Hail forms in strong thunderstorm clouds and has a diameter of at least .20 inches (or 5 mm). Hail is layered, irregular, clumped together mass of ice that falls to the ground. The average size of hail is between 1 and 1.75 inches; however hail .75 inch (or 2 cm) is large enough to cause significant damage to property and crops. As of January 2012, the US National Weather Service increased the threshold for issuing an alert for hail from .75 inch (2 cm) to 1 inch (2.5cm) or more in diameter. Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, farmers’ crops. Hail damage to roofs often goes unnoticed until further structural damage is seen, such as leaks or cracks. It is hardest to recognize hail damage on shingled roofs and flat roofs, but all roofs have their own hail damage detection problems. Metal roofs are fairly resistant to hail damage, but may accumulate cosmetic damage in the form of dents and damaged coatings.