Visibility of a polar aurora :
...that not everybody is able to see.
When we talk about visibility, others parameters can be noticed, but these ones disturb the show.
It is known that the solar winds, thus the cosmic particles, reach the earth constantly, creating polar auroras. The problem is that the human eye isn't sophisticated enough to see all of them. Actually, if their intensity is too weak, or the distance separating us from them too big, we can't distinguish them and think they're not here... What a mistake !
Whether it is day or night is the first disturbing parameter.
If it is day or night, we can single out an aurora.
Indeed the sun is such an intensive light source that it hides auroras (the light emitted by an intensive aurora is the equivalent of the one the moon emits).
That's why we can only observe auroras at night.
Fortunately, this phenomenon mainly occurs in high latitude areas, where it's night most of the time, which is rather useful!
For similar reasons, it's also hard to see auroras above enlightened cities.
However, when we stand far from lit up places during the night; we don't always distinguish a polar aurora. In fact, other parameters prevent us from seeing such a beautiful phenomenon.
To start with, the atmospheric pollution (the polluting gas of our atmosphere) also reduces the visibility of an aurora in two possible ways :
- On one hand, the absorption stops the light radiation, therefore the light source is less distinct. (Absorption comes from car gas).
- On the other hand, the diffusion diverts the light radiation, so the radiuses go away in other directions. Our eye receives fewer radiuses, so it considers the light source as less intensive. (Diffusion comes from dust particles and from sprays.)
| Atmospheric pollution |
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Then the intensity of an aurora can change, regarding the presence or not of the pollutants we refer to.
Nevertheless, there is in our atmosphere another component which acts on light radiuses : water.
Whether it's liquid or gas, the water attenuates the light.
It halts light, as the absorption does.
According to the water quantity gone through, a light source will be visible or not.
Do you find it weird that the atmosphere contains enough water to hide an aurora ?
Not at all in fact, if we consider the clouds.
The light is partly stopped by a cloud; yet they are made of water!
In the same manner, the ocean floor is very dark, because of the huge water quantity, which has stopped nearly all the radiuses!
So water attenuates light. But if there isn't any cloud, is there still water ?
Of course yes!
Indeed, the atmosphere always contains some water under the form of cloud, rain, or steam.
This water quantity in the atmosphere is the humidity (mass of water contained in each m3 of air).
It varies from one place to another, according to the temperature and pressure.
The relative humidity, which enables to rate humidity, corresponds to the ratio between the quantity of water in the atmosphere and the quantity of water of satured air.
It is a percentage.
In other words, if the air is saturated (it can't contain more steam), the relative humidity will be 100%.
If the relative humidity goes beyond 100%, there will be formation of water.
A fall of temperature raises the relative humidity, and reciprocally. The relative humidity is estimated thanks to a hygrometer.
| A hygrometer |
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Then, even without clouds, the light always goes through water and is attenuated by it, thus it seems less intensive. The relative humidity determinates the water quantity, so it influences the light radiation's attenuation.
As a conclusion, the visibility of a polar aurora depends on these parameters. As they are variable, a light source won't be inevitably visible at a given intensity. Then it is almost impossible to forecast a polar aurora... To facilitate seeing one, you will just have to be at the right place at the right moment!
