Hail occurs when water gathers on ice during the end, or backside, of a thunderstorm. An updraft pushes the particles from the backside to the top of the thunderstorm's front. The National Severe Storms Laboratory, or NSSL, reports that this theory differs from a past theory. The previous theory included super-cooled water droplets freezing instantly with nuclei such as dust or frozen rain, causing hailstones.
Hail freezes during one of two processes: dry growth and wet growth. Hail freezes during dry growth when air temperatures dips below freezing. Water droplets instantly freeze as they collide with ice particles. The droplets and ice particles create frozen bubbles that resemble cloudy ice similar to ice cubes. Wet growth also freezes hail when the temperatures fall below freezing but the temperature is not extremely cold. Nevertheless, the wet growth freezes hail when tiny pieces of ice crash with super-cool water. Hail doesn't freeze immediately. As the tiny pieces of ice fall from the sky, the water surrounds the pieces slowly freezing them. Unlike hail created during the dry process, this type of hail resembles clear ice.
All hailstones aren't the same. For instance, hail freezes in irregular shapes or becomes large or small. As hail travels up and down in an updraft, it can freeze and refreeze several times causing layers. Thus, you can tell what type of updraft the hail was in and how many times it froze before hitting the ground by counting the layers. Hail freezing in a balanced or steady updraft has zero to three layers. Also, the longer the hail stays in the updraft the larger it grows.
For hail to fall to the ground, it must become heavy enough to overwhelm the updraft keeping it in the atmosphere. Once it overcomes the updraft, gravity moves it toward the ground. How fast it falls depends on the thunderstorm accompanying the hail. When hailstones hit raindrops failing at the same time, it doesn't fall as fast. Thus, hail bumping into rain is similar to you hitting a wall as you fall. The wall breaks, or slows down, your fall. Wind is another factor in how hall falls to Earth. During a thunderstorm with strong winds, hail can fall at an angle instead of straight down.