wavelengths very effectively – leaving us with blue skies! However other wavelengths like those produced during sunsets or sunrise experience less scattering.

Scattering also explains why we perceive the sky as blue at ground level but whiter as we ascend a mountain or fly in an airplane, because more molecules exist at lower altitudes to interact with, leading to greater scattered blue light; at higher elevations however, fewer molecules interact and thus less of that blue light is scattered out of sight.

Sunlight appears brighter near the horizon than overhead because as it travels through thicker atmospheric layers closer to its destination, more blue wavelengths scatter out leaving behind stronger signals of reds and yellows, making the scene appear more vivid to our eye.

Leonardo da Vinci attempted to explain this phenomenon in his notebooks and, while not explicitly using the term “scattering”, had an understanding of its basic principle. For instance, his observation that sunlight passing through smoke looked blue proved his understanding.

As soon as you set foot on the Moon, its surface does not appear blue due to a lack of atmosphere – there is nothing but sunlight shining down from it, without an atmosphere to scatter light around and soften its brightness. Mars, with an atmosphere much thinner than our own has similar results – indeed astronauts who have visited have reported back that its sky appears black!

2. The Earth’s Atmosphere

The blue sky is formed by Earth’s atmosphere, not ocean water reflecting light back onto us. While many have long held this belief to be true, open water’s color comes from how different wavelengths interact with various substances; when light hits open water’s surface it gets scattered more readily due to shorter wavelengths being more easily redirected than longer ones; longer wavelengths are absorbed by molecules found within water molecules and remain as visible light thus leaving only bluer hues for us to witness.

At any point in time, most of the sunlight that reaches our eyes during a day will likely be blue due to Rayleigh scattering. White sunlight reaches Earth’s atmosphere where it is scattered by tiny particles such as gas and dust particles in our air; this process tends to scatter blue light more readily due to smaller molecules being smaller than wavelengths associated with blue and violet light wavelengths.

Other wavelengths cannot be as effectively diverted into space, which explains why we don’t often spot them during daytime; thus explaining why the sky only appears blue during daylight hours as opposed to at other times of the year.

At nighttime, the color of the sky changes due to absorption. As the Sun sets and rises, its rays pass through more atmosphere which absorbs much of its red and orange light; as a result, we often witness redder skies during evening and morning times than they otherwise would be.

Other planets with dense atmospheres will likely feature blue skies due to physics operating similarly on those planets. One difference could be that some Sun-emitting planets emit less violet or ultraviolet light; therefore bluer wavelengths of sunlight become more noticeable in the sky.

3. The Sun’s Radiation

As sunlight travels through our atmosphere, it interacts with gas molecules in the air. It interacts with them either by being scattered or absorbed – with smaller particles scattering more, while larger dust or air molecules absorbing it more readily than shorter wavelengths such as red and orange wavelengths that would normally cause red sky colors during daytime hours. Bluer hued wavelengths (blue/violet wavelengths) more likely being scattered by air molecules than longer ones – which is what creates blue skies during daylight hours.

Rayleigh Scattering was named for Sir John William Strutt, 3rd Baron Rayleigh of Sudbury who first observed it in the 1870s. He theorized that when light passes through a fluid with suspended particles in suspension, some waves will bounce off of these particles and scatter in different directions; Rayleigh observed that shorter blue wavelengths scatter more strongly than longer red wavelengths.

Due to Rayleigh Scattering being more easily scattered, blue light dominates visible illumination during the daytime sky. As the sun moves behind or in front, its proportion changes as more blue and violet wavelengths are absorbed than red or orange wavelengths; but Rayleigh Scattering remains the primary factor that accounts for its dominance as an explanation for our blue daytime skies.

As we ascend higher into the atmosphere, light begins to fade as there are fewer molecules present to scatter it. This explains why, at sunrise or sunset, skies become paler and paler as one looks towards their respective horizons.

If you were living in space, there would be no air to cause Rayleigh scattering and no blue hue from the sun’s light would appear; so, if just Rayleigh scattering were responsible, the sky would appear black; however, due to all other phenomena occurring simultaneously the color actually shifts towards blue. Since radiation from our star includes all colors on its spectrum but we only see blue due to other processes happening, some other aspect may account for its hue change as well.

4. The Earth’s Surface

Our sky’s distinctive blue hue results from how light scatters as it travels through our atmosphere, where shorter wavelengths like violet and blue light tend to be scattered by molecules more readily than longer wavelengths like red and yellow lights are. As such, blue and violet light tends to predominate our view of daytime skies.

Our planet is an ever-evolving environment, constantly shifting as gravity, wind, water, and tectonic plate movements shape its surface. The unit Earth’s Surface examines how these forces combine to shape our world’s landscape – something essential to its continued existence as life exists on this planet.

Atmospheric gases such as oxygen and nitrogen comprise our planet’s atmosphere, giving it its blue hue during sunlight penetration. Rayleigh Scattering occurs as sunlight hits these molecules, scattering light in all directions as sunlight passes through. Short wavelengths of light tend to be scattered more easily than longer ones, leading to blue and violet light being predominant during daytime hours.

Rayleigh Scattering is not the sole reason that our skies are blue, however. Other factors may also contribute to this appearance of our atmosphere – such as dust or aerosol particles of comparable or larger size to wavelengths of light that scatter all colors evenly (known as Mie Scattering).

These particles make our skies appear slightly bluer than they would be otherwise, while also increasing our eyes’ sensitivity to blue light over other colors, causing us to perceive it as blue rather than pink or green even without clouds present in the sky.

At nighttime, our skies become dark as the Sun has set and no more light can reflect off of Earth and reflect back onto it. On the moon too, its lack of an atmosphere leaves its sky black – yet when astronomers use satellite images of other stars they often notice that their skies appear blue because those stars contain large quantities of water in their atmospheres which cause blue light to be scattered further from these stars than usual.

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