The Nature of Hail

While we dodged tornados in Columbia yesterday, we did experience strong thunderstorms, one of which dropped penny-sized hail.  Fortunately, no damage occurred and the heavy rains were welcome following a relatively dry March.

Hail forms in thunderstorms that have an intense updraft, drawing warm, humid air into subfreezing layers of the atmosphere.  When super-cooled water droplets encounter ice crystals or specks of dust, they immediately freeze around that core; further movement up or across the thunderhead leads to additional coatings of ice as the small globe collides with other droplets of super-cooled water or with more ice crystals.  Once the hail fragments are too heavy to be kept aloft by the thunderstorm updraft, they fall to earth; these hailstorms may result in massive destruction of trees, crops, vehicles and man-made structures (especially if the hail has a diameter of 1 inch or more).  Thunderstorms generally produce hail with a diameter of 0.25 to 6 inches; the current record in the U.S. is an 8 inch hailstone, found in South Dakota.

Hail-producing thunderstorms are most common across inland areas of the Temperate Zone.  In addition to the strong updraft, the development of hail is favored by the presence of subfreezing air relatively close to the ground, preventing significant melting of the hail as it falls through the storm.  For this reason, hailstorms are uncommon in the Tropics and are especially common in higher elevations of the Western U.S., especially near mountain ranges which induce storm development and magnify the updraft; the Front Range region of Colorado and Wyoming and areas near the Black Hills of South Dakota are especially prone to hailstorms.