Hail is formed by rain drops that are lifted back up into a storm cloud by updraft winds. In the upper atmosphere, the rain turns to ice and the hail stone grows in size as it is repeatedly lifted and dropped out of the upper atmosphere back down into the lower, until it is ultimately too heavy for the updraft. The speed at which the hail hits the ground, or your roof, is dependent on the size and mass of the hailstone. Heavier, denser hailstones fall faster and impart much more energy into the surface they hit than smaller and/or lighter hailstones.
2-inch diameter hail, between golf ball and tennis ball size, normally falls from the sky at about 70 miles per hour (mph). With a steady 45 mile per hour wind, the combined horizontal and vertical components could result in a total speed at impact of about 85 mph. The additional speed imparted by the wind may not seem like much of a difference, but considering the impact is proportional to the square of the velocity, 15 mph can make a significant difference in the damage.

AEI Corporation built two devices to simulate hail impacts. 1-inch and 2-inch hailstones were shot from the devices at several different roof surfaces at varying speeds. For smaller hail traveling at lower speeds, a common slingshot type device was mounted to a frame. Using the device, 1-inch diameter ice balls were propelled at about 40 mph with little or no adverse effects on most roof surfaces. To move 2-inch hail at an appropriate speed, a pneumatic air cannon was designed and built. 2-inch diameter hail shot at 85 to 95 mph was enough to damage all but the most durable of roofing materials, concrete tile and single-ply EPDM rubber membrane.

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