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This week’s question was asked by a friend.

QUESTION: What makes hail?

ANSWER: Nothing good comes from hail. Hail damages roofs, dents airplanes and car tops, strips crops to bare stalks and can even kill people. Hail, yes!

Hail is born in thunderstorms. A thunderstorm, often called a cumulonimbus, is a towering cloud formed from water vapor carried by powerful upward air currents. These clouds can form alone or in groups or along a cold front squall line. They are characterized by lightning and tornadoes can emerge from these dangerous clouds.

A hailstone forms when rain freezes on a particle of dust, sand, and other debris carried in the air by strong gusts of wind. The hailstone grows as it travels up and down: up in the rising air currents and down by the pull of gravity. It’s a roller coaster ride.

Cut open a fallen hailstone and read the story. Rings, much like the tree stump rings, tell the tale. When the ice on the hailstone freezes quickly, at very low temperatures, it traps snowflakes and air bubbles. The ice looks milky. When the ice freezes more slowly around the hailstone, there is time for the tapped snowflake to melt and air escape. The finished ice is clear. The layers tell the story of different temperature regions the hailstone has traveled through in the cloud. Rain water collects on the hailstone as it falls, then that water will freeze as the hailstone rises into regions of freezing temperatures.

The hailstone gets heavier as it travels up and down. Very strong upward winds are required to create a large hailstone. When the hailstone is too heavy for the updrafts to support it, down it comes. A 4-inch hailstone needs updraft winds of about 180 mph. Updrafts of 20 mph will produce pea-size hailstones. The size of the hailstone gives a reading on the degree of violence of the thunderstorm.

The height of the thunderstorm determines if there will be hail. If that cumulonimbus thunderhead builds up to eight miles, there is a 50-50 chance of hail. One that makes it up to 11 miles high will almost certainly produce hail.

A storm cloud making hail will often take on a sick-greenish tint. The top theory is that the storm clouds briefly act as a filter, removing both the ROY (red, orange, yellow) and the BIV (blue, indigo, violet) colors, but not the green. Computer models show that greenish is possible with the right combination of water drop diameter and cloud thickness.

The largest hailstone ever recorded fell out of a severe super cell on July 23, 2010, in Vivian, South Dakota. It measured 8.0 inches in diameter, 18.62 inches in circumference and weighed 1.93 pounds. It was found by Lee Scott and turned over to the U.S. National Weather Service for certification in Aberdeen, South Dakota. It produced an impact pit about 10 inches across on the ground. It was irregular in shape and believed to form by several smaller hailstones freezing and fusing together.

When cold, dry air coming down from Canada meets warm moist air pulled up from the equator regions, the collision will make the weather report. The area of their impact is termed a front. A warm front occurs when a fast-moving warm air mass rides up and over the cold air. Clouds and rain form as the moisture in the warm rising air is wrung out. Cold air can’t hold the moisture that warm air can.

Sometimes the cold air is moving faster, pushing under the warm, air and giving it an upward boost. That’s a cold front. The cold air forces the warm air to rise much quicker than it normally would. Not only do we get clouds and rain but violent storms that may include tornadoes and hail.

Send questions and comments to: lscheckel@charter.net.

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Larry Scheckel is a retired Tomah High School physics teacher.

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Tomah Journal editor

Steve Rundio is editor of the Tomah Journal. Contact him at 608-374-7785.

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