Hail is frozen precipitation ranging from small pellets to large lumps of ice. The speed at which these ice chunks strike the ground determines their potential for causing damage. Understanding hail speed requires examining the interplay of physical forces in the atmosphere. This leads directly to the concept of terminal velocity, which dictates the maximum achievable speed for any hailstone.
The Physics Governing Hail Speed
A falling hailstone is acted upon by two opposing forces: the downward pull of gravity and the upward force of air resistance, also known as drag. Gravity provides the initial acceleration, trying to make the hailstone fall faster. As the stone’s speed increases, the opposing force of air resistance grows stronger, pushing back against the downward motion.
Terminal velocity is the point where the downward force of gravity exactly balances the upward force of air resistance. At this equilibrium point, the net force on the hailstone becomes zero, and it stops accelerating. It then continues its descent at a constant maximum speed. The size and mass of the hailstone are the primary factors influencing this maximum speed.
A larger, heavier hailstone has a greater gravitational force pulling it down, but the air resistance acting on its surface area does not increase proportionally to its mass. This imbalance means that the ratio of weight to surface area is higher for large hailstones, allowing them to overcome air resistance more effectively. Consequently, a large hailstone will fall much faster than a small one before reaching its terminal velocity. Factors like the hailstone’s irregular shape, its density, and the surrounding air density also affect the drag coefficient and the final fall speed.
Measured Speeds and Terminal Velocity Ranges
The terminal velocity of hailstones varies widely depending on their diameter and mass, with smaller stones falling significantly slower than larger ones. For a small hailstone, measuring about one centimeter (0.4 inches) in diameter, the fall speed is estimated to be around 20 miles per hour (9 meters per second). A golf ball-sized hailstone (approximately 1.75 inches) typically achieves a terminal velocity in the range of 25 to 40 miles per hour.
For truly large hailstones, the speeds can become comparable to highway velocities. Stones with a diameter between 2 and 4 inches have an expected fall speed between 44 and 72 miles per hour. The largest hailstones, those exceeding 4 inches in diameter, have been calculated to approach or even exceed 100 miles per hour. For instance, an 8 centimeter (3.1 inches) hailstone is estimated to fall at a rate of approximately 110 miles per hour (48 meters per second).
These values represent the theoretical maximum speed under ideal conditions, assuming the stone is a dense sphere. However, natural hailstones are often irregularly shaped and less dense than solid ice, which tends to increase the drag force. This results in a slightly slower actual fall speed. Despite these variables, the general principle remains: the larger the hailstone, the greater its terminal velocity.
Hail Size and Impact Energy
Meteorologists communicate hail size by comparing the hailstones to common objects, providing an easy-to-understand scale of potential severity. Quarter-sized hail (about one inch in diameter) is the threshold size considered severe due to its damage potential. This scale progresses through golf ball (1.75 inches), baseball (2.75 inches), and softball (4 inches) to describe increasing size.
The destructive capability of hail upon impact is directly related to its kinetic energy, which is a combination of its mass and the square of its velocity. Because kinetic energy increases exponentially with velocity, a small increase in size leads to a disproportionately large increase in impact energy. A hailstone that is twice as large as another can have an impact energy many times greater.
For example, the impact energy of a 3-inch hailstone is over a hundred times greater than a 1-inch hailstone. Small, pea-sized hail may only cause minor damage to plants, but golf ball-sized hail can shatter windows and severely dent vehicles. The combination of high mass and high terminal velocity means that larger hailstones transfer a substantial amount of energy on impact.