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A wonder of nature: Do you know how internal waves form in different fluids?
The waves we see in our daily lives are often the boundary between air and water, but in fact, there is a type of wave in nature that is not easy to detect - internal waves. This type of wave forms within the fluid medium rather than on its surface. To form internal waves, the fluid must have stratified properties, meaning that the density of the fluid changes as depth or height changes, usually due to changes in temperature or salinity.
The existence and behavior of internal waves fully demonstrate the complexity and wonder of nature.
Such waves form when density changes occur over small vertical distances. For example, in thermospheres in lakes and oceans or inverted layers in the atmosphere, these waves propagate horizontally like surface waves, but their speed is limited by the density difference between the interface above and below them. On the other hand, if the density changes continuously with height, then these waves can propagate not only horizontally but also vertically. Internal waves go by many other names, depending on the mechanics of their generation, their amplitude, and the influence of external forces.
Types of internal waves
Internal waves can be divided into various types depending on how they are generated and their propagation characteristics. If the waves propagate horizontally across an interface where density rapidly decreases, they are called interface (internal) waves; if the amplitude of these waves is large, they are called internal solitary waves or internal solitons. When these waves are affected by significant changes in air density, they are called inelastic (internal) waves. Characteristic examples include: the waves generated by the flow of landforms are called Lee waves or mountain waves. If such mountain waves break at high altitude, they may generate strong warm winds on the ground. This is called "Chinook" in North America. "Wind" is called "Fern Wind" in Europe.
Many natural phenomena, such as Chinook winds, are the specific effects of internal waves, which makes people marvel at the wonders of nature.
In addition, when tidal currents flow across submarine ridges or continental shelves, the waves generated are called internal tides; while those slowly evolving waves, when their dynamics are affected by the Coriolis effect, are called inertial gravity waves or inertial waves. . Internal waves are often distinguished from Rossby waves, which are affected by latitudinal changes.
Visualization of internal waves
In our daily life, the formation of internal waves can be observed through simple experiments. Tilt the salad dressing bottle slowly back and forth and you can clearly see the waves between the oil and vinegar. In the atmosphere, internal waves can be visualized through wave clouds: at the wave crests, the air rises and cools due to relatively low pressure. If the relative humidity approaches 100 percent, clouds of water vapor condense can form. These internal waves generated by flowing through the terrain form lens-shaped clouds, which are so-called レンズ clouds.
In northern Australia, these clouds are also called morning glow clouds and are often used by brave souls to glide, just like surfers gliding on the waves. Through satellite observations, the range of these waves can extend to hundreds of kilometers. Satellites can also observe that the ocean's thermospheric undulations change significantly as the waves move, increasing the surface roughness in the area and thus affecting the scattering of light.
Buoyancy and fluctuation frequency
According to Archimedes' principle, an object that is a fluid will lose weight due to the weight of the fluid it displaces, creating a buoyant force. This buoyancy force is particularly evident between fluids of different densities. Interestingly, for internal waves in water, because water is much denser than air, waves passing through the thermosphere will also feel more buoyant in effect. As a result, internal waves typically move slower than surface waves, making them an important and compelling subject in the study of fluid dynamics.
The characteristics of internal waves not only affect the movement of water bodies, but also profoundly affect surrounding ecosystems.
Through in-depth exploration of internal wave formation in different fluids, we can not only understand the magical workings of nature, but also understand the long-term impact of these phenomena on the environment and climate change. Faced with such a changeable nature, perhaps we can think about: Beyond how science explains these mysterious phenomena, how can humans better coexist in harmony with nature?
