Why Do Shockwaves from Explosions Shatter Windows?

When we watch a powerful explosion in an action movie — or during more serious times, on the news —  we often witness windows shattering in buildings and vehicles at a significant distance from the explosion. Sometimes, we can even see a visible wave racing outward from the explosion’s center. This is known as a blast wave, a specific form of a broader phenomenon called a shockwave.

To understand the effects of blast waves — and shockwaves in general — it’s important to first understand the speed of sound and what happens when something exceeds it. As the term suggests, the speed of sound in air — about 1,200 kilometers per hour (750 mph) — is the rate at which sound waves travel through that medium. More generally, physicists define the speed of sound as the speed at which mechanical vibrations propagate through a given medium. In air, these are vibrations of air molecules, but every material - whether solid, liquid, or gas - has its own speed of sound, determined by its properties, such as density and elasticity. For example, you can demonstrate this with a rope:  sound travels faster through a tight rope than through a loose one, because the tension affects how quickly vibrations propagate.

What happens when an object moves faster than the speed of sound in the material it’s passing through — like a high-speed aircraft flying through air? In this scenario, a problem arises: the particles in the material can only “get out of the way” at the speed of sound, but the object is approaching faster than that, giving them no time to move aside. As a result, the object forcefully compresses the particles in its path, generating a wave of extremely high pressure and density at the point of contact. This is known as a shockwave.  Shockwaves can form in various situations and materials. Blast waves from explosions are one example. Another is the shockwave created by a supersonic aircraft as it breaks through the sound barrier, producing the familiar and thunderous sonic boom.

The blast wave hurls objects away from the explosion source — just like in action films. A car is thrown into the air due to an explosion in an animated movie | Illustration: Shutterstock AI

Blown Away

In the case of a blast wave, there isn’t a physical object traveling faster than the speed of sound. Instead, a powerful explosion — such as those caused by ballistic missiles or improvised explosive devices — releases a tremendous amount of energy and heat in an instant. This rapidly heats the surrounding air and increases air pressure dramatically, sometimes tens or even hundreds of times above normal atmospheric pressure levels. The resulting pressure difference between the explosion's center and the surrounding area forces air to rush outward at incredible speeds, forming a blast wave.

Blast waves can travel long distances depending on the explosion’s strength and the surrounding environment, and they can cause widespread damage. This destruction is primarily due to the intense pressure difference between the wave and its surroundings. When a blast wave hits a window, it creates a pressure difference across the glass: the inside of the building still has normal atmospheric pressure, while the outside suddenly experiences a much higher pressure. The imbalance can force the window to shatter inward. It's a phenomenon often dramatized in action movies, where explosions hurl people or vehicles away from the blast site. 

However, after the shockwave passes, in its wake it often leaves behind a zone of low pressure — or underpressure. At that point, the pressure inside the building may become higher than the pressure outside. This reversed pressure difference can cause the window to shatter outward, as the higher internal pressure pushes the glass outward into the low-pressure zone. In real-world scenarios, both inward and outward shattering can occur in quick succession, depending on the structure, materials, and timing of the blast wave and its trailing effects.

One of the additional dangers posed by blast waves is that sound waves can travel from one material to another. For example, when we’re swimming in a pool, we can still hear sounds from above the surface even with our head underwater.  This happens because sound waves are transmitted through the physical motion of molecules: vibrating air molecules strike the water molecules and transfer part of the sound energy.  Similarly, a blast wave can propagate into solid materials like the walls of a building — and even into the human body — potentially causing significant damage due to the extreme pressure involved.

As with most complex phenomena, the damage caused by a blast wave depends on many factors. The wave can be absorbed by some materials, transmitted through others, or reflected off solid surfaces. The overall impact is shaped by the specific characteristics of the explosion, the surrounding environment, the construction of nearby structures — including windows — and other physical conditions.

Shattering windows during a blast can increase the danger significantly, as sharp, fast-moving glass shards can cause severe injuries or even fatalities. For this reason, Israel’s Home Front Command advises that in the absence of a designated protected space— such as a residential protected room (MAMAD), a communal protected space (MAMAK), or a shelter — people should seek refuge in an internal stairwell, with no external walls or windows. Alternatively, choose an interior room with as many surrounding walls and as few windows or openings as possible. Kitchens, bathrooms, and toilets should not be used as protected spaces, and it is important to stay away from ceramic tiles, glass, and other materials that might shatter.