Hail is a form of precipitation consisting of irregular lumps of ice that fall to the ground. Although it often occurs during warmer months at the surface, the temperature conditions needed for its creation are found high up in the atmosphere. Hail formation depends on the frigid temperatures within the upper reaches of a massive thunderstorm cloud. Here, air movement carries liquid water into regions well below the typical freezing point.
The Essential Cloud Structure
Hail production requires a specific type of storm cloud known as a cumulonimbus cloud. This cloud is characterized by its immense vertical development, often extending through all layers of the troposphere. The base can be relatively low, but its top can soar to altitudes of up to 52,000 feet (about 16 kilometers). This towering height allows warm, moist air rising from the ground to reach the extremely cold atmospheric levels necessary for ice formation.
The vertical structure ensures that developing ice particles have enough time and space to gather mass before gravity pulls them down to the earth’s surface.
Atmospheric Temperature Zones for Ice Formation
The initial requirement for hail formation is reaching the freezing level, the altitude where the air temperature drops to \(0^\circ\text{C}\). This level is the minimum temperature for water droplets to begin freezing into the initial ice particle, or hail embryo. However, the most active growth zone for hail often exists in a much colder region of the cloud.
This main growth area typically has air temperatures ranging from about \(-10^\circ\text{C}\) to \(-25^\circ\text{C}\). In this zone, supercooled water exists: liquid water that remains unfrozen despite being below the \(0^\circ\text{C}\) freezing point. These liquid droplets are a primary building block for hailstones, as they freeze instantly when they collide with an existing ice particle.
Hail growth virtually stops when temperatures fall below approximately \(-40^\circ\text{C}\). At these extremely cold altitudes near the top of the storm, all available liquid water has already frozen into tiny ice crystals. Hailstones need to continuously collect liquid water to grow, making the zone between \(0^\circ\text{C}\) and \(-40^\circ\text{C}\) the most important part of the cloud for their development.
The Role of Powerful Updrafts
The enormous size that some hailstones achieve is directly related to the strength of the air current that carries them. Updrafts are strong, vertical currents of rising air within the thunderstorm that suspend ice and water against the force of gravity. These currents act as a conveyor belt, lifting newly formed ice embryos from the lower cloud base and repeatedly circulating them through the hail growth zone.
The strength of the updraft determines the maximum size a hailstone can reach before its weight overcomes the upward push. For example, a supercell thunderstorm producing very large hailstones requires updraft speeds that can exceed 100 miles per hour. The longer an ice particle is held aloft by a strong updraft, the more time it has to collide with supercooled water droplets and accrete layers of ice, leading to significant growth. When the hailstone finally becomes too heavy for the air current to support, it falls to the ground.