Why can sound detect the weather?

Sound is the feeling of human ears to the fluctuation of air molecular density in the atmosphere. For example, when we ring a bell, it is rung and vibrates. This vibration pushes adjacent air molecules to change their density. This periodic change in density propagates in the air, forming sound waves. When the sound waves reach our ears, we hear the sound of the clock.

In recent years, the study of detecting the atmosphere with sound has attracted people's attention. As early as 190 1, it was found that there was a silent zone where the explosion could not be heard about 70 ~ 90 kilometers away from the shell explosion zone, and there was an audible zone outside this silent zone. During the First World War, this phenomenon attracted people's attention. People put many pickups at different distances around the explosion site to receive the explosion, in order to study the propagation phenomenon of this abnormal sound wave. During the Second World War, people also used rockets filled with explosives to explode at high altitude and used ground pickups for detection. These detections have proved that there is a high temperature area at an altitude of about 50 kilometers. The appearance of silent zone is due to the refraction of sound waves when they propagate at high altitude.

However, in recent years, "acoustic radar" has been used as the equipment to detect the atmosphere by sound waves. Acoustic radar can measure the variation of atmospheric temperature and humidity with height and its tiny pulsation below the surface 1 ~ 2 km, and can also measure wind direction, wind speed, front structure, convective hot air flow, inversion layer and so on.

Why can sound detect the nature of the atmosphere? This is because the atmosphere can affect the speed and path of sound waves and the frequency of sound wave vibration. As long as we can measure the changes of sound wave velocity, refraction and vibration frequency, we can understand the nature of the atmosphere.

For example, sound waves propagate in the air, and the propagation speed will be affected by air temperature, humidity and wind speed. The greater the temperature and humidity, the faster the sound travels. In addition, when the sound propagates along the wind, the speed of sound will increase; When sound travels against the wind, the speed of sound will decrease. The path of sound wave propagation is also curved due to the temperature distribution in the atmosphere, which is called acoustic refraction. According to refraction, we can infer the temperature distribution in the air. Sound wave is a dense wave with its vibration frequency. When the airborne plasmid that generates or scatters sound waves moves relative to the sound wave receiver in the sound propagation direction, the sound wave frequency received by the receiver will be very different from the original sound wave frequency, which is the so-called "Doppler frequency shift effect". By measuring the Doppler frequency shift effect of sound waves, people can measure the movement of air flow. Sound waves are also scattered by suspended matter in the air. This kind of scattering helps people to design instruments to receive scattered sound waves, so as to understand the influence of the atmosphere on sound waves when they propagate in the atmosphere, and then estimate the distribution of temperature, humidity and wind in the atmosphere.