Hail is a form of solid precipitation that falls as balls or irregular lumps of ice, known as hailstones. It occurs within specific atmospheric conditions, involving a complex interplay of air currents and temperature variations.
Essential Atmospheric Conditions
Hail formation requires strong thunderstorms, specifically towering cumulonimbus clouds. These clouds are characterized by powerful, rapidly rising air currents called updrafts. Updrafts within these storms can exceed speeds of 50 miles per hour, carrying water droplets and ice particles high into the atmosphere.
For hail to develop, a significant portion of the cloud must extend into regions where temperatures are below freezing, specifically 0 degrees Celsius (32 degrees Fahrenheit). A lower freezing level provides a greater vertical distance for hailstones to grow without melting. Within these sub-freezing areas, water droplets can remain in a liquid state even when their temperature is below freezing; these are known as supercooled water droplets. The availability of these supercooled droplets and a high liquid water content within the cloud are necessary for hailstones to accrue mass.
The Journey of a Hailstone
The formation of a hailstone begins with a small ice particle, often referred to as a hail embryo. This embryo is typically a frozen raindrop or a small pellet of soft hail called graupel. These initial ice particles are carried upward into the colder regions of a thunderstorm by powerful updrafts. As the embryo ascends, it collides with numerous supercooled water droplets.
Upon impact, these supercooled droplets freeze onto the surface of the hail embryo, a process known as accretion, causing the hailstone to grow in size. When a hailstone is cut open, it often reveals concentric layers of clear and opaque ice.
Clear layers form when water freezes slowly, allowing trapped air bubbles to escape, typically occurring in areas of the cloud with abundant supercooled water and temperatures just below freezing. Conversely, opaque layers form when water freezes rapidly, trapping tiny air bubbles and giving the ice a milky appearance, which generally happens in colder regions with fewer supercooled droplets. The hailstone continues to circulate and grow within the cloud until it becomes too heavy for the updraft to support its weight.
When Hail Falls
A hailstone eventually falls to the ground when its increasing size and weight overcome the strength of the updrafts that previously held it aloft. The speed at which a hailstone falls varies significantly with its size. Smaller hailstones, typically less than one inch in diameter, can fall at speeds ranging from 9 to 25 miles per hour. Larger hailstones, measuring between one and 1.75 inches, generally descend at speeds of 25 to 40 miles per hour. Exceptionally large hailstones, those measuring two to four inches in diameter, can reach 44 to 72 miles per hour, with some exceeding four inches potentially falling at over 100 miles per hour.
As hailstones descend from the cold regions of the cloud, they often pass through warmer air closer to the ground. During this descent, they may partially melt, reducing their size and potentially altering their shape. If the column of air below the thunderstorm is sufficiently warm and deep, smaller hailstones may melt completely, reaching the ground as rain instead of ice. Hail typically falls in concentrated areas known as hail swaths, which can be several miles wide and extend for many miles along the storm’s path.