Hailstone & Walnut Northfield, Minnesota August 24, 2006
True hail nearly always falls from thunderstorms, although it can fall from instability showers. On occasion, hail has also been know to fall from a clear sky, but this is the result of hailstones being catapulted far from thunderstorms by very powerful updrafts.
On occasion, hailstones may contain various kinds of foreign matter such as parts of leaves, twigs, pebbles, grass blades, nuts, insects, and even small frogs. Strong winds or tornadoes pick up these items which then become frozen into the ice as the hailstones are in the process of being formed.
Lower portions of a thunderstorm consist of liquid water droplets, the middle region of a thunderstorm is composed of supercooled water droplets – droplets which have been cooled below the freezing point without turning to ice. The upper part of a thunderstorm is composed of ice crystals.
Updrafts within a thunderstorm carry water droplets from lower regions up into the supercooled region and on up into the upper part of the storm where there are ice crystals. As the liquid water droplets go through these other areas of the thunderstorm, a layer of glaze or rime forms on the droplets depending upon several factors including the size of the droplets and the rate of accumulation.
Downdrafts then carry the droplets with their newly acquired coat of glaze or rime back down below the freezing level once again. Then, updrafts carry them back up again. The alternate freezing and thawing causes the rings (much like one sees in an onion) which are found when hailstones are cut in two. Sometimes, hailstones may contain layers of clear ice (glaze), layers of rime (milky or opaque ice), and even layers of snow. The longer the up and down process continues, the larger the hailstones grow and the greater number of rings they acquire.
The layers of rime ice have air bubbles in them and are formed by what is called dry growth. In dry growth, the developing hailstone’s surface is mostly dry and its temperature is below freezing when it collides with supercooled water droplets. These supercooled droplets then freeze onto the mostly dry surface of the hailstone. Because the surface is mostly dry, air bubbles form in the new ice layer.
However, the layers of clear ice (glaze) in a hailstone are formed by what is called the wet growth process. In this process, the temperature of the growing hailstone is at the melting point because the heat of fusion has been released by the freezing droplets. The surface of the stone at this time is mostly wet.
Some of the official sizes of hailstones are: Marble Size (.50 inch in diameter); Quarter Size (1 inch); Golf Ball Size (1.75 inches); Tennis Ball Size (2.50 inches); and Baseball Size (2.75 inches in diameter). Some violent thunderstorms may produce softball size hail of 4.50 inches in diameter. The term hailstorm is usually only used when either the amount or the size of the hailstones is considered significant.