## 212 Hail Size

The average hail fall duration in the Midwest is about 5 minutes and covers an area approximately 20 square miles, which is why it is always prudent to check for collateral damage to nearby similar buildings and structures when a hail damage claim requires verification. Hail falls, however, can last as long as 15 minutes.

The relative size range of hail within a particular hail fall area is usually no more than 1 to 3, that is, the largest hailstone will usually be no more than about three times the size of the smallest hailstone. Thus, the largest hailstones in a particular hail fall area can be readily estimated from the size of the smallest hailstones found there, and vice versa.

Hailstones range in size from less than that of a pea to several inches in diameter. A severe thunderstorm is defined as one that produces a tornado, horizontal surface winds of at least 58 mph, or one that produces hail at least 3/4 inches in diameter. A thunderstorm is said to approach the "severe" category when the associated horizontal surface winds are at least 40 mph, or there is hail with at least a diameter of 1/2 inch.

The largest hailstone recorded in the U.S. was 17.5 inches in circumference. It was found September 3, 1979, in Coffeyville, Kansas. The previous record holder had a maximum circumference of 17 inches, reportedly weighed about 1.5 pounds and fell in Potter, Nebraska, in November 1928.

Stories of hailstones as large as bowling balls have been anecdotally reported, but none have actually been found or otherwise verified. Such hailstones are in the same league with jackalopes and other exaggerations popular in the Great Plains. While hailstones the size of softballs, 4 inches in diameter or 12.6 inches in circumference, do occur, they are rare.

Most hailstones are less than an inch in diameter. Based on possible updraft wind speeds and the air resistance of hailstones, hailstones with 5.5-6 inch diameters are considered to be the theoretical limit. This theoretical limit excludes bowling ball-sized hailstones.

The theoretical upper bound limit for the size of hailstones is derived as follows. First, the relationship of hailstone mass to its radius is determined.

where mhail = mass of hailstone, pice =density of hail ice, and R = mean radius of the hailstone.

When a hailstone is supported and lofted upward within a strongly convective cloud system, the air friction between the updraft and the hailstone has to exceed the downward force exerted by gravity. Thus, the following relation between the force of gravity and the force created by frictional drag is derived:

where F = downward force due to gravity, g = acceleration of gravity, Afrontal = projected area of hailstone facing updraft, R = mean radius of hailstone, pair = density of air around hailstone, C = coefficient of drag, and V = velocity of air supporting hailstone.

Solving for the velocity term in Equation (ii) yields the following:

As a rule, the ratio of the density of hail ice to air at sea level is about 660 to 1. Of course, at higher elevations where hail is formed, the ratio is greater because the density of air is lower. An average ratio value of about 1376 to 1 is assumed for the higher elevations where hail is usually formed. For a spherical hailstone and assuming a Reynolds number larger than 105, a typical value for "C" in Equation (iii) is about 0.2. Substituting in the other values accordingly, Equation (iii) reduces to the following:

Given that the other factors such as the coefficient of friction or the ratio of hail ice density to air density do not change very much, Equation (iv) indicates that the updraft velocity needed to suspend a hailstone within a cloud is proportional to the square root of the radius of the hailstone. Each doubling of the radius of a hailstone requires that the associated updraft velocity increase at least 41.4%.

Using Equation (iv), when R = 1 inch, the minimum required velocity of the updraft, "V," is about 221 ft/sec or 150 mph. Likewise, when R = 3 inches, "V" is minimally about 383 ft/sec or 261 mph.

Table 21.1 shows the relative ranking of wind intensity in tornadoes and windstorms. For a 2-inch diameter hailstone, winds corresponding to an F-2-ranked storm is required. For a hailstone diameter of 6 inches, Table 21.1 shows that winds corresponding to an F-5-ranked storm are needed, which are as big as they come on earth.

Rank |
Wind Speed (mph) |
Level of Damage |

F-0 |
up to 72 |
Light |

F-1 |
73 to 112 |
Moderate |

F-2 |
113 to 157 |
Considerable |

F-3 |
158 to 208 |
Severe |

F-4 |
209 to 250 |
Devastating |

F-5 |
251 and more |
Incredible, Atomic bomb-like effects |

Bowling balls, by the way, have a diameter of 8.5 inches, or an "R" of 4.25 inches. Using Equation (iv), this computes to an updraft velocity of at least 455 ft/sec or 310 mph. This is about Mach 0.41, or41%of thespeed of sound at sea level. A convective updraft of this magnitude has yet to be measured on earth. This is why reports of hailstones the size of bowling bowls, without actually having one to provethepoint, shouldbe considered an exaggeration. Of course, if such bowling ball-sized hailstones really did fall, the accompanying F-5 magnitude tornado or windstorm would make damage claims from the hail a moot point. Hail damage would be the least of their concerns.

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