Today we’re talking about weather and sound. What is the connection ?
Well, you may have already noticed that in winter the voice carries further than in summer. The reason is simply because the propagation of sound through the air depends on the weather. The first factor that comes into play is temperature. The hotter it is, the faster the sound travels. You may know that the speed of sound is 340 m/s, that is to say 1224 km/h, but that is at 15°. Well at 30°, it goes 30 km/h faster, and at -10°, it goes 50 km/h slower.
It does not make big variations anyway, 50 km / h out of 1200.
It’s true, but small variations in the speed of propagation of a wave can have important consequences, and in fact, I have already spoken to you about it in Bring back your Science, with the fata morgana and the mirages.
Ah yes, but it was about the light, not the sound!
It’s true, but in fact, it doesn’t matter, it’s waves in both cases, and the speed of light also varies with temperature. The mirage that we see most often in the region are these puddles of water on the roads in summer. They are caused by temperature variations near the ground. Above an overheated road, the air temperature varies rapidly, from very hot near the asphalt, to much cooler at a few tens of cm. And one of the consequences of the variation of the speed of light with the temperature of the air, is that the light bends towards the colder air, so that the rays of light that we seen when looking at the road actually come from the sky. We see on the surface of the road a piece of sky, which our brain generally interprets as a puddle of water: this is called a mirage.
And the same thing happens with sound?
Yes, but with different consequences, and on different scales. Because the variations in the speed of light as a function of temperature are minute compared to that of sound. Imagine, for light, the variation between 15° and 30° is less than 5 km/s at a speed of around 300,000 km/s. That’s a relative change more than 1000 times smaller than for sound. And so even very small variations in temperature at ground level are enough to modify the propagation of sound. Normally, the temperature decreases with altitude, you know, the higher you go in the mountains, the colder it gets. On our scale, this means that it is a little warmer at ground level, and a little cooler a few meters above the ground. Sound waves, like light, are deflected towards cooler areas, and therefore upwards. And so if you are a little far from the source of a noise, you cannot hear it, it passes above you!
Okay. So that’s what limits the range of the sound?
It is one of the factors. And in winter, you can have the opposite effect. After a long period of cold, it often happens that the ground remains colder than the air, especially when it has frozen. And suddenly, the air temperature increases when you move away from the ground, at least for the first few meters, and so this time, the sound is deflected towards the ground. And we can have sound like that which “falls” in a way on earth, sometimes up to several kilometers from the source, and therefore we can hear sounds coming from very far away. I don’t know if it’s true, but Canadians say that in 1947, during the record cold of -63°, you could hear dogs barking 6 km away, and people talking several km away!
That’s a bit extreme though! It is not likely to happen here, especially with global warming!
In fact, even in summer, there are situations where one has a temperature inversion. For example on the surface of a body of water: the water just needs to be colder than the air, which is quite often the case.
Okay, so that’s why the sound carries further to the surface of a pond!