Why do I always hear remote train horn at night?

Question

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.

Answer 1

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.

EDIT: For an excellent visualisation of this effect, see these animations produced by Daniel Russell (Penn State)

Answer 2

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.

Answer 3

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.

Answer 4

i don't think it has anything to do with quiteness, a typical surburban area, the noise difference during day and night ain't that great to mask train horn (which is pretty loud). It's the air temperature at work.

In the daytime the ground air is warm, bends airwaves upwards so for certain distance, you ain't gonna hear any sound at all, but during the night, the wave bends downward, bounced off ground and then bends downward again, I can even hear train rumbling at 8km away during night, and I live in rural which means the day&night noise difference ain't that high, but in the day time, there's nothing I can hear