Hail is a dramatic meteorological event, a form of solid precipitation that descends from severe thunderstorms, often causing widespread damage. Understanding the speed of falling hailstones is important because this velocity directly translates to the potential for destruction to property and agriculture. This speed is not constant, but is governed by complex atmospheric physics and the hailstone’s own physical characteristics.
The Formation and Characteristics of Hail
Hail begins high in the atmosphere within powerful cumulonimbus clouds, characterized by intense, rising air currents known as updrafts. Raindrops or small ice particles are carried upward into regions where temperatures are well below freezing. Here, they encounter supercooled water droplets, which are liquid water existing below the normal freezing point.
These supercooled droplets freeze onto the suspended ice particle on contact, causing the hailstone to grow. The powerful updraft continuously holds the developing hailstone aloft, allowing it to accumulate more ice. This process results in the signature layered structure seen when a large hailstone is cut open, with alternating bands of clear and opaque ice. The longer a hailstone remains suspended, the greater its mass and size become, which determines its eventual falling speed.
Determining Hail’s Falling Speed
The speed at which any object falls through the atmosphere is limited by terminal velocity. This is the maximum speed an object can achieve when the downward force of gravity is perfectly balanced by the upward force of air resistance, or drag. Once a hailstone’s mass overcomes the strength of the thunderstorm’s updraft, it begins to fall, accelerating until it reaches terminal velocity.
The terminal velocity for hailstones varies significantly based on their size. A smaller hailstone, about one centimeter in diameter (pea-sized), falls at approximately 20 miles per hour. Hailstones the size of a golf ball (roughly four centimeters in diameter) increase considerably in speed, often reaching 70 to 80 miles per hour.
Very large hailstones, those with diameters around eight centimeters or more, can achieve speeds exceeding 100 miles per hour, sometimes reaching 110 miles per hour. This demonstrates that the speed is not fixed, but is heavily dependent on the stone’s dimensions and the balance of forces acting upon it. The destructive potential is directly tied to this increasing speed as the size of the hailstone grows.
Key Factors Influencing Descent Speed
The most significant factor influencing a hailstone’s terminal velocity is its mass and size. Since mass increases as the cube of the radius and surface area increases as the square of the radius, a larger stone has a greater weight-to-surface-area ratio. This higher ratio means the gravitational pull overcomes air resistance more effectively, resulting in a faster descent.
The shape and surface texture of the hailstone also play a substantial role in determining the amount of air resistance, or drag, it experiences. Hailstones are rarely perfect spheres; many are irregular, lumpy, or have spike-like protrusions. These non-spherical shapes increase the drag coefficient, creating more resistance and leading to a slower terminal velocity compared to a smooth, spherical stone of the same mass.
Atmospheric conditions along the hailstone’s path also modify its speed. The density of the air, which decreases with altitude, affects the amount of drag. As a hailstone falls from high altitude, the increasing air density near the ground tends to slow its descent. Strong horizontal winds and powerful downdrafts within the storm can add a horizontal or vertical component to the hailstone’s velocity, increasing its overall speed and impact angle.
The Impact of High-Speed Hail
The consequence of a hailstone’s speed is quantified by its kinetic energy, which measures the energy of motion. Kinetic energy is proportional to the object’s mass and the square of its velocity. Because velocity is squared, a small increase in falling speed leads to a disproportionately large increase in impact force.
This exponential increase in kinetic energy explains why large, high-speed hailstones inflict severe damage. Hailstones falling at over 100 miles per hour can easily shatter vehicle windshields, dent car bodies, and tear through the siding of homes. On agricultural land, the concentrated impact force shears crops and can cause total yield loss.
The effect is amplified when wind is involved, as the horizontal wind speed is added vectorially to the vertical terminal velocity, increasing the resultant impact speed and force. For roofing materials, the threshold for damage begins with hail about one-and-a-quarter inches in diameter falling at terminal velocity. Hailstones with irregular shapes can concentrate their kinetic energy over a smaller contact area, increasing the likelihood of localized damage like punctures.