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This is definitely not an illusion, as many people have the same experience. I have usually lived in places miles away from train stations, which makes it unlikely to hear any train horns during the day. However at night, occasionally train horns can be heard. I hope if any one can explain the physics of this effect, like possibly sound travels faster at low temperature.

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Not exactly a duplicate, but closely related: How far can one hear sound? –  John Rennie Aug 30 at 10:59
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Maybe you are actually a character in a novel. Do you also sometimes notice a breeze blowing, or a dog's bark? –  rob Aug 30 at 11:31
    
Maybe it's not a horn. Could you record it? I don't know how far a train can be heard, but it could possible that during the day there is too much noise. –  jinawee Aug 30 at 11:51
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What country do you live in? There may be a non-physics answer here -- some places don't require horns for level crossings during the day but do at night when visibility is lower. So the horns just may not be sounding or may not sound as loud during the day when they aren't needed. –  tpg2114 Aug 30 at 12:20
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It could also be that the train only comes through at night –  Jim Aug 30 at 21:15

3 Answers 3

up vote 22 down vote accepted

There are two things that can be considered: one is trivial - that it is quieter at night so you are more likely to hear the horn.

The second is physics: the speed of sound depends on the square root of temperature, so the refractive index is proportional to $T^{-1/2}$.

At night it is quite possible to get a temperature inversion, such that air near the ground is colder than higher up. This would normally occur in still conditions and I think is more common in winter.

As the refractive index decreases with height it means that sound waves propagating upwards at some angle to the horizontal will be bent back towards the ground. The sound waves at some distance from the source will be more intense than you might expect if the waves propagated isotropically.

The contrast with the daytime situation would be enhanced by a more normal temperature gradient where the refractive index increases with height.

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+1 This is the better answer and close to what I was typing, you typed faster. –  paisanco Aug 30 at 13:12
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That's because I've answered it before. This is really a duplicate. –  Rob Jeffries Aug 30 at 13:16
    
I do not think that hearing better at night is anything "trivial", because that may imply that the human ear may apply a kind of CFAR (constant false alarm rate) detector. How does it do it? –  user31748 Aug 30 at 17:50
    
maybe also the chance that the train will sound the horn at night –  ratchet freak Aug 30 at 17:56
    
@user31748 I don't think either. You hear everything louder at rest, in sleep or in meditation. At least that is my experience. It might be because your brain don't have to process visual information, so it can concentrate on audial information. I don't think the real solution is related to physics, but I am much more involved in biology... ;-) –  inf3rno Oct 18 at 1:16

It might not have to do with day/night physics at all. It is possible that the train doesn't run during the day so it is unlikely you would even hear the horn.

I live in a city, about 2.5 miles from the rails, where during the day the train tracks are used only for public light rail transit. The large locomotives only run at night after the light rail transit has stopped and therefore I only hear their horns at night.

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The same thing happens some places even when the tracks aren't needed during the day for public transit; running trains at night avoids needing to hold up as much traffic at crossing. –  Jefromi Aug 30 at 21:25

There are several explanations, depending on a lot of factors.

At night, the air near the ground can have a different temperature than air only a few hundred feet above1. This affects the transmission of sound waves.

There is often less wind at night. If the wind is blowing towards the train it can be harder to hear

There is usually less ambient noise after dark, so the distant train sounds louder.

As pointed out elsewhere, maybe the trains don't use the horn in daytime.

  1. I taught skydiving for 10 years. I can conclusively state that it is possible to have a substantial temperature differential, in both directions, between the ground and 3000 feet above the ground.
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The lapse rate is often of order 1 degree per 100m. But usually in the wrong direction to cause the phenomenon described. –  Rob Jeffries Aug 30 at 13:10
    
That's the standard adiabatic lapse rate. I'm talking sudden transitions, like "pleasant at 2000 feet, f**ing cold on the ground. Or the other way. –  paul Aug 31 at 0:52

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