MADISON, Wis. (WMTV) – The second topic of Severe Weather Awareness Week is hail. During severe thunderstorms, hail is often one of the biggest threats, and the way that hail forms is quite a neat process.
Hail Formation
Hailstones are formed when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere, causing them to freeze. Hailstones then grow by colliding with supercooled water droplets that freeze onto the hailstone’s surface. Eventually, the hailstone becomes heavy enough to overcome the strength of the storm’s updraft, and it falls to the surface due to Earth’s gravity.
Hail Growth Regimes
Hail doesn’t grow the same way every time. There are actually two different types of hail growth: wet growth and dry growth. It all has to do with latent heat, which is the energy absorbed or released by a substance during a change in its physical state, such as melting, boiling, condensing, or, in this case, freezing.
Wet growth happens when the latent heat released as water freezes is enough to warm the hailstone’s surface to around 32 degrees. Because the surface is slightly warmer, not every droplet freezes instantly. Instead, a thin layer of liquid water forms around the hailstone before freezing. This creates hailstones that are clear, smooth, and often layered like an onion.
Dry growth is different. This occurs when the freezing process does not release enough latent heat to warm the hailstone. The surface stays below freezing, so droplets freeze immediately on contact. Air gets trapped between the frozen droplets, creating a milky, cloudy appearance. This also leads to hailstones that are rougher and sometimes spiky…