Hail: Summer's Ice Storms

Hail is a form of precipitation that falls from the sky as ice pellets that can range from small pea-sized projectiles to humongous hailstones as large as grapefruits. Hail generally forms when there's a severe thunderstorm in the vicinity and can be a warning to monitor your local weather situation closely for lightning, torrential rain—and possibly even tornadoes.

Not a Winter Weather Event

Because it's made of ice, hail is often mistaken as a cold-weather event, but in reality, it's not winter weather. While the thunderstorms with which hail is associated can happen any time of year and at any time of day, they are most likely to take place in the spring and summer months, especially from May through August. 

Likewise, hailstorms can occur year-round, however, the most destructive hail events usually occur at the height of summer. This makes sense since the most destructive thunderstorms tend to take place when there's an abundance of atmospheric heat to fuel their development. 

Hail usually forms over an area and leaves within a few minutes. However, there have been instances when a hail storm has stayed in the same area for several minutes, leaving several inches of ice covering the ground.

Hail Forms High Up, in Cold Clouds

Okay, but if hail is a summer weather event rather than a winter one, how do temperatures get cold enough to form ice, you ask?

Hailstones are formed inside cumulonimbus storm clouds that can measure towering heights of up to 50,000 feet. While the lower portions of these clouds contain warm air, the temperatures in the upper portions are below freezing.

Strong updrafts inside the storm system whisk raindrops up into the sub-zero zone, causing them to freeze into ice crystals. These ice particles are subsequently carried back down into the cloud's lower levels by a downdraft, where they thaw a bit and collect additional water droplets before being wafted back up to the deep freeze a second time.

This cycle can continue multiple times. With each trip above and below the freezing level, a new layer of ice is added to the frozen droplet until it finally grows too heavy for the updraft to lift it. (If you cut a hailstone in half, you'll see alternating concentric layers inside that resemble tree rings.) Once this happens, the hailstone falls out of the cloud and heads to the ground. The stronger the updraft, the heavier a hailstone it can carry and the longer a hailstone cycles through the freezing process, the larger it grows.

Hailstone Size and Speed

Hailstones are measured according to their diameter. But unless you have a knack for eyeballing measurements or are able to slice a hailstone in half, it's easier to estimate its size by comparing it to everyday items.

To date, the largest hailstone recorded in the United States fell in Vivian, South Dakota, on July 23, 2010. It measured eight inches in diameter, 18.2 inches circumference, and weighed one-pound-15-ounces.

Hail Damage

The velocity of hail varies by shape and size. The largest and heaviest hailstones can fall at speeds upwards of 100 mph. With their hard exteriors and relatively fast speed of descent, hailstones can cause extensive damage. On average, over $1 billion dollars in damage to crops and property is sustained each year in the United States alone. The most susceptible objects to hail damage include vehicles and roofs. 

One of the most costly hail events in recent weather history occurred in June 2012 when severe storms crossed over the Rockies and Southwestern United States causing over $1 billion dollars in damage in the state of Colorado.

The Top 10 Hail-Prone Cities in the U.S.

• Amarillo, Texas
• Wichita, Kansas
• Tulsa, Oklahoma
• Oklahoma City, Oklahoma
• Midwest City Oklahoma
• Aurora, Colorado
• Colorado Springs, Colorado
• Kansas City, Kansas
• Fort Worth, Texas
• Denver, Colorado