why is the sky blue?

Hello. Welcome to solsarin. This post is about “why is the sky blue?”.

sky is blue

It’s a common misconception that the sky is blue because it reflects the blue of the seas and oceans.
In fact,

it’s the Earth’s atmosphere, and a process known as ‘scattering’,

that causes our skies to be blue.

Why is the sky blue?

Here’s the short answer…
As white light passes through our atmosphere, tiny air molecules cause it to ‘scatter’.
The scattering caused by these tiny air molecules (known as Rayleigh scattering) increases as the wavelength of light decreases.
Violet and blue light have the shortest wavelengths and red light has the longest.
Therefore, blue light is scattered more than red light and the sky appears blue during the day.
When the Sun is low in the sky during sunrise and sunset, the light has to travel further through the Earth’s atmosphere.
We don’t see the blue light because it gets scattered away, but the red light isn’t scattered very much – so the sky appears red.

What is light?

The Sun gives out or emits all the colours of visible light which we see as being approximately white.
As demonstrated by Sir Isaac Newton with a triangular prism,

when white light passes through the prism it separates out into the colours of the rainbow.

This experiment demonstrates

This experiment demonstrates that white light is composed of all the colours of visible light in roughly the same amounts.
These different colours have different wavelengths,

and this affects how they interact with different substances.

Violet and blue light have the shortest wavelengths and red light has the longest.

How is light scattered?

The Earth’s atmosphere is composed of lots of different air molecules.

Sunlight can be redirected by the air molecules and this is known as ‘scattering’.
The size of these molecules is much smaller than the wavelengths of visible light.

The type of scattering that occurs is known as Rayleigh scattering named after Lord Rayleigh (John William Strutt) who discovered it.
This type of scattering increases as the wavelength of light decreases, so blue light is scattered more than red light by the tiny air molecules in our atmosphere.

why is the sky blue?

The sky during the day

At noon, when the Sun is overhead it appears white.

This is because the light travels a shorter distance through the atmosphere to get to us;

it’s scattered very little, even the blue light.

During the day

During the day the sky looks blue because it’s the blue light that gets scattered the most. It’s redirected into many different directions all over the sky,

whereas the other wavelengths aren’t scattered as much.
In reality,

violet light has a shorter wavelength compared to blue light and therefore it’s scattered more –

so why isn’t the sky violet?

It’s because our eyes are actually more sensitive to detecting blue light,

and more of the sunlight coming into the Earth’s atmosphere is blue rather than violet.

Why does the sky look red during sunrise and sunset?

During sunrise or sunset, the sky appears to change colour.
When the Sun is low in the sky, the light has to travel a longer distance through the Earth’s atmosphere so we don’t see the blue light because it gets scattered away.
Instead we see the red and orange light that travels towards us since this light hasn’t been scattered very much.

Hence the Sun and skies look redder at dawn and dusk.

why is the sky blue?

Skies on other planets

Other planets don’t have an atmosphere exactly like ours,

and so their skies would look different.
Mars’s atmosphere is much thinner than the Earth’s – less than one per cent.

The low density of air molecules means that the Rayleigh scattering that causes our skies to be blue on Earth has a very small effect on Mars.
We might expect it to have a very faint blue coloured sky,

but due to the haze of dust that remains suspended in the air the daytime sky on Mars appears more yellow.

This is because the larger dust particles absorb the short wavelength blue light, and scatter the remaining colours to give a butterscotch hue over the Martian sky.

during a sunrise

However during a sunrise and sunset on Mars, the sunlight travels a longer distance through its atmosphere and

it’s alike to the thickness of the atmosphere on Earth.

As such, the blue light gets scattered in all directions and the longer wavelengths of light don’t get scattered much at all – providing a blue glow to the sky around the Sun in the hours around dawn and dusk.

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If you were standing on the Moon

If you were standing on the Moon, the sky wouldn’t appear to have any colour except black.
The Moon’s atmosphere is so thin that it virtually doesn’t have one.

When the air is too thin for gas molecules to collide with each other,

we call it an ‘exosphere’ instead.
Because of the lack of an atmosphere, sunlight isn’t scattered, so whether it’s daytime or nighttime on the Moon,

the sky appears black.

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Why is the ocean blue?

So the Earth’s sky isn’t blue due to it reflecting the colour of the seas and oceans. But what makes the sea blue –

is it reflecting the blue of the sky?
It’s not the sky that makes open water appear blue.

It’s once again due to how different wavelengths of light interact with different substances.
Water molecules are good at absorbing longer wavelengths of light,

so when sunlight hits the water the reds and oranges get absorbed.
The shorter wavelength blue light is absorbed very little and much of it is reflected back to our eyes.

It’s possible to see hues of green and sometimes other colours in the water,

but that’s due to sunlight bouncing off other particles or sediments within it.

How many of these weather sayings do you recognise and is there any truth to them?

In this article we look at some of these sayings and whether there is any science to back them up.
Dating back thousands of years,

weather forecasting had to rely less on scientific data and more on human experience.

The sayings became particularly important in sailing and agriculture, as they looked for reliable forecasts ahead of time.
From this,

developed the old weather sayings and phrases we see and hear today.

Red sky at night

The concept of “Red sky at night, shepherd’s delight.

Red sky in the morning, shepherd’s warning” first appears in the Bible in the book of Matthew.

It is an old weather saying often used at sunrise and sunset to signify the changing sky and was originally known to help the shepherds prepare for the next day’s weather.
Despite there being global variations in this saying such as “Red sky at night, sailors delight.

Red sky in morning, sailors warning”, the scientific understanding behind such occurrences remains the same.

why is the sky blue?

Why does a red sky appear at sunrise and sunset?

The saying is most reliable when weather systems predominantly come from the west as they do in the UK.

““Red sky at night, shepherds delight” can often be proven true, since red sky at night means fair weather is generally headed towards you.
A red sky appears when dust and small particles are trapped in the atmosphere by high pressure.

This scatters blue light leaving only red light to give the sky its notable appearance.
A red sky at sunset means high pressure is moving in from the west,

so therefore the next day will usually be dry and pleasant.

Red sky in the morning(why is the sky blue?)

“Red sky in the morning, shepherds warning” means a red sky appears due to the high-pressure weather system having already moved east meaning the good weather has passed,

most likely making way for a wet and windy low-pressure system.
St Swithun’s Day

The saying goes:(why is the sky blue?)

St. Swithun’s day, if thou dost rain, For forty days it will remain; St. Swithun’s day, if thou be fair,

For forty days ’twill rain no more
This story originated with St. Swithun,

the Bishop of Winchester in the Anglo-Saxon era. He initially requested to be buried outside where he said he might be subject ‘to the feet of passers-by and to the raindrops pouring from on high.’
On 15 July, more than a century later, his body was moved to an indoor shrine and so began the heavy shower. This was said to be a result of the saint’s anger at being moved.
The rain continued from 15 July for 40 days and nights. This led to a folklore myth that whatever the weather is like on 15 July will be how it is on the following 40 days and nights.

Does it really happen?(why is the sky blue?)

The jet stream does play an important part in predicting how the weather would be for the next 40 days

and nights from the end of June/early July.

The location of the jet stream shortly after the summer solstice largely determines the following summer’s weather.
If the jet stream is located southerly then it is likely to be a more unsettled summer.

If the jet stream is in a northerly position then the weather is likely to be brighter and dry throughout summer.
However, this does not prove that heavy rainfall for St.

Swithun’s Day and the following 40 days and nights could happen.

In fact, since records began there has been no occurrence of rainfall for such a prolonged period of time.

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