Where is the blue sky located?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

30 January 2011

How high is the blue sky from the ground? Now that’s not an easy question. Think about it: the most straightforward method of measuring the distance to a place is to travel there while counting the measurement units.

For example, to measure the distance from Nairobi to Kisumu, just get into a car, zero the odometer, and driver to the town. Check the count of kilometres when you get there and there you have it!

But the sky presents a difficulty in that it is not easy to say exactly where it is located. This is one of the paradoxes of nature: here is something that anyone can see from virtually anywhere on Earth, but no one can say where it is!

To understand the paradox, we need to know why the space above the clouds looks blue. As explained in this column in June 2005, the quick answer to that question is that air is blue in colour. But this colouration is very faint and can only be observed when looking through a thick section of air, for example, the atmosphere.

So, even though the blue sky appears like a thin blanket wrapped around the Earth, the truth is that blue colour is everywhere: Extending from the ground all the way up to “the end of the atmosphere”.

Can we then measure the distance to the end of the atmosphere and say that this is the location of the blue sky? No! The reason is that if you flew up a few tens of kilometres and looked back downwards, you would see a blue colouration appearing between you and the ground. From that altitude, you might begin to wonder how far down this blue haze is from you!

Secondly, the atmosphere really has no end. The air thins out gradually as one goes higher above the ground. So how far up would you go to reach the end – 10km or 100km or 1,000km, or 10,000km or…?

For these reasons, my quick answer is that the blue sky just in front of your eyes. However, if you flew vertically upwards facing the sky, you would notice that the blue colour fades off gradually the higher you go.

Somewhere between 50km and 100km above the ground, the blue colour will no longer be visible in front of you. But you cannot say that this is the location of the blue sky! What has happened is that you have reached a point where the air above you is too thin for its blue colour to be visible.

So, rather than talk about the distance to the blue sky, it makes more sense to ask about its extent from the ground. The answer to that would be between 50km to 100km above the ground.