Hail are pellets of frozen rain that fall from cumulonimbus clouds. They are formed by violent updrafts which carry water droplets to the very cold air at the edge of the troposphere where they freeze. On their way down, hail collide with liquid rain drops which then freeze to the surface of the hail stone, causing them to grow and occasionally form obscure shapes. Under particularly violent convection, hail stones may be recirculated aloft by updrafts multiple times, providing plenty of opportunity for growth, and resulting in hail stones the size of golf balls, tennis balls, or sometimes even larger (the largest recorded hailstone fell in South Dakota, USA and had a diameter of approximately 20 cm). Updraft cycles and hail growth will continue in a cumulonimbus until the hail stones have become so heavy that the force of gravity acting on the stones is stronger than the updrafts. Then, they fall to the ground. The forces needed to keep even moderately sized hail stones in the air are immense. For example, a 2 cm hail stone requires an updraft velocity of approximately 90 km/h to remain aloft, while an 8 cm hail stone would require an updraft velocity of 180 km/h!
Hail is the most common in areas where the freezing level is relatively close to the ground because the hail does not have an opportunity to melt as it falls. These conditions are the most common in high altitude regions of continental interiors. Interestingly, hail is rare when temperatures are very cold because there are so few water droplets in the cloud (the droplets are mainly ice), and hail stones need liquid water to grow. Furthermore, hail is fairly rare in the tropics. This is because the high freezing levels mean that most hail melts before it reaches the surface. In the mid-latitudes, you can expect to experience the occasional hail storm in coastal regions and hail in the open ocean is rare.
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