A cumulonimbus cloud is a massive, vertically developed storm system capable of generating intense weather. These towering structures, often visible as the classic “thunderhead” or anvil shape, are unique in their ability to produce both liquid rain and solid hail. The coexistence of these two distinct forms of precipitation requires exploring the extreme conditions operating within the cloud. This article details the separate processes that allow for the simultaneous delivery of rain and ice to the ground.
The Internal Environment of a Cumulonimbus
The scale of a cumulonimbus cloud enables diverse precipitation types. These clouds extend vertically through the entire troposphere, with tops frequently reaching altitudes of 12 to 16 kilometers. This immense height creates an extreme temperature gradient: temperatures at the cloud base can be warm, while the upper reaches are far below the freezing point of water.
The cloud structure is powered by powerful, sustained vertical air currents. Warm, moist air rises rapidly through a strong updraft, which can exceed 175 kilometers per hour in severe storms. This upward flow transports moisture and water droplets from the lower levels to the sub-zero upper atmosphere. Simultaneously, compensating downdrafts of cooler air carry heavier precipitation, both liquid and frozen, back toward the surface.
The Process of Raindrop Formation
Rain is produced through two primary mechanisms operating in different thermal regions. In the lower, warmer sections of the cloud (above \(0^\circ\text{C}\)), the collision-coalescence process dominates. Larger water droplets collide and merge with smaller ones, forming progressively heavier drops. This growth continues until the droplets become too heavy for the updraft to suspend, and they fall as warm rain.
Much of the rain also begins as ice higher up in the cloud, forming through the Bergeron process. In this method, ice crystals grow rapidly at the expense of supercooled water droplets because the saturation vapor pressure over ice is lower than that over liquid water at the same temperature. As these ice crystals descend below the \(0^\circ\text{C}\) isotherm, they encounter warmer air and melt completely before reaching the ground. This melting contributes significantly to the liquid rainfall associated with the storm.
The Extreme Conditions Required for Hail
Hail formation requires intense conditions, primarily the presence of supercooled water. Supercooled water is liquid water that remains fluid even below \(0^\circ\text{C}\), often existing down to \(-40^\circ\text{C}\). This water is abundant in the mid-to-upper levels of a strong cumulonimbus cloud.
The process begins with an ice embryo, such as a frozen raindrop or graupel, held aloft by the powerful updraft. The embryo is repeatedly lifted into cloud regions filled with supercooled water droplets. As the particle travels through this environment, the supercooled droplets impact its surface and instantly freeze, a process known as accretion.
The hailstone grows in layers, similar to an onion, with each pass through the supercooled water zone adding mass. The layers alternate between opaque (from rapid freezing) and clear (from slower freezing), recording the stone’s journey through the cloud.
A hailstone’s ability to reach a large size is proportional to the strength and duration of the updraft. Updrafts must be strong enough to counteract the growing gravitational pull, suspending the ice mass for many minutes. For a hailstone to grow to the size of a golf ball, an updraft speed of approximately 88 kilometers per hour is required. The hailstone finally falls when it becomes too heavy for the updraft to support its weight.
Why Rain and Hail Fall Together
The simultaneous descent of rain and hail results from the spatial separation of their formation processes and their interaction with the atmosphere below the cloud. Raindrops, formed by collision-coalescence or the melting of smaller ice particles, are generally released from the cloud base or the edges of the precipitation shaft. Hail forms much higher up, often in the core of the storm, and is released when the central updraft weakens or is overcome by the stone’s weight.
Both types of precipitation are carried downward by the storm’s downdraft, which is intensified by the weight and drag of the falling moisture. As the hailstone descends, it enters the warm air below the freezing level, known as the melting layer. This layer is a zone of ablation where the ice begins to turn back into liquid water.
The size of the ice particle determines its fate upon exiting the cloud. Smaller hailstones or graupel pellets melt completely in the warm air column, converting into large raindrops. Only the largest, most dense hailstones, which accumulated mass in the most powerful updrafts, resist complete melting and reach the surface as ice. The rain experienced on the ground is a mixture of melted ice from the cloud’s upper reaches and liquid drops from the lower levels, falling alongside the surviving hailstones.