Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
What is the Effect of Temperature on the Atmosphere?
John Tyndall was the first to demonstrate refraction of sound through a balloon filled with carbon dioxide (CO2). The balloon, surrounded by air, behaves like a convex lens of glass surrounded by air for refraction of light. This is because carbon dioxide is heavier than air. If a clock is placed at one side of the balloon [Fig.], the sound rays produced by the clock meet at a particular point on the other side after refraction through the balloon. If we place our ear at that point, the sound of the clock is heard distinctly. But sound will not be heard if we place our ear elsewhere.
It is an important point to note that, the velocity of sound in air is nearly 330 m ᐧ s-1, whereas that in water is about 1500 m ᐧ s-1. As a consequence, unlike in the case of light waves, air is a medium denser than water for sound waves. So, when sound enters water from air, the rays deviate away from the normal. Steel is a still rarer medium, as the velocity of sound in it is about 5000 m ᐧ s-1. A convex-shaped steel lens or water lens placed in air would, thus, show the properties of a concave lens, i.e., would behave as a diverging lens.
Refraction of sound in the atmosphere
Effect of temperature: The whole atmosphere over the surface of the earth may be supposed to be divided into layers, one above the other [Fig.]. At day time, the layer of air adjacent to the surface of the earth has the highest temperature and as we move upwards the temperature falls gradually. So, the density of air increases with increase of height from the surface of the earth. Thus, the upper layers of air are denser than the layers near the surface of the earth. Hence, sound emitted from a source near the surface of the earth gets refracted.
While propagating upwards due to refraction, it bends towards the normal at every layer, and eventually travels upwards almost vertically [Fig.(a)]. For this reason, only a negligible portion of the sound emitted from the source reaches a distant listener standing on the surface of the earth, i.e., at day time sound cannot travel a large distance in the forward direction.
Just the opposite happens at night. The density of the lowermost layer of air is maximum because its temperature is minimum. So, sound emitted from a source on the surface of the earth gets refracted while propagating upwards. In this case, sound moves from denser to rarer medium. So it bends away from the normal at every layer [Fig.(b)]. At one stage, total internal reflection occurs and it reaches a listener. So, sound emitted from a distant train, low voices of persons sitting in a boat floating in the middle of a river, etc., are very clear to our ears at night.
Effect of wind: Generally, the velocity of wind near the surface of earth is less than that in the upper layers. If wind blows from the source to the listener, the wavefronts bend towards the earth and help the listener to hear the sound. So, under this condition, sound coming from a distant source is heard distinctly.
On the other hand, if the wind blows in the opposite direction, i.e., from the listener to the source, an opposite incident happens. In this case, the wavefronts at the upper levels have comparatively smaller velocities [Fig(b).]. Hence, the wavefronts bend in the upwards direction. So a very negligible portion of the sound can reach a distant listener standing on the surface of the earth. Therefore, under this condition, sound coming from a distant source is not heard distinctly.