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July/August 2008

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The Return of the Anti-Hail Cannons

Hailstorms have been the bane of farmers since man first began trying to coax a living off the Earth. Average yearly property and crop losses to hail currently stand at $500 million in the United States. Elsewhere, estimates are more difficult to obtain, but known losses from single severe hailstorms demonstrate the severity of the problem on a worldwide scale. For example, on July 12, 1984, Germany experienced a hailstorm that caused US$950 million in damages. Likewise, a hailstorm in Canada on September 7, 1991, caused US$500 million in damages, and another in Australia on April 14, 1999, also caused US$500 million in damages. While hail generally does not result in famine, it has a tragic impact on those whose crops or properties are struck by the ice. An entire season’s work and investment might lie in shreds and, for fruit crops, subsequent seasons’ crop production could also be compromised due to breakage of tree branches and vines. As a result, farmers have always struggled to find ways to fend off the constant threat of hail, even when their understanding of the science behind this meteorological phenomenon was limited.

The growth of hail in clouds is now fairly well understood. The principle factor is the existence of deep layers in thunderstorms, where condensed water is in a liquid state, at temperatures below 0°C (even as low as -40°C), and in vertical air currents strong enough to hold the hail aloft while it grows.  The hailstone begins as a small ice particle (known as an embryo), which falls through the air faster than the liquid particles that surround it.  As it falls through the cloud, it collects the liquid droplets, which freeze immediately on contact with the ice surface. Once the growth of the hailstone is well under way, its temperature rises above that of the cloudy environment, due to the release of the latent heat of freezing of the captured water.  If the rate of capture of the cloud water is greater than the capacity of the air to carry away the released heat, its surface temperature can rise to 0°C and the hail is said to undergo “wet growth.” Its surface will be wet, and it might even shed some of the captured water to the air. If the rate of capture and freezing of the water is less than the ability of the air to carry away the heat, the surface of the hailstone remains dry (ice) and it undergoes “dry growth.” 

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