Hail! Hail! Summer’s Almost Here

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A softball-size hailstone. Photo credit: NOAA

A softball-size hailstone. Photo credit: NOAA

Of all the types of frozen precipitation, hail is the most mysterious.

Sleet is just frozen raindrops. Freezing rain is liquid raindrops freezing on contact with a surface. Snowflakes form high in clouds and simply flutter to the ground. But why should chunks of ice the size of baseballs, or bigger, fall from the sky?

Dynamics Of Hail Formation

There is no mystery about how a hailstone is constructed. If you slice one in half, you will see concentric rings of ice. Just as the rings of a tree indicate the growth during successive years, the rings of a hailstone indicate successive layers of freezing.

But why would this be so? A raindrop or snowflake simply falls to the ground. If a small hailstone is in the vicinity of other frozen matter, it will not accrete ice. If it falls to where the temperature is warmer than freezing, some liquid drops could freeze, but the small iceball would fall to the ground, unless…

Vertical Currents In The Atmosphere

Most of the motion in the atmosphere is horizontal. Typically vertical (up and down) motions are less than ten percent as large as horizontal motions. But occasionally strong updrafts and downdrafts occur. This happens when the atmosphere has become unstable to any vertical displacement. This means if you lift a parcel of air it will be warmer than its surroundings and keep rising. This happens most often when the lower layer of air is saturated and dry air lies above.

Under unstable conditions, powerful updrafts and downdrafts (what goes up must come down) can lift and lower a small hailstone, and each time it comes into contact with raindrops above freezing (or even below freezing where raindrops are ‘supercooled,’ still in a liquid state), the raindrops freeze on the surface of the hailstone. Alternate lifting and lowering of the baby iceball can create a frozen baseball. A hailstone can go up and down until it is so heavy that not even the most powerful updraft will keep it aloft.

If a large hailstone gets caught in a downdraft, it can hit the ground at 100 miles per hour, about the speed at which a major league baseball pitcher’s fastball approaches home plate.

Hail alley. Graphic courtesy of NOAA

Hail alley. Graphic courtesy of NOAA

Where Are The Conditions Favorable For Hail Formation?

The United States midland is a perfect location for the conditions that lead to large hail. Near the surface, warm, humid air streams north from the Gulf of Mexico; dry air comes eastward from the Rocky Mountains and lies on top of the saturated air, creating conditional instability. If a cold front comes through and lifts the whole column of air, or even if daytime heating creates localized updrafts, violent overturning can result. The proximity of mountains with their upslope currents increases the number of lifting mechanisms.

The map of hail occurrence in the United States looks strikingly like that for tornado frequency — not surprising since each phenomenon is born of conditional instability.

Hail Alley includes parts of Texas, Oklahoma, Arkansas, Kansas, Nebraska, Colorado, and Wyoming.

Every continent except Antarctica experiences hail, and there are several hotspots besides the American plains. The general condition of mountains in close proximity to a large body of water exists in south China and India. Each has its own ‘Hail Alley.’

How big is the hail? Meteorologists have size ratings based on easy-to-visualize measurements. Image by Decoded Science, all rights reserved.

How big is the hail? Meteorologists have size ratings based on easy-to-visualize measurements. Image by Decoded Science, all rights reserved.

Categorizing Hailstones By Size

Meteorologists have developed a highly sophisticated nomenclature for the different sizes of hailstones.

Soccer balls, basketballs, and footballs are obviously too big to qualify for a name, but Decoded Science is unable to expoain why the hockey puck, at 3 inches, has been bypassed in favor of afternoon tea.

In the interest of bringing some uniformity to this list, Decoded Science recommends that countries either issue more coins of different sizes, or invent more sports with different-sized equipment.

Maximum Hail: The Records For Weight, Diameter, And Circumference

Measuring the largest hailstone is not as easy as it sounds; a hailstone begins to melt as soon as it reaches the ground. On July 23, 2010, Les Scott of Vivian, South Dakota recovered a large hailstone that had fallen in a violent storm. He stored the iceball in his freezer, and after due examination by National Weather Service personnel, the National Climate Extremes Committee validated the stone’s measurements.

The record hailstone’s diameter was eight inches and it weight one pound, 15 ounces – it holds the current world record for ‘largest hailstone.’

Because of the variability in the shapes of hailstones, a seven-inch-diameter stone that fell in Aurora, Nebraska in 2003 still holds the record for largest circumference at 18.75 inches.

Damage From Hailstorms

Hail does approximately a billion dollars worth of damage to crops and property in the United States each year. In April, 2001, an outbreak of severe hailstorms caused $1.9 billion in damage from Texas to Pennsylvania over six days. And in April, 2003, a three-day outbreak of hailstorms did $1.6 billion in damage from Texas to Tennessee.

As global warming increases the ability of the atmosphere to hold moisture (warm air holds more water vapor than cold — more water vapor, more raindrops), an increase in the number of hailstorms — and the damage they cause — is a distinct possibility.

 

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