Why is the sky blue ?

Synonymous with good weather, even when it’s the winter season, we look at it with a smile because it’s a color that is good for morale. But do you know why is the sky you gaze at so happily blue?

Atmosphere, atmosphere, do I have a mouthful of atmosphere?

Is it the sun that causes this beautiful blue color ? Not directly, because it only diffuses white light or at least that’s how we perceive it with our human eyes.

If we could see it as it really is, we would find that this light is composed of a multitude of colorswhich inevitably makes us think of the rainbow.

Passing through our atmosphere, these colored lights are blocked by it because they have waves of more or less long range and the atmosphere is made up of somewhat heavy gaseous molecules that will somehow absorb them in part and create as many obstacles to deviate their trajectory

Instead of being in straight beam which is their natural way of arriving on Earth, they spread in all directions.

Because of its range length, blue light is scattered more widely than others.

The light will therefore be sometimes blue (when the weather is nice), gray or white when there is rain or clouds. But why the sky finally changes color depending on the weather ?

And when is the frog party?

Again, there is a very simple explanation called Rayleigh scattering. While solar radiation is long range, the particle radius is small. The scattering intensity is inversely proportional. We no longer see blue light if you look away from the sun.

Moreover, even if the human eye is less sensitive to it, what we take for a uniformly blue sky actually includes a share of purple up to 30% (i.e. 10% more than the share of blue!).

If the atmosphere were lighter and thinner, that would change the situation: solar radiation would have less difficulty passing through it and the light would be less diffuse. Instead of having a blue sky as we like to watch it so much, we might have a darker blue sky, even bordering on black. In the mountains, this is true: the sky seems darker.

The thickening of the atmosphere is observed in the morning and in the evening, during sunrises and sunsets. It is for this reason that we have a sun that we see of another color and that we rejoice in these incredible colors that the sky takes.

While the sun is perfectly white, the fact of having lost some blue during the passage through the atmosphere gives us the illusion when we look at it that it is yellow. That is why, since we were little, we are used to drawing it in this way when we manage to distinguish colors.

Characteristics of Rayleigh scattering change when molecules in the air become waterlogged ; when the rain comes or the storm comes. We no longer speak of Rayleigh scattering but of Mie broadcastwhich gives for example their white color to the clouds.

Caroline Tellier
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