In this blog post, we’ll take a step-by-step look at the principles behind the formation of shock waves during supersonic flight, the process by which the funnel-shaped clouds around aircraft form, and the various effects of shock waves.
How Do Shock Waves Form?
The sounds we hear in everyday life spread out in all directions from a sound source and travel to our surroundings. Generally, since moving objects travel slower than the speed of sound, the sound they produce reaches us before the object itself does. This is precisely why we can hear the engine noise of a car approaching from behind in an alley and avoid it.
To understand this more easily, let’s imagine a 100-meter dash. Suppose a runner is sprinting toward you as you stand at the finish line, shouting all the while. The sound the runner makes spreads in all directions at every moment and reaches the finish line first. Therefore, you hear the sound before you see the runner. This is similar to the circular ripples that spread across the surface of a calm pond when you throw a stone into it.
However, the situation changes as the runner accelerates and approaches the speed of sound. When the runner reaches the speed of sound, the sound traveling forward can no longer stay sufficiently ahead and begins to overlap continuously in front of the runner. As these multiple sound waves overlap simultaneously, a very abrupt change in pressure occurs—this is what is known as a shock wave. In other words, a shock wave is a phenomenon that goes beyond the simple overlapping of sound waves; it involves very abrupt changes in pressure, density, and temperature occurring within an extremely thin region.
The speed of sound varies depending on temperature, but at 15°C near sea level, it is approximately 340 meters per second, or about 1,225 kilometers per hour. In aviation, this speed is defined as Mach 1. For reference, the KTX typically operates at around 300 kilometers per hour, which is much slower than the speed of sound.
If a runner moves faster than the speed of sound—that is, at supersonic speeds—they will outpace the sound waves they generate. In this case, an observer will hear the sounds in a different order than the actual sequence of events, depending on their position relative to the runner. For example, the observer might hear the sound produced near the finish line first, and the sound produced immediately after the start later. This occurs because, as the sound source moves faster than the sound waves, multiple sound waves converge in a conical shape to form a single shock wave front (Mach Cone). When this shock wave passes the observer, it produces a very loud sound known as a “sonic boom.”
The Clouds Created by Shock Waves, the Physical Processes Involved, and Their Effects
You may have seen funnel- or cone-shaped clouds forming around a supersonic fighter jet in flight. These clouds are not the shock waves themselves, but rather condensation clouds formed when water vapor condenses due to a sudden drop in air temperature caused by rapid pressure changes around the aircraft.
When air expands rapidly, its internal energy is used for the expansion, causing the temperature to drop sharply. If the temperature of the water vapor in the air falls below the dew point at this point, tiny water droplets form, creating a visible cloud. Conversely, when the pressure rises again and the air temperature increases, the water droplets evaporate. Therefore, these clouds are not caused by fighter jets “dragging” clouds behind them; rather, they are a phenomenon that appears and disappears as condensation and evaporation occur very rapidly around the aircraft. Furthermore, these condensation clouds can only be clearly observed under atmospheric conditions where humidity is sufficiently high.
The shock wave itself consists of a very thin layer, and within it, variables such as velocity, pressure, density, temperature, and entropy change almost discontinuously. The actual thickness of a shock wave is very thin—roughly equivalent to the average free path of air—making it difficult to distinguish with the naked eye. However, when the density of the air changes due to the shock wave, light is refracted, which can make the air appear to ripple; these density changes can be clearly observed using specialized optical equipment such as Schlieren imaging.
Shock waves are also a very important factor in aircraft design. When a shock wave occurs, drag increases sharply, and it can affect the aircraft’s stability and maneuverability. Furthermore, on the ground, a loud explosive sound known as a sonic boom can be heard; in some cases, this may cause building windows to rattle or cause discomfort to people. For these reasons, supersonic flight over land by civilian supersonic aircraft has been restricted in many countries.
Meanwhile, even in aircraft that do not fly at supersonic speeds throughout their entire flight path—such as commercial airliners—the airflow can locally reach supersonic speeds on the upper surface of the wings or near the leading edge. When this occurs, small shock waves form, resulting in “shock wave drag,” which increases aerodynamic drag. The wings of modern airliners incorporate various aerodynamic designs to minimize this effect.
Finally, photographs and videos showing shock waves and condensation trails together serve as excellent visual aids for understanding the complex aerodynamic phenomena that occur during supersonic flight. When flight conditions and atmospheric humidity are right, these changes in airflow can be observed directly around fighter jets or high-speed aircraft, providing a prime example of how invisible shock waves alter the surrounding air.
