The Process of Hail Formation (Credit: World Atlas/ Weather Underground/ The Weather Channel)

Hailstones measured after a thunderstorm in Zurich, Switzerland.
Credit: Arnd Weigmann/ Reuters (Newsweek)
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Hail is a type of precipitation that forms from water droplets within cumulonimbus clouds. During the updraft of a thunderstorm, water droplets become supercooled as they rise up into temperatures below freezing. At such cold temperatures, these water droplets condense onto condensation nuclei, such as dust particles within the cloud, and then freeze onto this condensation nucleus. The small ice particle that is formed during this condensing process within the cloud then begins the growth process before precipitating from the cloud as hail.

Credit: Daphne Johnson (WeatherOps)
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One way that hailstones can grow is throughout a process known as wet growth. During wet growth, supercooled water droplets continue to collide with the ice particle formed during the initial condensing onto the condensation nucleus. During the collision, the water from the water droplets spreads out surrounding the ice particle, causing the ice particle to freeze slowly. This type of growth creates ice particles that are relatively clear, as the supercooled droplets slowly freeze onto the particle. Hailstones can also grow throughout a process known as dry growth, in which the supercooled water droplets collide with the ice particle and freeze immediately on contact. Hailstones formed during dry growth formation tend to have air bubbles from the supercooled droplets trapped and frozen into the ice particle. These air bubbles can occur because the freezing process occurs so quickly. This process can create hailstones that appear cloudy as well as have irregular shapes. 

Large hailstones measured after a severe thunderstorm in South Dakota.
Credit: NOAA
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Hailstones remain within the below freezing area of the cloud until they collide with more and more supercooled water droplets, and thus grow greatly in size. When a hailstone becomes too large or too heavy to be supported by the updraft of the thunderstorm, the hailstone falls from the cloud due to gravity during the downdraft. Hailstones vary from an average size of approximately 5 mm to 15 mm, but records show that they can grow much larger. The hailstone that holds the record for being the largest hailstone in the United States is a hailstone from South Dakota that is over 8 inches in diameter. Hailstones that grow to these sizes can cause severe damage to cars, trains, livestock, crops, and airplanes. Cloud seeding is a process that is currently aiming in part to prevent hail from occurring in certain areas by shooting chemicals, such as silver iodide, into the sky. These chemicals act as the condensation nuclei for precipitation formation within the cloud, and thus can alter the type of precipitation that falls from the cloud. Hailstones propose high hazards to people located wherever they may fall, especially due to the fact that they are able to grow to such destructive sizes. 

To learn more about hail and other severe weather phenomena, be sure to click here!

©2019 Weather Forecaster Christina Talamo