Hail can absolutely break windows, though the likelihood of damage depends on physical forces and the specific vulnerability of the glass. Hailstones are frozen precipitation that act as high-velocity projectiles during severe weather. When one of these ice chunks strikes a window, the resulting damage is a direct outcome of the energy delivered during the impact event. The material integrity of the glass must withstand this sudden force to prevent fracture.
The Physics of Glass Fracture
The ability of a hailstone to break glass is governed by the principles of energy transfer, specifically kinetic energy. This energy is calculated using the hailstone’s mass and the square of its velocity, meaning that a small increase in speed can dramatically increase the destructive power. A heavier, faster-moving hailstone delivers significantly more energy to the window surface than a lighter, slower one.
When the hailstone strikes the glass, the focused force creates mechanical stress waves that radiate outward from the point of impact. These waves travel rapidly through the glass material, causing it to deform momentarily. Fracture initiation occurs when the localized tensile strength of the glass—its ability to resist pulling apart—is exceeded by the force of the impact.
If the localized force is high enough, the glass cannot absorb the energy without failing. This failure typically begins with radial fractures, which are cracks that move outward from the impact point. Subsequently, concentric fractures may form in circles around the impact point as the stress waves constructively interfere. The entire process is a rapid material failure resulting from the delivery of excessive kinetic energy.
Factors That Increase Hail Damage Risk
The probability of a window breaking during a hailstorm is determined by several variables related to the hailstone and the window’s construction. Hail size and velocity are the most significant factors in determining the damage threshold. While pea-sized hail, which is about a quarter-inch in diameter, rarely causes breakage, stones that are one inch or larger, roughly the size of a quarter, begin to pose a significant threat.
Once hailstones reach the size of a golf ball, approximately 1.5 to 1.75 inches in diameter, the risk of window failure increases exponentially. Wind speed also plays a major role, as strong winds increase the horizontal velocity of the hailstones. This combination of large size and high wind velocity can easily exceed the tensile strength of standard glass.
The composition and construction of the window introduce a second layer of vulnerability. Standard, single-pane windows made from annealed glass are the most susceptible to breakage; this glass is the weakest type and shatters into large, sharp shards upon impact. Tempered glass is manufactured to be up to four times stronger than annealed glass, but when it fails, it crumbles entirely into small, relatively harmless granules.
Laminated glass, which uses a plastic polyvinyl butyral (PVB) interlayer sandwiched between two glass sheets, offers superior impact resistance. Even if laminated glass cracks, the interlayer holds the fragments together, preventing the window from opening up. Modern double or triple-pane units are generally more resilient than older single-pane windows due to the additional layers, though the type of glass in each pane remains the ultimate determinant of strength.
The angle at which the hailstone strikes the glass also affects the outcome. An angled impact, common during storms with high winds, often causes more damage than a direct, perpendicular hit. This is partly because angled impacts can strike the weaker points of the window frame or sash.
Strategies for Protecting Windows
Homeowners can employ both permanent and temporary measures to mitigate hail damage risk. For long-term protection, upgrading to impact-resistant windows, such as those with laminated glass, is highly effective. Installing storm shutters made from durable materials like aluminum or steel provides a physical barrier that can be closed immediately before a severe storm.
Specialized window films can be applied to existing glass to enhance safety properties. Although these films do not prevent the glass from cracking, they are designed to hold the shattered pieces together, reducing the risk of injury and preventing the window from completely collapsing inward. Mesh protective screens can also be installed to absorb some of the kinetic energy from smaller hailstones.
When a severe storm is imminent, temporary measures can be quickly deployed. The most effective technique involves boarding up windows using at least 5/8-inch thick plywood panels. These panels should overlap the window frame and be secured using lag screws to withstand high winds and repeated impacts. Parking vehicles in a garage or away from the house also prevents the need to cover additional glass surfaces.